In the sparkling sunshine yesterday, runners at the finish line of the Boston Marathon said they had taken very much to heart the new warnings about drinking too much water during a race. ”I was conscious of not taking huge amounts of water,” said Ian Bloomfield, 52, of England, who pronounced himself ”quite pleased” with his time of 2 hours 45 minutes. ”I was very aware of hyponatremia,” the potentially fatal result of overhydration.  Brian Paff, 24, of Chicago said he had had hyponatremia in a college race. ”I passed out from it,” he said. Yesterday, he was careful not to drink too much.

In a dramatic turnabout, sports doctors now say that drinking too much water can be as bad or worse than not drinking enough. This holds for both endurance athletes and weekend warriors, though most people who exercise for just an hour or so don’t really need to worry about either dehydration or overhydration.

In a study published last week in the New England Journal of Medicine, Boston doctors, who have long made Marathon runners their living laboratory, found that hyponatremia (a low concentration of sodium in the blood because of too much water) occurred in 13 percent of participants. Of the 488 marathoners in the 2002 study, three had such low blood sodium that they were at risk of severe brain swelling, which can lead to brain damage, coma and death.It was during the 2002 Boston Marathon that 28-year-old Cynthia Lucerodied of hyponatremia. Worldwide, some researchers believe there may be as many as one death per marathon, though not all are due to hyponatremia. Sports drinks, once thought to be an athlete’s protection against hyponatremia, turn out not to be. Even though they contain some sodium, it’s not enough to make a difference, the study showed.Many endurance athletes, especially smaller, slower runners who take longer to finish and therefore have more time to drink, actually gain weight during marathons, sometimes as much as 8 to 10 pounds, said Dr. Christopher S.D. Almond, lead author of the study and a cardiology fellow at Children’s Hospital. Women are at greater risk because they tend to be smaller and slower and also, perhaps, because they may more diligently adhere to the old belief that it’s wise to drink lots of water.Excessive water drinking among athletes has become such a concern among sports doctors that guidelines are rapidly changing. At major endurance races around the country, including yesterday’s marathon, doctors now do an on-the-spot test for blood sodium levels on collapsed runners to differentiate between hyponatremia and dehydration. Giving fluids to someone with hyponatremia could be fatal.           

The Boston Marathon has had these tests at the finish line for several years and this year, for the first time, had them along the course as well. In addition, Marathon officials provided scales along the route and at the finish line, and encouraged runners to write their pre-race weight on their bibs to check for weight gain.”This has been a major paradigm shift in the last few years,” said Dr. Benjamin Levine, coauthor of an editorial in last week’s medical journal and a cardiologist at the Presbyterian Hospital of Dallas. ”It used to be thought that thirst was a poor measure” of impending dehydration and, therefore, that athletes should drink as much as they could before thirst set in. ”That is clearly not correct,” he said.In 2003, USA Track and Field, which governs track-and-field events, began saying that athletes should use thirst as a guide for fluid replacement. The International Olympic Committee Medical Association began recommending caution in fluid consumption last year, just prior to the Athens Games. The International Marathon Medical Directors Association now advises drinking no more than about 12 to 25 ounces of fluid an hour, especially for slower, back-of-the-pack runners.Other guidelines now in the works are likely to follow suit, including some from the American College of Sports Medicine and a consensus statement from experts who met recently in South Africa and will publish their recommendations this summer. This shift in thinking reflects the growing awareness the dangers of excess fluidconsumption.

”Hyponatremia is less common, but more dangerous” than dehydration, said Dr. Peter Moyer, medical director of Boston Fire, Police and Emergency Medical Services.  

Dehydration does have clear, adverse effects on performance, which is a major issue for elite athletes, said Michael Sawka, chief of the thermal and mountain medicine division at the US Army Research Institute of Environmental Medicine in Natick. 

”But if you’re not doing high-level performance and you’re not losing a lot of water, it’s not important to drink,” he said. ”There are no real adverse health consequences” to dehydration, except low blood pressure, ”susceptibility to heat exhaustion and, if you’re out there long enough, heat stroke.”    Like hyponatremia, heat exhaustion does make people feel ill, get nauseous, have muscle cramps and feel dizzy upon standing up quickly; heat stroke includes all that plus mental-status changes, such as confusion about one’s identity and location.  But dehydration ”would have to be extreme and prolonged, like being lost in the Sahara for days, to become fatal,” said Dr. William G. Goodman, a kidney specialist at the David Geffen School of Medicineat UCLA.  So how can you tell how much fluid you need during exercise? The best approach is to weigh yourself before and after exercise, said Dr. Arthur Siegel, chief of internal medicine at McLean Hospital in Belmont. If you lose, say, 3 percent of your body weight after a workout, ”that represents mild dehydration,” he said. It is enough, though, to be a ”green light” to rehydrate slowly, over eight to 12 hours. If you gain 3 percent of body weight, that’s serious — a red flag or hyponatremia.So, whether you’re a marathoner or a casual athlete, make it a point to weigh yourself before and after workouts. If you do gain weight, don’t drink fluids (salty broth is OK if you have a headache or salt craving) until after you have urinated spontaneously. The point is to let your body get rid of excess water.

This column is for everyone who hates to exercise, or would like to exercise, sort of, but really, truly, deeply believes they don’t have enough time or just can’t do it.

First, if you’re in this category, take heart: You’re not alone. Two-thirds of Americans are now overweight or obese, according to government figures, and more than half do not get enough physical activity – and that’s according to the old, wimpier guidelines.

Two weeks ago, federal health officials upped the ante, issuing tougher, new exercise standards as part of the Dietary Guidelines for Americans 2005. Like the old guidelines, the new ones recommend at least 30 minutes of moderate activity on most days of the week.  This figure is based on solid science and translates to walking two miles in 30 minutes, or the equivalent.

This will not make you buff and beautiful. But it will reduce your risk of high blood pressure, stroke, coronary artery disease, type 2 diabetes, colon cancer and osteoporosis.

Now – and here’s the tough part  – the new guidelines also say it may take an additional 30 minutes a day, for a total of 60, to prevent weight gain. And another 30  – 90 minutes in all – to sustain weight loss in previously overweight or obese people.

Granted, that does sound daunting. But here’s how to make it less so.

For openers, the 60 minute recommendation is not absolute and is not based on as much data as the 30 minute recommendation for general health or the 90 minute advice to prevent weight regain, said Steve Blair, president and CEO of the Cooper Institute in Dallas, a non-profit exercise research organization.

While some people may need 60 minutes a day to prevent weight gain, he said, others may hold the line with 40 and others may need more than 60.

“It’s not a flat 60 minutes for everybody,” explained Russell Pate, an exercise physiologist at the University of South Carolina and a member of the committee that advised the government on the new recommendations. “There is a wide range of activity levels that different people need to prevent weight gain.”

Okay, the time issue. You probably have more than you think: 24 hours a day, like everybody else.

The problem is that while “people’s leisure time has increased, most of that goes into watching television,” said Susan Hanson, a geographer at Clark University in Worcester, MA who chaired a committee convened by the National Research Council, part of the National Academy of Sciences, to study how the “built” environment affects physical activity. Studies reviewed by her committee show that the average American watches TV for three hours a day.

So, one obvious step is to skip 30 to 60 minutes of TV and walk instead – around the house, in the mall, outside, wherever.  Get a pedometer (they’re cheap – $15 and up) and see how many steps you take on an average day. If it’s 3,000, try to make it 4,000. If it’s 4,000, try to make it 5,000. At 10,000 a day, you’ll be close to the basic guideline.

