Whether you do it to lose weight, maintain weight loss, or just have fun, exercise is essential for good health.

As a nation, we are obviously getting fatter and fatter. Yet we seem ever more confused about how to lose weight. We’re particularly fuzzy on the question of how big a role exercise plays, or whether we just have to count calories.

So here’s the deal. Yes, you can count calories or weigh yourself every day. If your weight is up today compared with yesterday, you probably ate more calories than you burned. If it’s less, you burned more than you ate – provided you didn’t drink gallons of liquid the day before, throwing the scale off.

It comes down to simple arithmetic, and you’ve heard it before: Calories in, calories out. You will absolutely, inevitably gain weight if you eat more calories than you expend in basic metabolism – breathing, digesting, sleeping, etc. – plus whatever else you do, such as chasing the kids around, walking, vacuuming, or going to the gym.

Except that most of us can’t – or won’t – do the math, probably because it’s so depressing.

We routinely overestimate the number of calories we spend in physical activity, and underestimate the calories from food. For instance, when I swim hard for an hour, which I do regularly, I probably use up 400 to 600 calories. But when I eat a blueberry muffin, which I’m afraid I also do regularly, I take in nearly 400 calories. So, I have to swim pretty fast for 40 minutes just to offset one lousy muffin. It’s not fair! I swim for health (and fun). But if my only goal were weight loss, it would be easier to just not eat that muffin.

“The problem is people’s inability to know how many calories they burn and eat,” says Dr. George Blackburn, associate director of the Division of Nutrition at Harvard Medical School. “If you put a person in a metabolic chamber, where you know exactly what they eat and what they burn, the calories in, calories out idea is always reconfirmed.”

So, if it takes an awful lot of exercise to make a dent in the calories in-out equation, is exercise pointless? No way. It’s essential for good health. Regular physical exercise reduces the risk of early death, coronary heart disease, stroke, high blood pressure, type 2 diabetes, colon and breast cancer, and depression, according to the 2008 Physical Activity Guidelines for Americans (www.health.gov/paguidelines).

Moreover, even if exercise doesn’t help much in the battle to lose weight, it is essential to maintain weight loss, says Dr. Timothy Church, director of Preventive Medicine Research at the Pennington Biomedical Research Center in Baton Rouge, La.

“This whole thing is not rocket science,” he says. “You can take weight off through a whole variety of strategies. But people don’t lose weight and keep it off unless they are physically active. There are tons and tons of studies on this.”

Among them is a series of studies by researchers at the University of Pittsburgh who showed last year that it takes a considerable amount of exercise – expending 2,000 calories, which requires four or more hours of exercise – per week to maintain a 10 percent weight loss, even on a low-calorie diet.

If your goal is weight loss, as opposed to overall health, does it matter what you eat? No. And yes. And it goes without saying that any diet should involve lots of fruits and veggies, whole grains, and reasonable, not gigantic, portions.

But it still comes down to calories. In February, a two-year study of more than 800 overweight adults showed that people can lose weight if they reduce calories, regardless of the percentages of fat, protein, and carbohydrates in their diets. The study, by researchers from the Harvard School of Public Health, the Pennington Biomedical Research Center, and the National Institutes of Health, was published in the New England Journal of Medicine.

Some foods are more “addictive” than others because they have a bigger effect on the brain chemicals that control the “reward” circuits in our brains. From a neurobiological point of view, sweets, fats, and salty foods make us want to eat more of the same, as Dr. David S. Kessler, the former head of the US Food and Drug Administration, makes clear in his new book, “The End of Overeating.” Obviously, eating more leads to weight gain.

Overall, says Blackburn, the body can’t store calories from protein as amino acids, so it either makes protein from them or converts them to carbohydrates. Excess dietary carbohydrate is first stored as glycogen, but if the body already has enough of that, excess carbs are stored as body fat. Excess dietary fat also gets stored as body fat.

And what about the question of whether exercise increases or decreases appetite?

Exercise can suppress appetite, says Blackburn, because it triggers not only the chemical dopamine, which governs the brain’s reward system, but also endorphins, those feel-good brain chemicals. These substances act on the hunger and satiety areas of the brains for as long as four hours afterward. “You don’t need cigarettes, or drugs, or food, all those things in the pleasure areas of the brain, because exercise has already activated them,” says Blackburn.

A review article in 2007 from researchers at Tufts University also concluded that there is a “spontaneous reduction in hunger associated with participation in exercise.”