If you absolutely can’t give up TV time, turn it to your advantage. Get up and walk during the commercials. Get a treadmill or exercise bike (used ones can be fine) and work out while you watch.

Buy light weights to lift as you watch your favorite shows. (They’re roughly $3.50 for a 5-pound dumbbell and $24 for a 10-pound ankle weight.) The best use of a weight “is to throw it through the front of the TV,” joked Dr. Christopher B. Cooper, an exercise physiologist and professor of medicine at the David Geffen School of Medicine at UCLA.

Used as intended, weights can build muscle mass. This doesn’t directly boost cardiovascular health, but every pound of muscle burns roughly 280 calories a week because muscle is metabolically active 24/7, not just while you’re pumping iron. A pound of fat, Cooper noted, burns less than 20.

There are other easy steps, too. If your goal is 60 minutes of exercise a day, break it into six 10-minute bouts. Take a walking break from the computer every hour.  Get off the bus a few stops earlier and walk the rest of the way. Go for a walking meeting with your boss.

If child care is your excuse, exercise with the kids, said Johanna Hoffman, an exercise physiologist at Johns Hopkins Weight Management Center.  Kids love those elastic resistance bands for building strength, she said. Or create a “walking schoolbus” whereby you walk your kids (and your neighbors’) to and from school when the weather’s decent.

If you’ve failed at exercising on your own, get a buddy and commit to walking or working out together. If lack of energy is your problem, don’t despair.  You may feel tired at first and only be able to walk a few blocks. But if you stick with it, you’ll find that exercise actually gives you energy.

Too much housework? Give yourself some credits for vacuuming, raking leaves, cutting grass, even ironing. These activities probably won’t equal walking briskly, but they’re better than sitting on the couch.

Finally, recognize that you will have setbacks. Don’t let these depress you into giving up. It’s a sad fact of life that if you weigh, say 150 pounds, walking a mile in 20 minutes will only burn about 100 calories (probably less than a cookie), said William J. Evans,  chief of the nutrition, metabolism and exercise lab at the University of Arkansas for Medical Sciences. If you’re out of shape and overweight, “the actual amount of calories you burn won’t be huge until you get better trained,” he said.

But, except for “the sickest of the sick,” it pays to start, even for people with severe heart failure, said Dr. Robert Sallis, a sports medicine expert fellowship at the Kaiser Permanente Medical Center in Fontana, CA. Sallis “prescribes” exercise for all his patients “except for those I’m admitting to the ICU,” or intensive care unit in a hospital.

Start off by just doing as much as you can, he said, and by following the “FITT” mnemonic, which stands for “frequency” (preferably, most days a week); “intensity” (hard enough so that you can’t sing but you can talk);  type (exercise that works major muscles groups like the arms and legs); and time (at least 30 minutes a day).

As with much else in life, the key is to just start. Then keep going.

I have been standing on my head, off and on, for about 35 years now,  as well as sitting cross-legged breathing through one nostril at a time, and — my favorite — lying flat on my back, utterly relaxed, in the so-called “corpse pose.”I am, in other words, one of the 15 million Americans who, according to a Harris Interactive Service Poll done in 2003 for Yoga Journal, have fallen in love with this ancient Indian practice that’s part meditation, part exercise. To the cognoscenti — and our numbers grew by nearly 29 percent between 2002 and 2003 — yoga is a pleasant practice that seems to enhance physical and emotional strength, flexibility and balance. But does it?

Well, to the extent that yoga overlaps with what the so-called “relaxation response,” it’s no leap at all to conclude that yoga is good for you. The “relaxation response,” a term coined years ago by Dr. Herbert Benson, president of the Mind/Body Medical Institute in Boston, consists basically of quieting the mind and body through prayer, contemplation or focusing on something simple, like breathing.

The relaxation response has been shown to lower blood pressure, heart rate and respiration; to reduce anxiety, anger, hostility and mild-to-moderate depression; to help alleviate insomnia, premenstrual syndrome, hot flashes and infertility; and to relieve some types of pain, like tension headaches.

But for yoga itself, there’s not much scientific evidence that the practice confers its own specific health benefits — though that doesn’t seem to dampen anyone’s enthusiasm, including my own.

“There is not enough really good research to draw strong conclusions about anything about yoga,” said health psychologist and yoga teacher Roger Cole of Synchrony Applied Health Sciences, a health promotion consulting firm in Del Mar, Calif.

Take standing on your head. There is some data suggesting that inversion may slow heart rate and make people secrete less of a stress hormone, norepinephrine. “The question,” said Cole, “is whether that amounts to clinical benefits.”

Some yoga teachers claim that standing on your head increases blood flow to the brain, a supposedly good thing. That’s nonsense, said Dr. Timothy McCall, a slim, Somerville physician and yogi who writes a health column for Yoga Journal. “Blood flow to the brain is tightly regulated,” he said, so going upside down probably doesn’t bathe the brain in extra blood. And standing on your head could worsen glaucoma (increased pressure within the eye) and or problems with the retina. That said, McCall is still convinced that headstands “have a profound effect on slowing the body down.”

McCall, who, perhaps more than anyone else, has tried to assess the science of yoga, has visited research institutes in India, where most of the yoga studies are being done.

Though some of the research — both in India and the West — is methodologically flawed, yoga has more than 50 documented effects, including greater strength, increased flexibility, better balance, better cholesterol levels and better mood.Of five studies on asthma since 1985, three showed statistically significant benefits. One 1998 study on carpal tunnel syndrome (pain caused by pressure on a nerve in the wrist) found yoga could reduce some symptoms and improve grip strength. A couple of small studies found that yoga can lead to both subjective and objective improvements in COPD, or chronic obstructive pulmonary disease, a disease in which the airways and air sacs in the lungs lose their elasticity, making it difficult for air to flow in and out.

Another small study found that the slow, diaphragmatic breathing of yoga helps increase oxygenation in some patients with congestive heart failure. Several studies show yoga helps improve symptoms of coronary artery disease, though the patients also made other changes — like switching to low-fat diets. One study of severe depression found that the deep breathing (pranayama) of yoga, electric shock therapy and drugs all improved scores somewhat on a standard depression test.

At Boston’s Brigham and Women’s Hospital, senior neuroscientist Sat Bir Singh Khalsa is studying yoga as a treatment for insomnia. At the University of Texas MD Anderson Cancer Center in Houston, Dr.Lorenzo Cohen, director of the integrative medicine program, hasfinished a 7-week study of 39 men and women with lymphoma that waspublished in April. Cohen randomized patients to receive instruction in Tibetan yoga or no special intervention. Those who practised yoga slept better than those who didn’t, though there were no differences in other measures such as anxiety, depression or fatigue.

Researchers at the UCLA Jonsson Cancer Center are about to start a study of yoga to combat fatigue in women with breast cancer. Put bluntly, yoga may indeed have more health benefits than have been documented so far. But the skimpiness of solid data so far means: Buyer Beware. If you’re in a class and the teacher makes absurd claims — like a certain pose will make your spleen happy — smile serenely and think of the claim as a metaphor.

And choose your teacher with care. Unlike hairdressers and manicurists, yoga teachers are not licensed. Anational organization called Yoga Alliance (www.yogaalliance.org) lists teachers who have completed various levels of training, but it provides no real evidence of competence. Some methods of yoga, like the Iyengar system, do have a rigorous, multi-tiered system of certification, says Patricia Walden, director of the BKS Iyengar Yogamala of Cambridge. But it’s still a crapshoot. So good luck. And, as they say in Sanskrit when one person puts her palms together and offers a humble greeting to another, Namaste.