Psychologically, as opposed to biochemically, some experts theorize that exercise might lead people to believe they can reward themselves with treats afterward or that they may be tempted to be less active for the rest of the day. And some studies, says exercise physiologist William J. Evans of the University of Arkansas for Medical Sciences, suggest that if you exercise, say, for 40 minutes a day, you will “then compensate by decreasing how active you are at other times of the day, leaving total energy expenditure unchanged” or that you might reward yourself with extra food. Then again, other studies say both of those theories are wrong.

One factor that matters without question, in terms of controlling food intake, is how fast you eat. “It takes about 20 minutes for food to get digested and formulated into hormones for your brain to know what you did, to get that signal to the brain,” says Blackburn. If you wolf down your food, you’ll finish your second helping before your brain has registered your first.

As for the perennial question of how much exercise you need, the new federal guidelines, released last year, say adults “gain substantial health benefits” from getting 2 1/2 hours a week of moderate intensity aerobic activity, or 1 hour and 15 minutes of vigorous physical activity.

Moderate activity means walking briskly, water aerobics, ballroom dancing, and even gardening. Vigorous activity means racewalking, jogging or running, swimming laps, jumping rope or hiking uphill. The guidelines also recommend weight training at least two days a week.

Boiled down, my personal mantra is this: You have to do both – diet to keep caloric intake under control, and exercise for fitness and fun. 

A week from today, 10,375 women – and 14,737 men – are expected to run in the Boston Marathon. The presence of so many women – the most ever entered in the historic race – is a sure sign of how far women have come in athletics.

So is this: In 1972, before Title IX, the law that spurred women’s athletics, fewer than 300,000 high school girls played sports, according to the National Federation of State High School Associations. Now it’s more than 3 million.

But there’s a dark side to this terrific news: The more girls and women play sports, the more they, like boys and men, get hurt. And – attention athletes, coaches, and parents – they get hurt in different ways.

Last week for instance, researchers from the Center for Injury Research and Policy at Nationwide Children’s Hospital in Columbus, Ohio, reported on a stunning injury rate for young gymnasts, 82 percent of whom are female.

Using data from 100 hospitals across the country, the team reported in the journal Pediatrics that every year, roughly 26,600 children ages 6 to 17 get injured badly enough doing gymnastics to wind up in the emergency room. This is a “very high” injury rate, roughly equivalent to ice hockey, said public health specialist Lara McKenzie, the lead author.

And gymnastics is just the tip of the iceberg. Cheerleading is now the leading cause of direct fatal and nonfatal injuries among high school and college women, according to the Center for Catastrophic Sport Injury Research at the University of North Carolina. “Cheerleading used to be about shaking pompoms,” said the center’s director, Frederick O. Mueller. “Now, it’s about throwing people 20 feet in the air.”

Basketball is not exactly benign, either: It’s brutal on that most vulnerable part of the female athlete’s anatomy: the knees.

And even plain old running seems to be tougher on women’s legs than on men’s, though it’s not clear why.

This “absolutely does not mean that women should not play sports,” said Dr. Lyle Micheli , a consultant to the International Olympic Committee on Women’s Sports Issues and director of sports medicine at Children’s Hospital Boston. After all, in two of the most popular high school sports, soccer and basketball, no significant differences existed in injury rates between boys and girls, according to figures released by the federal Centers for Disease Control and Prevention for the 2005-2006 school year.

But athletic activities can cause different injuries in women and girls than in men and boys. Female athletes and their coaches should pay increased attention to the anatomical differences that put girls and women at disproportionate risks for certain injuries.

The sports injuries that seem to disproportionately affect women include:

• Tears in the ACL, or anterior cruciate ligament, in the knee, a problem that hits basketball and soccer players especially.
• Plantar fasciitis, in which activity can cause tiny tears in heel tissue.
• Compartment syndrome in runners, in which lower leg muscles get too tight for the surrounding space, causing severe pain.
• Shin splints in runners, in which repetitive stress can lead to pain and even fractures in the lower leg.
• Kneecap pain.
• Pain along the outside portion of the leg in the iliotibial band, between the knee and the hip, known as iliotibial band friction syndrome.

Women’s knees are more vulnerable than men’s because the “Q-angle” – the angle between the hip and the knee – is greater in women than in men. While wide hips are good for childbearing, they mean there is more stress on the knee in moves like landing from a jump and twisting. This torque can shred the ACL, a ligament that helps stabilize the knee. This is a major reason why female athletes have four times more ACL tears than men do, said Dr. George Theodore, a Red Sox team physician and sports medicine specialist at Massachusetts General Hospital.

Making matters worse for women’s knees, especially among basketball players, is that women tend to “cut,” or suddenly change direction, differently from men, said Micheli.

“There’s more of a tendency for women to change direction on just one foot, maybe because of the width of the pelvis, while men often use two feet.”

Wider hips also mean that there may be more “pull” on the kneecap, which can cause pain when running uphill. Kneecap cartilage also seems to wear down more in women than in men. And women’s kneecaps slide around more from side to side, in part because women have more estrogen, which can make women’s ligaments more flexible than men’s.

Fortunately, solutions exist for these problems. One is better coaching for females – to teach them how to run, jump, land, and twist safely. Another is orthotics, devices that can be put in shoes to minimize stress on knee, ankle, and hip joints. And, of course, everyone should use appropriate safety equipment, such as thicker mats for gymnasts, helmets for cyclists, and headgear in soccer, which obviously benefit men as well as women.

Strength training, or weight lifting, is an absolute must for female athletes, just as for males, though the emphasis may be on different muscle groups. To help prevent knee injuries, for instance, female athletes need to work extra hard to build up a muscle on the inside of the knee called the vastus medialis and to build up hamstring muscles on the back of the thigh.

“We can’t really change people’s anatomy, but we can rebalance muscles with proper stretching, strength training, and orthotics,” said Theodore of Mass. General.

It’s clear that physical differences are not a reason to keep women from sports or from playing just as hard as men, said Laura Pappano, coauthor with Eileen McDonagh of “Playing with the Boys: Why Separate is Not Equal in Sports.” Pappano said her daughter, now 13, was told that girls play nine-hole golf while boys play a full 18.

“The idea that women are doomed because of injuries is just absurd,” Pappano said. “The whole ‘girls will get hurt’ argument has done more to limit women than anything else.”

I couldn’t agree more. Regardless of your gender, get out there and play hard. But train hard, too. And be sure to get the excellent coaching you need to keep you safe. 

I’m an exercise junkie — and proud of it. I swim, I run, I bike.

But, like many other people, I’m a disaster when it comes to lifting weights, also called strength, or resistance, training. The closest I come is lifting a few tiny dumbbells at home in front of the TV. And that’s only when the Red Sox are on.

This is about to change, and not just because of lingering New Year’s resolutions.

A growing body of evidence shows that strength training not only provides many benefits that aerobic workouts alone cannot, but also offers some of the same health benefits as aerobic conditioning.

It’s long been known that weight lifting becomes more important as you get older to prevent injury and preserve the strength to do normal things like climbing stairs, hauling groceries, and chasing grandchildren.

What’s comparatively new is that it does much more than that, potentially reducing the risk of developing heart disease, relieving neck pain, improving balance, and making it easier to battle the bulge — though it needs to be done properly to avoid injury.

The evidence for the value of strength training has grown so much that last year, the American College of Sports Medicine and the American Heart Association issued new recommendations for healthy adults 65 and older that stressed emphasized the importance of weight lifting.

The groups now recommend that all older Americans do eight 8 to 10 repetitions for each of the major muscle groups (biceps, quadriceps, hamstrings, etc.). Resistance exercises should be done on two or more non-consecutive days of the week.

The idea is to lift a weight that’s heavy enough to work each muscle group until it is fatigued, so the amount you lift will increase as your strength grows. Weight-bearing exercise, like walking or running, does not count as weight lifting — that means you really have to lift weights or work out on a resistance machine.

One of the biggest benefits of strength training is that it dramatically increases muscle mass, which aerobic exercise does not, noted William J. Evans, director of the Nutrition, Metabolism, and Exercise Laboratory at the University of Arkansas for Medical Sciences. More muscle mass is good not just because it makes you stronger but because it increases basal metabolic rate — muscle cells even at rest burn more calories than fat cells.

Moreover, while aerobic exercise can significantly, although temporarily, increase blood pressure, a potential concern for some heart patients, resistance training does so only minimally, Evans said. Weight training also gets results fast — it only takes resistance training twice a week for a few weeks to begin to see a significant effect, compared with three days a week with aerobics.

Indeed, the more researchers probe the benefits of weight training for specific conditions, the stronger the case they can make, said Miriam Nelson, director of the John Hancock Center for Physical Activity and Nutrition at Tufts University.

Although studies have not yet proven that strength training lowers the risk of osteoporosis, Nelson said, they do show it lowers the risk of fractures by improving balance, bone density, and muscle mass. Weight training is also good for people with arthritis, she said, because stronger muscles can take the pressure off inflamed joints.