It can be rather bewildering, frankly. We all know by now — duh — that we’re supposed to exercise.But how? By lifting weights? Or running? How much? A short walk from the parking lot to the office, or miles and miles a day? How often? Once a week on Saturday mornings, if someone else can watch the kids, or every day? How hard? Til we’re a breathless, sweaty mess or just pleasantly glowy? And most of all, in our crazy, overscheduled-lives, when?

Great minds have tackled these questions, and are tackling them again, with new federal exercise guidelines due out this summer. But one of the conclusions is already known: Exercise is just about the closest thing to a magic bullet that modern medicine can recommend. It’s very, very good for you for a long list of reasons that we’ll get to in a minute.

Think evolutionarily. We are all animals, not that different from zebras, lions, dogs, etc. Animals do not lie on the couch all day eating potato chips and watching TV. (Okay, maybe your zebra does, but he probably wouldn’t if he had the choice.)

Animals are built to run around, hunt, eat, sleep, procreate and then do it all again. They probably even like the running around part. Studies suggest that rodents, when put in cages with little wheels to run on as well as toys to play with, prefer the exercise wheels. In other words, our animal souls think of exercise as fun and essential, not some unwanted burden imposed by experts.

Okay, so you’re not buying that. But you’re still determined to exercise and you can’t possibly do it all — the strength training with weights, the yoga for flexibility, the brisk walking or running or swimming for cardiovascular fitness. What’s most important? Simple: The aerobic, cardiovascular stuff, in other words, getting your heart rate up. This doesn’t mean strength training is not important — it is, for building muscle mass and stronger bones. Stretching and flexibility are important, too. But if you have to choose, do the exercise that has been shown overwhelmingly to prolong life and reduce chronic disease.

A number of epidemiological studies show that 30 minutes a day of moderate intensity physical activity is enough to significantly reduce the risk of many diseases, including heart disease, hypertension, stroke, diabetes, certain cancers and hip fractures, said Dr. I-Min Lee, an associate professor of medicine at Harvard MedicalSchool

That may not be enough to achieve and maintain weight loss, cautioned William J. Evans, chief of the nutrition, metabolism and exercise lab at the University of Arkansas for Medical Sciences. “But there is no question that 30 minutes a day is enough to make a significant difference in life expectancy and health.”

So how hard do you have to work at it? The experts waffle a bit on this. But very roughly, if you’re a 30 year old, you should exercise aerobically at a heart rate of between 155 and 165; if you’re 40, it’s 145 to 155; if you’re 50, it’s 140 to 150. If you’re 60, it’s 130 to 140. If you’re 70, it’s 125 to 135. (The actual formula is slightly complicated, but here it is. Take the number 220 and subtract your age, say, 60. That gives 160. That’s an estimate of your maximal heart rate. Now subtract your average resting heart, say 60, from this, which gives 100. Then you take 70 percent of that number (100), which gives 70. Add that number (70) to your restingheart rate (60), and you get 130, the heart rate you should be aiming for during aerobic exercise.)

The goal of 30 minutes a day comes from guidelines set in 1995, when a team of exercise physiologists and doctors from the federal Centers for Disease Control and Prevention and the American College of Sports Medicine and others tried to figure out what to recommend for average Americans, most of whom, then as now, didn’t exercise. Though they recommended 30 minutes a day, they softened this by saying people could “accumulate” 30 minutes in three bouts of 10 minutes each, a moreachievable goal for many people.

The new guidelines expected out this summer will probably recommend the same. The difference, said Dr. Bill Kraus, a cardiologist and exercise guru at Duke University Medical Center, is that “when those [1995] guidelines were written, no one had a clue.” Today, they do, especially for the question of intermittent versus continuous exercise. In one study published in 1999, exercise physiologist John Jakicic at the University of Pittsburgh found that for weight loss, body composition, improvements in blood pressure, cholesterol and blood sugar, it made no difference whether the exercise was continuous or intermittent. That makes sense, he said, because those factors “are all about energy burn.” The only place in which continuous exercise was better was in fitness measures (like one that physiologists called “VO2 max”), a measure of the efficiency with which the heart and lungs getoxygen to the muscles and the efficiency with which the muscles use oxygen.

For weight control, a report in 2002 from the Institute of Medicine, an arm of the National Academy of Sciences, concluded that 30 minutes of exercise a day is “insufficient” to maintain healthy weight. It probably takes 60 minutes, especially if you’ve been overweight Steven Blair, president of the Dallas-based Cooper Institute, a leading exercise research center, is a case in point. “I need even more than an hour of vigorous exercise. I have run nearly every day for 35 years and now, at 65, I run 25 miles a week, which is a lot for a fat, old man.” Even so, he’s gained 25 to 30 pounds.

But exercise clearly helps to some extent with weight control, even if you make no changes in diet. At Duke University, cardiologist Kraus and his team took 240 typical Americans — sedentary, overweight men and women aged 40 to 65 with mild to moderate cholesterol problems — and divided them into four groups for a 6-month study. There was no change in diet for anyone.

Over the six months of the study, the control group — typical, non-exercising Americans — gained three pounds. The other groups lost weight, with those who exercised more at a higher intensity losing more weight than the gentle exercisers.

The bottom line, says Kraus, is this: Any exercise is better than none, and more is better than less.

What this fat, out of shape country needs is a Diet and Exercise Czarina. I hereby volunteer.

First, let’s get two philosophical things straight. Eating too much and exercising too little is obviously a matter of individual responsibility. Nobody’s force-feeding us, or tying us down so we can’t exercise.

But the fact that 64% of Americans are now either overweight or obese, the fact that physical inactivity combined with bad diet is overtaking tobacco as our leading killer, and the fact one in every four adults does little or no exercise all suggest that other things are at play, too.  We live in what some call a “toxic” environment – surrounded by junk food and stressed by too much work and too little time to work out.

In other words, the forces beyond our immediate control – government, employers, the food industry, health insurers, schools, city planners – could do a lot more to supplement our individual efforts to get trim and fit.

For instance, Congress could pass and the president could sign something like the bill recently sponsored by Sen. Tom Harkin (D-IA) that would force chain restaurants – and vending machines – to display the calories, sodium, fat and transfat content of menu items. (Instead, the House recently passed the so-called “cheeseburger bill,” which bans lawsuits that blame the food industry for making us fat.)

Restaurants could offer lower-calorie kids’ meals. A recent study by the Washington-based Center for Science in the Public Interest showed that some kids’ meals have enough fat and calories to choke an adult. The group tested food from chain restaurants and found that a “Boomerang Cheese Burger with Fries” at the Outback Steakhouse contained 840 calories, more than half of what a sedentary kid aged four to eight needs a day.

Schools could add a BMI, or body mass index, score to kids’ report cards.  Arkansas schools already do this, says exercise physiologist William J. Evans, chief of the nutrition, metabolism and exercise lab at the University of Arkansas for Medical Sciences. (The BMI is a ratio of height to weight. To calculate it, take your weight in pounds and multiply by 703; then divide this number by height in inches squared. Ideally, BMI should be about 23, maybe slightly older for older folks.)

Schools could also bring back physical education. Only 8 percent of elementary schools, 6.4 percent of middle schools and 5.8 percent of high schools provide daily physical education, according to the federal Centers for Disease Control. Another no-brainer: Schools could restrict the sale of sugary drinks, as the American Academy of Pediatrics recommends.

Employers could do a lot more, too. Helen Darling is the president of the Washington, D.C.-based National Business Group on Health, which represents 207 of the country’s largest employers. Big employers are just “starting to wake up” to the impact that America’s obesity and flabbiness have on the bottom line, she said.

For years, “the view among enlightened employers was that things that have to do with one’s person – what you eat, whether you exercise or not – was none of their business. It would be considered invasive. But as of just about a year and a half ago, our organization looked at what drives health care costs and disability costs and productivity,” she said. Members were “stunned.”

Medical claims for an obese employee are typically twice as high as those of a non-obese employee, her group found. Indeed , obesity costs American business an extra $13 billion a year in health, disability and life insurance, as well as in paid sick leave. (Overall, obesity costs the country $117 billion a year.)

Even more startling, the biggest increases in both medical and disability claims are in younger employees. A recent study by the Rand Corporation, a nonprofit California think tank, found a 40 to 50 percent increase in recent years in the number of people aged 30 to 49 who are too disabled to care for themselves or do routine tasks. “The only factor researchers could identify that would explain such a large jump in disability is obesity,” the Rand researchers said in a prepared statement.

So what can businesses do? Plenty, said Darling.  “The number one way to have any kind of fitness as part of the [corporate] culture is if the CEO and the CFO and the senior VPs are fitness buffs.”  Behavior change is tough, but “the workplace is the facilitator,” she said. After all, workplace smoking bans helped many Americans quit. To encourage fitness and weight loss, employers can make stairwells well-lit and cheerful, they can provide healthy food choices in the cafeteria and offer to pay for health risk appraisals from an employee’s own doctor, and give employees who follow their doctors’ plans $100 or extra benefits.

Company gyms can help, but not if  “people are looked down on” for taking time from work to exercise, noted James Hill, director of the Center for Human Nutrition at the University of Colorado Health Sciences Center in Denver. A better approach may be “walking meetings.”  One CEO in Colorado, he said, goes for a walk every day at 3 and employees are free to join him.

City planners and civic groups could help by making streets more pedestrian-friendly. Nationally, a group called Walkable Communities, Inc. helps civic groups do just that. In Boston, a group called WalkBoston worked with city planners to make sure that some of the green space freed up by the Big Dig construction project becomes walkable, says Liz Levin, the group’s president. [Note: I have just been put on the WalkBoston board.]

Health insurers, too, are beginning to create incentives to boost the survival of the fittest. HealthCare Dimensions in Tempe, Arizona, sells a program called “Silver Sneakers” to 21 health insurers in 14 states, including Fallon Community Health Plan in Massachusetts. Companies like Fallon buy the program for their customers on Medicare, and the Silver Sneakers folks contract with private health clubs and not-for-profit fitness centers like YMCAs to offer classes – free – to enrolled seniors.

Mohit Ghose, a spokesman for America’s Health Insurance Plans (AHIP), a Washington-based industry group for the insurers who cover more than 200 million Americans, says many health plans already offer some kind of exercise benefit. About 18 percent offer exercise counseling or cardiovascular fitness programs as a basic benefit. Roughly 76 percent will help customers get access to a gym at a discount, but “people need to ask the question by calling the 800 numbers of their plans,” he said.

Starting in January, PacifiCare, a consumer health organization in Cypress, CA, has been offering a “health credits” program that offers prizes – like exercise bikes or treadmills – to members who earn credits by participating in health promotion programs, such as exercise or smoking cessation.

None of this is rocket science. But as Czarina, I have hope, and not just because I’m an idealist. As the big players figure out how badly obesity and sloth affect the bottom line, things will have to change.

Hardly anyone thought it was possible for a human being to run a mile in less than four minutes – until Roger Bannister did it in 1954. Within 3 years, nine other men had done it, too.

In the dark old days, some people thought women shouldn’t compete at all for a variety of silly reasons, including the belief that too much exercise might dislodge the uterus, rendering a woman infertile.

Once women finally did start running marathons, it was considered a given that none could run one in less than 2 hours and 20 minutes –  until Paula Radcliffe ran the (flatter-than-Boston) London Marathon exactly one year ago today (April 13th) in 2:15:25.

Athletic records are made to be broken – that’s the fun of it. But there must be some limits to human performance, right? After all, as Steven Blair, president and CEO of the Cooper Institute, an exercise research center in Dallas, put it, “A time of zero seconds will never be achieved in the Boston Marathon, to state the ridiculous.”

So what are the factors that limit performance, especially in endurance events like the marathon? There are many, though some exercise physiologists nonetheless believe someone someday just might run a marathon in under two hours.

“We still don’t completely know” what all the limits of human performance are, said Miriam Nelson, director of the John Hancock Center for Physical Activity and Nutrition at the Friedman School, Tufts University. “World records will be set for many years to come.”

But in general, human performance depends heavily on genetics, in particular the genes that govern cardiac output, and on training, the physiological adaptations the body makes to respond to the stress of intense, prolonged exercise. Nutrition, motivation, equipment (like better running shoes) all count, too. So does the ruggedness of joints.

For endurance events, “the first limit is the ability of the heart to pump enough blood and to deliver oxygen to the peripheral, skeletal muscles,” said geneticist and exercise physiologist Claude Bouchard, executive director of the Pennington Biomedical Research Center in Baton Rouge, LA. “Cardiac output is extremely important,” and good cardiac output (as well as bad) has a strong genetic component – it tends to run in families.

“The second determinant is the efficacy of the skeletal muscle machinery” to use that oxygen, to combine it with fuel (carbohydrates or fats) to make ATP, adenosine triphosphate, the energy molecule that allows muscle filaments to contract.  The production of ATP takes place inside cells in an organelle called the mitochondrion; the more mitochondria a person has, and the more efficiently they work, the better the ATP production.

In other words, the two most important factors, at least for endurance events, are getting enough oxygen into muscles and the ability of muscles to use this oxygen to make ATP. This combination is often referred to as VO2 max, or maximum volume of oxygen.

Training increases both the number and efficiency of mitochondria, Bouchard said. And like cardiac output, the ability to respond favorably to training – “how trainable you are” – also runs in families.  “You can’t be an elite athlete if you don’t have both sets of conditions – [being] highly endowed and highly trainable.”

And while two athlete wannabes might look the same on some physiological measures, “you can’t tell until you train someone how well the mitochondria will respond,” said David Costill, now partially retired but formerly the director of the Human Performance Laboratory at Ball State University in Muncie, IN. In genetically-favored people, he said,  “you see a large increase at the cellular level in mitochondrial number and all the enzymes in mitochondria.”

Training also produces an increase in capillaries – tiny blood vessels that bring oxygen to cells. And of course, an increase in muscle strength.

Fuel matters, too. For optimal endurance, athletes need to be able to burn both carbohydrate, which is stored in muscles in a form called glycogen, and fat, which is stored everywhere. “We can store a functionally infinite amount of energy in the form of fat, but we are limited in the form of energy as carbohydrate in the muscle itself,” said Russell Pate, a professor of exercise science at the University of South Carolina. That’s why marathoners spend the last two or three days before a race eating carbohydrates and letting glycogen build up in their muscles.

When marathoners “hit the wall,” it’s usually because they are running out of glycogen. The way to avoid this is to “be well-adapted for fat metabolism,” said Pate, which means teaching the body to burn fat to supplement waning carbohydrate stores, which can be done by endurance training.  (Elite runners do this by training 120 to 140 miles a week.)

Genetically gifted marathoners are also endowed with an ideal ratiosof fast-twitch to slow-twitch muscles.  The best endurance athletes have lots of slow twitch, or Type I, muscles, which look red and have a high oxidative capacity – a good ability to use oxygen efficiently. (Ducks, which, like marathoners, travel long distances, are also loaded with slow-twitch muscles, which is why duck meat is red; chickens, not exactly endurance champs, are rich in white, fast-twitch, or Type II, muscles.)  Toward the end of a marathon, when most racers are running out of glycogen in their slow-twitch muscles, the lucky ones can recruit fast-twitch muscles for a final kick.

In the long run, and a marathon is clearly that, being an elite athlete takes a combination of good genes and grueling training. Dr. Lisa Callahan, a sports medicine physician at the Hospital for Special Surgery in New York, put it this way:  “If you take two people with the same physique, the same running style, the same motivation and drive, who train exactly the same and one always beats the other, that may be the genetic edge.”

But David Costill of Ball State begged to differ: “It’s not genetics. Most winners will tell you it’s having a killer instinct, and truly believing they are the best.”

There we stood in our color-coded bathing caps, 1336 women — nervous, excited and all lined up in “waves” on a recent summer Sunday morning on the shores of (I kid you not) Lake Chargoggagoggmanchauggauggagoggchaubunagungamaugg in Webster, MA.

For each of us – we ranged in age from 27 teenagers to five hardy souls in their early 60s – the goal was at least to finish this Danskin triathlon, a mere “sprint,” or short, triathlon in the world of more or less extreme sports, but a daunting enough challenge for many participants who, like me, had never done such a thing before. We were about to wade into the 77 degree, beautifully clear water of the lake, trying not to drown as sleeker, faster bodies swarmed over us, to swim a half-mile course around a series of orange buoys.

Dashing out of the water at the finish, we were then supposed to step quickly over the mats that would record our individual times for the swim from the little electronic “chips” fastened around our ankles. From there, we were to run or walk fast to the “transition area,” where our 1336 bikes were lined up by number on racks. This first transition, which the winner of the overall event (in her early 20’s) managed in two minutes and thirty-four seconds, took me six minutes and five seconds – including a quick wash of the sandy feet, donning of the bike helmet and sneakers, and pushing the bike to the start line.

We would then bike 12 miles around the lake, uphill steeply in the beginning (uphill enough that some people had to get off and push and at least one, not me thankfully, stopped to throw up). Then it was downhill as fast as one’s courage would allow, with faster bikers yelling encouragement – “You go, girl!” – as they whizzed past. At the bike finish, the mats again recorded our times electronically, bikes were stowed, helmets removed, “Hammer gel” or other so-called food gulped with a sip of water, then we were off on the truly hellish bit, the 3.1 mile run. Then back, mercifully, to the finish.

It was glorious, a total high, just to be part of this mob of slightly-crazy women with gorgeous legs, pretty, muscular arms and energetic, delighted faces. There was a sense of unrestrained exuberance in the air, and a feeling of immense gratitude, at least on my part, to have the good health to even contemplate such an event.

Which made me wonder. Why do so many people hate exercise? And why do doctors’ urgings about the medical benefits of exercise fall on so many deaf ears?

My new theory is that we’ve been misguidedly selling exercise as a medical duty, a “should,” when it’s really a form of constructive selfishness, a short cut to joy and self-affirmation in lives that have too many “shoulds” already.

Granted, the medical reasons to exercise are endless. Exercise physiologist Bill Evans of the University of Arkansas for Medical Sciences estimates that regular exercise can prolong life by two and a half years, if you start at age 35, and by six months even if you start at 75.  A Harvard study that has followed 17,000 men for decades has found that vigorous exercise (expending 1,500 calories a week or more in physical activity,  including brisk walking) reduces the risk of mortality in any given year by 25 percent.

“Physical activity is the closest thing that we have to a magic bullet for health,” says Dr. I-Min Lee, associate professor of medicine at Harvard Medical School. “There is no single drug that can give the same overall benefit to health that physical activity does. Everything that gets worse as we get older gets better with exercise. Even moderate intensity exercise such as brisk walking, 30 minutes a day, is sufficient to lower the risk of stroke, heart disease and diabetes, in both men and women.”

The medical benefits go on and on. Studies at the Cooper Institute in Dallas, TX.  show that the death rate from all causes is about 50 percent less in moderately fit men and women than in the non-fit at any given point in time.

Indeed, an article in the New England Journal of Medicine last year concluded that “poor physical fitness is a better predictor of death than many other factors, including smoking, hypertension and heart disease.” Other studies show that exercise dramatically improves the quality, and well as the duration, of life, not just be reducing the risk of physical diseases but by reducing depression, stress and anxiety as well.

And yet, many Americans don’t do it.  Figures released earlier this year by the National Center for Health Statistics show that while one in five Americans do engage in a high level of regular physical activity, an astounding one in 4 “engage in little or no regular physical activity.” The data come from 32,000 interviews conducted in 2000.

That’s appalling, given that it doesn’t take all that much exercise to meet the government’s recommendations, which say, among other things, that a person should engage in physical activity, at work or at leisure, that causes light sweating or a slight to moderate increase in breathing or heart rate five times or more a week for at least 30 minutes each time.

The excuses people offer for not doing this minimum are legion. Too busy with kids, work and housework. Too fat to look decent in exercise clothes. No place to walk in dangerous neighborhoods. Too tired. Too impatient to build up stamina slowly. Etc, etc.

So take it from a newly minted triathlete who, like all but eight women in the Danskin triathlon, finished the course, with glee (and great admiration for the winner, who finished in an hour and five minutes, 29 seconds).

Don’t think of exercise as one more “should” in your life. Think of it as a treat, a break from the kids and the computer. A time to get away by yourself, or to socialize with fellow walkers. It’s not just for your body. It’s for your soul.

Kelly Hall, 34, was in fantastic shape, routinely biking 100 to 200 miles a week in preparation for last year’s AIDS Ride from Boston to New York. Usually, she trained with other riders, who made it a point to take food and hydration breaks. But one day last June, Hall, a strategic planner at Partners Community Health Care in Needham, decided to ride alone, despite the 95 degree heat.

The first 50 miles, around Concord, were a breeze. “I thought I’d do another 25,” she recalls. She drank a little Gatorade, the sports drink designed to restore normal blood levels of electrolytes, but relied mostly on water, constantly sipping from the 70-ounce Camelbak water supply on her back.

 Halfway through the last loop, hyponatremia, a relatively rare but potentially fatal condition in which blood levels of sodium sink dangerously low, “hit like a ton of bricks,” says Hall. She finished her ride “really, really slowly,” got back to her car, threw up, drove home and threw up again. She made it to Brigham and Women’s Hospital, where she had a grand mal seizure.

Nearly the same thing happened to Sarah Snyder, 45, a Globe editor, who was training with friends for another long, benefit ride.  She, too, was in good condition, and she too,  dutifully sipped water for 56 miles. “What a good doobie I’m being,” she remembers thinking.

But after her ride, she threw up violently and was rushed to the hospital, where she drifted in and out of consciousness for 18 hours. “You drank too much water. You screwed up your sodium,” the doctors told her. When asked to say her name, “It came out like Swahili.”