Weight training has been shown to have other benefits, too.

Research by Steven N. Blair, an exercise scientist at the University of South Carolina, suggests that people with greater muscle strength may be somewhat less likely to develop metabolic syndrome, a cluster of factors that raise the risk of heart disease and diabetes, such as increased waist size, high fasting blood sugar, high triglycerides, low HDL or ”good” cholesterol, and high blood pressure. More studies are needed to confirm this association.

For older people with physical disabilities, 66 trials reviewed by Cochrane Collaboration, an international nonprofit group that evaluates health treatments, increasing strength and, to a lesser extent, function. A different 2007 Cochrane review of 34 studies showed that exercises, including strength training, can improve balance in women age 75 and older. Yet another 2007 Cochrane review of 34 studies on fibromyalgia (musculoskeletal pain) showed strength training may improve physical capacity.

And a Danish study just published last week showed that strength training aimed at shoulder and neck muscles can diminish the chronic neck pain that many people get from working of at computers.

I could go on. But I’m convinced. Weight training may not be as much fun as a run in the park. But I need it. I’m guessing you do, too.

Today, as an estimated 20,000 runners begin their mad dash from Hopkinton to Boston, Dr.  Wood, a cardiologist, four-time marathoner and co-director of the Massachusetts General Hospital Women’s Cardiovascular Health Center, will be setting up shop in the corner of the medical tent at the finish line.

As soon as they’re finished, about 25 amateur runners will stroll or hobble over to Wood’s corner to let her take a sample of their blood. They will also get a non-invasive test to see how well their hearts are working after the stress of running for about 4 hours. As they have done every year since 2003, Wood and her MGH colleagues will then compare these post-race test results to the pre-race exams done two weeks earlier.

The MGH findings on Boston marathoners  – three published papers to date and two pending – are sobering and lend support to the idea that while moderate exercise is perhaps the most important thing a person can do for health, taking it to extremes, like a marathon, may not be.

Among marathon runners, the biggest cardiac risk seems to arise in people who train the least. People who worked up to a marathon by running at least 45 miles a week for at least three to four months “were golden,” said Wood. “They didn’t get into any trouble at all. If they trained less than 35 miles a week, they were in big trouble.”

Translated for the rest of us, this means that “sudden, strenuous activity can trigger a heart attack,” said Dr. Arthur Siegel , a 20-time marathoner and director of internal medicine at Harvard’s McLean Hospital.

Roughly 450,000 Americans now run in marathons every year. And 325,000 do triathlons, which involves swimming, biking and running, according to USA Triathlon, the sport’s organizing body. Many of these are not well-trained athletes but newcomers who race to raise money for charities. That means, said Siegel, that in many such events, participants “are getting older and slower. That’s where the cardiac risk comes in, especially for middle-aged men with previously silent heart disease.”

The key to healthy exercise, in other words, is moderation and consistency, especially if you are new to a sport.

Moderate exercise is unarguably good for you. “The greatest hazard of exercise is not doing it,” said Dr. Harvey Simon.   Simon is an avid runner, former marathoner, MGH internist and author of “The No Sweat Exercise Plan,” which advocates very moderate exercise – even as moderate as gardening and housework – instead of extreme exertion like marathoning.

Study after study has shown that moderate, regular exercise can indeed reduce the risk of heart disease, diabetes, stroke, hip fracture and some kinds of cancer.

But exercising moderately takes patience and persistence. If you have not been exercising regularly,  you should work up over several weeks to walking 45 minutes a day at least five days a week, said exercise physiologist Kerry Stewart  at Johns Hopkins School of Medicine.

At first, you may have to stop every few minutes and rest, he said. That’s fine, just start up again. If you get chest pains or severe shortness of breath, of course, stop and call your doctor. Obviously, if you have heart disease or have had a heart attack, check with your doctor before starting or substantially increasing your workouts. 

To gauge whether you’re working hard enough, you can take your pulse or use a strap-on heart monitor, available at many sports stores. If you’re healthy, the goal is to work out at about 70 percent of your maximum heart rate, which can be determined by a stress test in a doctor’s office.

You can also estimate your maximum heart rate by taking the number 220 and subtracting your age; if you’re 60, your maximum heart rate is 160, so your “target” heart rate during exercise should be about 112 beats per minute. You can also use the subjective “perceived exertion” scale, which runs from 6 (no effort) to 20 (your absolute max). The goal is to have your perceived exertion be about 12 to 14. An even simpler way is to use the “sing/talk test:” Work hard enough that you can’t sing but can talk.