Full-blown cases of hyponatremia (sometimes called water intoxication) are relatively rare, roughly 0.1 to 4 percent of people who sweat steadily for hours in grueling, long distance events, says Scott Montain, a research physiologist at the US Army Research Institute of Environmental Medicine in Natick. The incidence of hyponatremia appears to be highest in events lasting more than four hours, especially at high temperatures.

But the prevalence of warning symptoms is much higher – up to 27 percent of athletes who seek attention in a medical tent during a long race – and appears to be rising.

In the 1996 New Zealand Ironman Triathlon (3.8 kilometer swim, 180 kilometer bike and 42 kilometer run), researchers found that 9 percent of athletes who showed abnormalities on medical tests had hyponatremia.

Typically, conscientious athletes get in trouble because they adhere too diligently to one recommendation  (drink lots of fluids) but ignore another (keep electrolytes up). (Electrolytes are charged particles such as sodium, potassium, calcium and bicarbonate that must be kept in near-perfect balance) Indeed, for most marathoners, Montain notes, the real problem is drinking too much water, not failing to take in enough sodium.

Women in particular, may be at risk, some researchers believe, in part because they may too dutiful about drinking water.

Some studies suggest that certain drugs may also increase risk, among them ibuprofen and other NSAIDS (non-steroidal anti-inflammatory medications), acetaminophen, some cancer drugs, nicotine, diuretics, narcotics and some antidepressants and anti-psychotic drugs. On the other hand, data presented recently at the American College of Sports Medicine meeting suggest that ibuprofen and other NSAIDS may not increase risk after all.

“When you sweat, you lose both water and salt,” says Dr. Soheyla Gharib, medical director of the women’s health center at Brigham and Women’s Hospital. “If you replace only water, the salt level in the blood gets diluted.”

And that can be disastrous. Normally, the body tries to keep positively and negatively charged electrolytes in balance to keep cells electrically neutral, says Dr. Ronenn Roubenoff, associate professor of medicine and nutrition and director of human studies at the Jean Mayer USDA Human Nutrition Research Center at Tufts University. Though sodium is probably the most important electrolyte for endurance athletes to worry about, he says, “an imbalance of any one of the electrolytes can be harmful.”

Normally, sodium is plentiful in the blood and relative low inside cells. But when the concentration in the blood gets too low compared to the amount inside cells – either because a person drank too much water, took in too little sodium, or both – water rushes into cells. “Water follows sodium as day follows night,” Roubenoff says.

The result is dangerous swelling, particularly in the brain, that can lead to brain damage, coma and death.

Curiously, hyponatremia can occur whether a person is dehydrated, normally hydrated or overhydrated because any of those conditions can happen while blood levels of sodium are too low, adds Roubenoff.

Further complicating things is that the symptoms of  hyponatremia can be easily confused with those of heat stroke and heat exhaustion. With heat exhaustion (also called exertional heat injury), people feel ill, get nauseous, have muscle cramps and may feel dizzy standing up quickly.

With heat stroke, people have all those symptoms plus another one: mental status changes, that is, confusion about who and where they are and what day it is. People with genuine heat stroke also typically have extremely high body temperatures

With hyponatremia, people also feel very ill and may have mental status changes, but don’t have the high temperatures of heat stroke. They also vomit forcefully and repeatedly and, unlike those with heat exhaustion, do not feel better by resting and cooling off.

Treatment of hyponatremia may involve restricting fluid intake and promoting urine production, but some people also need intravenous (IV) salt water (saline) with a high concentration of salt, until blood electrolytes return to normal. Re-setting the water-salt balance “must be done carefully and slowly,” cautions Dr. Gabriel Danovitch, professor of medicine at the University of California , Los Angeles, School of Medicine because overly-rapid correction of electrolyte imbalance can cause further problems.

To protect yourself against hyponatremia, start by paying attention to how much you sweat. In general, women sweat less than men and their ability to regulate core body temperature (and sweating) may also vary with different phases of the menstrual cycle.

Individuals also vary considerably in how much sodium they lose in sweat. You may be a heavy sodium loser if your sweat burns your eyes, tastes salty or leaves a cakey-white residue on your skin. 

Sweat contains between 1 to 2 grams of sodium per liter – and since it’s easy to lose a liter of perspiration in a long race, that means you’re losing this much sodium, too. You can replace 2 grams of  sodium with about a half-teaspoon of salt.

You can also make sure you’re getting enough sodium by drinking sports drinks like Accelerade, Cytomax, Gpush, Gatorade and the like instead of plain water during long events. (Check the labels for sodium content; some brands have twice the sodium of others.) Sports drinks also typically contain carbohydrates as well, which provides energy and helps with water and sodium absorption.). These sports drinks are similar to Pedialyte and other oral rehydration solutions used to treat children who become dangerously dehydrated.

If you don’t like sports drinks, you could take salt tablets, but they may make you nauseous.  At the very least, you can eat salty foods prior to and during a big event.

You can gauge how much fluid you’re losing by weighing yourself before and after half an hour of exercise. (Don’t drink anything between weigh-ins.) If  you lose a pound in half an hour, your sweat rate is two pounds per hour, which means you need to replace that much fluid. (Two pounds of fluid is 32 ounces.)

As a practical matter, you’re probably not in any danger from dehydration unless you lose more than  2 to 3 percent of your body weight during exercise, but you can’t rely on thirst to guide you because by the time you’re thirsty, you may already be dehydrated. So the goal is to adjust your fluid intake to how much you sweat.

In fact, for ordinary mortals who work out for less than an hour, sports drinks are unnecessary and plain old water (along with the salt you normally get in food) will do just fine, says Roger Fielding, an exercise physiologist and associate professor of health sciences at Boston University.

For endurance athletes doing events longer than an hour and sweating profusely, it’s probably wise to drink 6 to 8 ounces of fluid every 20 minutes to prevent both excessive dehydration and hyponatremia, says Montain of the US Army lab. For events lasting longer than two hours, you’re probably better off with a carbohydrate-electrolyte sports drink than plain water. But don’t drink more than 40 ounces (1 and

Remember how good it used to feel, hanging out in the sun, letting your face acquire that nice, ruddy glow?

Then came all those depressing public health messages telling us that the sun was dangerous, that we should feel guilty about even the slightest tan.

Well, fellow sun worshipers, the sad truth is that as a general rule, we should still practice “safe sun” – including hats and sunscreen, especially for little kids – much of the time.

But there’s a new ray of hope – dare we say “sunshine?” – in the form of a modest but significant shift in medical thinking toward the view that SOME unprotected sun exposure may actually be a good thing, like 15 minutes or so a day in the summer for adult Bostonians who tan well, less for those burn easily.

“In my opinion,” says Dr. Robert Stern, chief of dermatology at Boston’s Beth Israel Deaconess Medical Center, “it’s probably true that for people over 40, even people who have had a non-melanoma skin cancer, we have oversold the idea of having to be sun-phobic. For them, modest exposure has little risk.”  For kids, cautions Stern, it’s another matter – excessive childhood exposure to sunlight has been linked to later basal and squamous cell skin cancers, as well as to melanoma, a more serious form of skin cancer.

The rationale for the some-sun-is-good point of view, supported by a number of recent articles in medical journals, is that the vitamin D made in the skin in response to ultraviolet B radiation may protect against certain diseases, including cancer of the breast, colon and prostate.

Other diseases, most notably MS (multiple sclerosis), also show a “latitude effect,” that is, they are less prevalent among people in sunnier climes, though, as with cancer, whether this is truly due to vitamin D or to some other factor that varies by region such as diet, behavior or genetics is unclear.  In rodents, high doses of vitamin D can actually prevent MS.

Sunlight may even be an effective treatment for some diseases. In a recent study published in the journal Lancet, Dr. Michael F. Holick, an endocrinologist and leading vitamin D researcher at Boston University School of Medicine and others showed that exposing people with mildly high blood pressure to UV-B can lower blood pressure, perhaps by correcting an underlying vitamin D deficiency.

Before we get in too deep here, let’s be clear. It’s vitamin D that has the real benefit, not sunlight per se, which means you can take vitamin D supplements, especially during the winter if you live at higher latitudes and especially if you have dark skin (which makes less vitamin D). By contrast, people in Florida typically make plenty of vitamin D all year in their skin. It’s very tough to get enough vitamin D from your diet unless you consume lots of fish liver oil, the flesh of fatty fish like salmon and fortified milk and cereals.

Actually, vitamin D is not a vitamin at all in the normal sense, but is really a steroid-like hormone made, after exposure to the UV-B rays from sunlight,  from a precursor of cholesterol in the skin. After an inactive form of vitamin D is made in the skin, it is transformed in the liver and kidney to the active or hormonal form called 1,25 dihydroxy vitamin D. Indeed, several teams of researchers have recently found that the organs such as the breast, prostate and colon in which vitamin D seems to reduce cancer risk can also make their own stores of the vitamin’s active form, an important finding.

Like other hormones, vitamin D works by fitting into specialized receptors on cells in many organs of the body and has numerous biological effects, the most important one being to aid in the absorption of dietary calcium. When a person has enough vitamin D in his system, the intestines can absorb 30 percent of the calcium available in the diet; without enough vitamin D, this drops to 10 percent, notes Holick.

The consequences of insufficient vitamin D can be serious. When the body can’t absorb enough calcium from the diet, it steals calcium from the bones to restore proper levels in the blood, a process that weakens bones, often leading to osteoporosis. Low levels of vitamin D can also lead to weak and achy muscles, as well as generalized bone pain, symptoms often misdiagnosed as fibromyalgia.

Just as important as its effects on calcium and bone is the fact that  vitamin D helps regulate many basic cell processes, notes Dr. David Feldman, an endocrinologist and vitamin D researcher at Stanford University School of Medicine. By acting on specific regions of DNA called vitamin D response elements, it helps control the biochemical signals that tell cells when to divide, when to stop dividing and when to die – all processes that are crucial in both normal and malignant cells.

In the early 1990s, for instance, researchers showed that adding the active form of vitamin D to cancer cells in the test tube inhibits their growth, a finding that has now been shown in breast, prostate and colon cancer cells as well as leukemic cells.

Currently, researchers at several labs around the country are testing whether a high dose, prescription form of vitamin D called calcitriol can slow the progression of prostate cancer in men with the disease. This treatment could increase the risk of kidney stones, says Feldman, but it’s promising enough that numerous drug companies are pursuing closely-related versions (or analogues) of active vitamin D that would be active against cancer without triggering kidney stones.

Vitamin D as a supplement has also been shown to be extremely effective at preventing Type I diabetes. A study by Finnish researchers published in Lancet last fall shows that vitamin D (2000 International Units a day) in infancy can reduce by 80 percent the risk of Type I diabetes 30 years later, perhaps because vitamin D slows the body’s immune attack on its own insulin-producing cells. This fits with data from mice showing that high doses of activated vitamin D will markedly reduce the risk of Type I diabetes.

But perhaps the most intriguing evidence of vitamin D’s importance comes from studies of sun exposure and cancer.

In the March 15 issue of Cancer, William Grant, by day an atmospheric scientist at NASA’s Langley Research Center in Hampton, VA and by night, an independent researcher, published a study showing that the geographic distribution of many cancers varies with UV-B exposure. 

Since the early 1980s, Grant notes, scientists have been gathering evidence that some types of cancer – most notably, cancer of the breast, colon, ovary and prostate, as well as non-Hodgkin’s lymphoma – are higher in Americans who live in the least sunny regions.

“What I did was basically take two maps and put them together,” says Grant of his latest study. This showed that in addition to the cancers already known to vary with UV-B exposure, there appear to be many others (bladder, esophagus, kidney, rectum, stomach and uterus) that also increase as sunlight decreases. 

In fact, more than 30,000 Americans die prematurely every year from cancer that may be attributed to low levels of UV-B exposure, Grant estimates.

Other researchers, too, have found links between sun exposure and cancer.  In 1999, a team led by epidemiologist Esther M. John of the Northern California Cancer Center in Union City reported on a study of more than 5000 white women, 190 of whom developed breast between the time they were first interviewed by government researchers in the early 1970s and 1992.

The team correlated various measures of sun exposure and found that the women with the highest levels of sun exposure were the least likely to get breast cancer.

This March, researchers from the National Cancer Institute led by Dr. Michal Freedman, an epidemiologist, found that Americans living in sunny areas were significantly less like to die from (not just get) cancers of the breast, ovary, prostate and colon. Not surprisingly, her team found, high levels of sun exposure were also linked to the milder (non-melanoma) types of skin cancer, too.

The bottom line ? If you’re white, all it takes for your skin to make enough vitamin D is about 15 minutes a day in the sun, without sunscreen, at noon, says Holick.  If you’re black, it may take considerably more than that. How much sun you should get depends on your skin type and sensitivity to sun – if you burn readily, you may only be able to tolerate five minutes in the sun, and that would be enough.

And if you’d rather just take vitamin D supplements? That’s fine – the general guidelines are 200 IUs a day if you’re 50 or under, 400 if you’re between 50 and 70, and 600 IUs if you’re over 70, says Tufts University epidemiologist Susan Harris. Some researchers even recommend 800 to 1000 IUs a day. (Since there’s almost no vitamin D in breast milk, Harris notes, breast fed babies living in less sunny regions of the country should probably take baby vitamins.

The risks of overdosing are small, adds Reinhold Vieth, a biochemist and vitamin D researcher at Mount Sinai Hospital in Toronto, who believes it would take tens of thousands of IU s for long periods to become a problem.. Still, to be on the safe side, many researchers suggest limiting vitamin D intake to 2000 IUs a day.

Judy Foreman is  Lecturer on Medicine at Harvard Medical School and an affiliated  scholar  at the Women’s Studies Research Center  at Brandeis University.. Her column appears every other week. Past columns are available onwww.myhealthsense.com.

SIDEBAR

Vitamin D and the Evolution of Skin Colors

• Vitamin D is part of one theory for the evolution of different skin colors among humans.

• Human beings arose with black skins in Africa and then migrated outwards from there. Black skin is rich in melanin, a pigment that acts as a natural sunscreen, protecting against sunburn.

• But, just like sunscreen of SPF 8 or higher, melanin reduces the amount of vitamin D the skin can make. That’s fine for someone in Africa who spends lots of time in the sun. Farther from the equator, however, people with light skin gain an evolutionary advantage. With less available UV-B, light skin probably evolved so that humans migrating northward would still be able to make enough vitamin D.

• Indeed, people who failed to make this adaptation would have had a difficult time reproducing. Insufficient vitamin D can lead to rickets, which causes defective bone growth. In women, this can mean such poor pelvic development that babies could not be borne – and the mother’s genes would not be passed on.

Marybeth Turner, a 35-year-old Lexington mother of two young children, has great legs. And arms. And abs. And strength. And balance. And endurance.

Perhaps not surprisingly, given all that, she also has a personal trainer, in her case a huge, gentle guy named John Damon, co-head trainer at the Mt. Auburn Club in Watertown.

“Squeeze. Don’t move your neck,” he coaches as Turner does abdominal crunches while balancing on her back on a big fitness ball. Moments later, he urges, “Keep going; you’re looking good!” as she steps up repeatedly onto a 20-inch-high platform.

“I thought I was in shape until I started working out with John. . . . I was a runner. I was very active,” says a smiling Turner, who looks dazzlingly fit. But she’s become even fitter since she started personal training eight months ago. “I’ve become physically stronger and I have more energy.”

Personal trainers are to fitness what San Pellegrino water is to dining out – a bit expensive, perhaps, but definitely capable of adding a certain fizz to things.

The question is, do personal trainers really know what they’re doing? The short answer is, some do, but it’s hard to tell which ones because personal training is a totally unregulated industry. Unlike physical therapists or dieticians – or even hairdressers and manicurists – personal trainers are not licensed by states. That means anyone with visible biceps, a business card and a clipboard can call himself a personal trainer, and charge anywhere from $35 to $125 an hour or more to watch you work out.

To be sure, some trainers are, or claim to be, certified, but the certification process ranges from rigorous to bogus. Some trainers have not only been to college but have doctorates in exercise physiology. Others have barely made it out of high school, have trained at a weeklong workshop and would be hard pressed to tell a fast-twitch muscle from a slow one, much less advise a heart attack survivor about how to walk her way back to health.

What’s more, the risk of getting a know-nothing trainer may increase as the health club industry booms, outstripping the supply of qualified trainers. The number of Americans who belong to clubs grew from 24 million in 1995 to 31 million in 1999, according to the International Health, Racquet and Sportsclub Association, a Boston-based trade group for the country’s 17,000 health clubs.

To keep up with the demand, more than 300 organizations are churning out personal trainers and there are 65,000 of them in the country, according to IDEA, a San Diego-based organization for fitness professionals. The Aerobics and Fitness Association of America alone certifies about 8,000 new trainers a year.

So, with all those folks out there just waiting to buff you up, what should you ask for? For starters, all this:

Certification. Obviously, it’s no guarantee, but if the trainer is not certified at all, keep looking. If he says he is, ask by whom. The consensus among trainers is that the toughest standards are those of the American College of Sports Medicine, followed, in roughly this order, by the National Strength and Conditioning Association, the American Council on Exercise, the Aerobic and Fitness Association of America and the Cooper Institute.

Education. Ask what academic degrees and courses your trainer has taken. An advanced degree may not be essential, but some study of health or physiology is important.

A trainer should ask you for a detailed medical history. If he doesn’t, “that’s a red flag,” said Walt Thompson, professor of kinesiology and health at Georgia State University and chairman of the certification and education committee for the American College of Sports Medicine. If you have or have had serious health problems, you should also get clearance from your doctor for the type of exercise you’re planning to do. Ask if the trainer is certified in cardiopulmonary resuscitation and first aid. If he or she is not, go elsewhere, said Damon of the Mt. Auburn Club.

A good trainer shouldn’t let you work out on the first visit. Instead, he should ask about your goals (and make sure they’re realistic) and assess your condition – testing your strength, taking your blood pressure, measuring body composition (fat versus lean tissue), joint flexibility, and perhaps putting you on a treadmill, stationary bike or rowing machine to gauge cardiovascular fitness.

If a trainer tries to make you progress too fast, be wary. A general rule of thumb is never to increase workouts by more than 10 percent from one week to the next. For instance, if you did 20 minutes on the exercise bike each session last week, do two minutes more each time this week. The same goes for increasing weights in strength training. A good trainer should change your routine (so you continue to progress) every eight weeks.

If you’re very sore after the training sessions, or if you get injured, that’s another red flag. Mild soreness after strength training (weight lifting) is good: It means the muscle is rebuilding itself. But extreme soreness – if you can’t comb your hair after upper body workouts or walk upstairs after leg lifts – it means your muscles are seriously inflamed.

If your trainer leaves you alone on any of the equipment, either during an assessment or a real workout, fire him. A personal trainer should be just that – not dividing his attention between you and other clients or wandering away to chat with his chums. He should also offer a truly personalized program, not a canned list of exercises you could get from a book.

If your trainer offers nutritional advice, guarantees you’ll lose weight, or worse, tries to sell you supplements, dump him.

“The biggest problem is personal trainers giving advice about dietary things,” said Bill Evans, director of the nutrition, metabolism and exercise lab at the University of Arkansas for Medical Sciences. “They have no training in that. Their information can be just wrong and sometimes dangerous,” including telling older people who may be undernourished to cut food intake.

The same goes for medical advice. Though some trainers do have some health training, many don’t, which means they’d better not give medical advice, said Wayne Wescott, fitness research director at the South Shore YMCA in Quincy. “If someone comes in and says, `I’m taking these medications. What does that mean for exercise?’ I say, `Let’s talk to your physician together.’ “

Before you sign up, ask your trainer for names of other people he or she worked with, and call them, said Roger Fielding, associate professor of health sciences and exercise physiology at Sargent College at Boston University. Be specific: If you’re an older person, ask for names of other older people he’s worked with; the same goes if you’re recovering from a heart attack or have an underlying illness such as diabetes.

Ask if the trainer carries liability insurance. He or she should for your protection and that of the trainer.

When a trainer writes up your exercise program, he or she should make clear what the fee is, along with hours and rules about whether you have to pay if you miss a session. Many offer one session free. It’s a warning sign if a trainer tries to sign you up for a very long-term commitment or asks you to pay cash under the table.

Finally, pay attention to the social and emotional signals the trainer is sending. A big part of the trainer’s job is to encourage and motivate you. If the trainer does all the talking, doesn’t listen to your concerns, or seems mostly interested in wearing revealing clothes and strutting his or her stuff, find someone less narcissistic.

 

Judy Foreman is a Lecturer on Medicine at Harvard Medical School. Her column appears every other week. Past columns are available on www.myhealthsense.com.

SIDEBAR 1: Walking Toward Fitness

A personal trainer may be just what you need, but you can get many of the health benefits of exercise by plain old walking.

In March, for instance, Harvard researchers published results in the Journal of the American Medical Association of a study of nearly 40,000 women showing that even light-to-moderate exercise lowers the risk of heart disease.

“The evidence continues to mount that even moderate intensity exercise like brisk walking for 30 minutes a day – can lower the risk of heart disease, Type II diabetes, stroke, osteoporosis, colon cancer, breast cancer and premature death,” said Dr. JoAnn E. Manson, chief of preventive medicine at Brigham and Women’s Hospital.

In her new book, “The 30-Minute Fitness Solution,” Manson and her co-author, Patricia Amend, take the guesswork out of figuring out a moderate exercise program. They also highlight nutritional tips, strength-training exercises and specific fitness goals you should set for working out on exercise equipment.

SIDEBAR 2: Information Available On The Internet

There’s no shortage of information on personal trainers available from health clubs and on the Internet. Here’s a sampling:

•   Aerobics and Fitness Association of America: www.afaa.com

• American College of Sports Medicine: www.acsm.org

• American Council on Exercise: www.acefitness.org

• Cooper Institute: www.cooperinst.org

• IDEA, a membership organization for fitness professionals: www.IDEAfit.com

• National Strength and Conditioning Association: www.nsca-lift.org