Moderation is the key, said Simon of MGH. “I used to preach ‘No pain, no gain,’ but now I say, “No pain, big gain,’ ” he said. The whole “aerobics doctrine” that a person needs a lot of strenuous exercise “inspired the few, but discouraged the many,” he said. Even just walking at the extremely leisurely pace of half an hour per mile has benefits.

In other words, you shouldn’t under-do exercise, but you shouldn’t overdo it, either. Chronic fatigue, trouble sleeping, muscle tiredness, nagging congestion or sore throat, persistent aches and pains and depression are common signs that you may be working out too hard, said Siegel. To avoid this, try not to increase your exercise duration or intensity by more than 10 percent over any two-week period.

Experienced athletes “know how delicate the balance is between training to obtain optimal performance and overtraining to the point where muscle function begins to deteriorate,” said Dr. Christopher Cooper , an exercise physiologist at UCLA. But for amateurs, finding that balance point can be hard.

As for marathoners, Wood and her MGH colleagues have found that running 26.2 miles can lead to clear signs of cardiac stress. They have found that cardiac troponin, a chemical that only shows up in blood tests when heart muscle is damaged, rises in 60 percent of runners, and in some, it rises so high that “if you had just looked at these scores, these people would have been admitted to the hospital for heart attacks,” Wood said.

They’ve found that another chemical, BNP (for brain natriuretic peptide), another red  flag for cardiac dysfunction, also goes up after a marathon in 60 percent of runners. Platelets also become activated and more likely to form the clots that can trigger heart attacks, according to a just-published paper by Siegel and Alexander Kratz, director of the hematology lab at MGH.  And, as shown on echocardiograms, the heart’s ability to relax after each beat remains impaired for at least several weeks in most marathoners.

Bottom line? You don’t have to run a marathon to get into good shape. Just put on comfortable shoes, get out and walk. Moderately. And consistently.

In a few weeks, millions of us will be glued to our TV sets, watching the best athletes in the world ski, skate and slide their way into Olympic history in Turin, Italy.

We will certainly be dazzled, as always, by the sheer physical skill of these folks who have pushed their bodies so hard for so many hours a day, year after year.

But just as important as physical training, say those who study elite athletes, is the mental training that goes into a peak performance. If two athletes are equally fit, the edge often goes to the one with the better emotional skills — not a do-or-die focus on winning, but a set of habits that all of us can learn, including positive “self talk,” maintaining an energy level that is neither too excited nor too relaxed and, perhaps most important, a Buddhist-like ability to focus totally on the moment at hand, on this particular breath, stroke, turn.

So useful are these techniques that sport psychologists say their coaching is increasingly being sought by surgeons, trial lawyers, musicians, public speakers, business people and others who need to perform at their best in high stress situations. Partly because of this increasing demand, the Association for the Advancement of Applied Sport Psychology, the major professional organization in the field, has grown from a few hundred 20 years ago to 1,300 today, said the group’s president, Craig Wrisberg, a sport psychology professor and mental training consultant at the University of Tennessee.

Nowhere has the teaching of mental skills become a finer art than at West Point, where Nate Zinsser, director of the performance enhancement program, runs a sophisticated lab that is the envy of sports teams around the country. He’s building better athletes (Army must beat Navy) and also better soldiers, who have imagined every possible thing that might go wrong with a military operation. “You don’t want to experience anything for the first time in combat.”

Among other things, Zinsser has what he described as “very cool”  ergonomically designed chairs in which cadets sit and, through biofeedback techniques like monitoring heart rate, learn to relax and ignore potential distractions — such as crowd noise — piped in through speakers.

 “The process of training and learning to compete competently is a much more valuable lifetime lesson than simply the accomplishment of having won something on a given day,” said Zinsser. The key, for Olympic athletes as well as weekend warriors, is to learn to juggle two contrasting disciplines. “You have to be almost an obsessive-compulsive workaholic to get yourself ready to be good. But then you have to be this relaxed, Buddha-like Zen master, which allows all the stuff you have been training to come out.”

In other words, you train your body, especially your nervous system, so that you can automatically do your best on every step, jump, start or landing. Then you get your mind and its anxious chatter out of the way, go on “autopilot” and let your body “fly itself,” said Jim Bauman, a sport psychologist for the US Olympic Committee who has been working this year with the men’s alpine ski team.

Naturally, you can’t will yourself into the zone. But you can set the stage for it, in an athletic event, public speaking or any potentially stressful performance. Here’s how: