It’s not clear whether vitamin D specifically protects against H1N1, a novel virus, but there’s growing evidence that it does protect against a number of respiratory infections – and that many Americans do not get enough of the vitamin.

One study showed that people taking supplements containing 2,000 international units of vitamin D a day suffered fewer respiratory infections than those not taking supplements. Another study showed the obverse – that people with low blood levels of vitamin D were somewhat more likely to have had a recent upper respiratory tract infection than people with higher levels (24 percent vs. 17 percent). Vitamin D boosts the activity of a gene that makes cathelicidin, a natural antimicrobial compound that is part of the body’s defenses against infections, says Dr. Carlos A. Camargo, an associate professor of medicine and epidemiology at Harvard Medical School.

When there’s lots of sunshine, people make vitamin D naturally. But in New England, most people have low levels of vitamin D, especially in winter. The problem is a national one as well: A study being published today in Pediatrics shows that about 20 percent of children ages 1 to 11 have suboptimal levels of vitamin D.

You can get a blood test to determine your vitamin D level. People with darker skin are at extra risk because highly pigmented skin requires more sun exposure to obtain a healthy level.

The vitamin has so many benefits – including lowering the risk of osteoporosis, heart attacks, and colon cancer – that “I am encouraging everyone to increase their vitamin D intake, especially children,” says Dr. Michael F. Holick, a professor of medicine, physiology, and biophysics at Boston University. He suggests that children take a minimum of 400 IUs a day and preferably 1,000. “Adults should take at least 1,000 IUs and preferably 2,000 IUs a day,” he says.

Daniel Perlman, a senior scientist at Brandeis University, says 2,000 IUs a day is safe: “In the summer sun, the body itself is known to produce far higher levels.”

Yes – a blood test called 25-hydroxyvitamin D, which, though expensive ($20 to $100 a pop), is covered by insurance. Your level should be at least 30 nanograms per milliliter of blood and not more than 100. If your level is way below that, say around 18, you need to take 1,200 international units or more a day of vitamin D supplements, either the type called D2 or D3, said Dr. Michael F. Holick, an endocrinologist and leading vitamin D researcher at the Boston University School of Medicine.

“Everyone should be tested,” said Holick, who consults for a lab that conducts the tests but does not get royalties. Many doctors hesitate to order the test because they still believe that rickets, a serious but now, extremely rare bone disease in children, is the only manifestation of vitamin D deficiency.

But even mild levels of vitamin D deficiency have been linked to cancers of the breast, prostate, and colon, as well as to multiple sclerosis, both type 1 and type 2 diabetes, heart disease, and stroke. A meta-analysis involving data on more than 57,000 people in 18 studies showed that those taking vitamin D supplements had a lower risk of dying for any reason during the study.

Vitamin D is a steroid-like hormone that the body makes after skin exposure to UV-B radiation from sunlight. After an inactive form of vitamin D is made in the liver, it is transformed in the kidney to an active form.

Many studies show that people, especially African-Americans and those in northern latitudes like New England, are at high risk of being deficient in vitamin D. A 2004 study of 307 healthy teenagers led by Dr. Catherine M. Gordon, an endocrinologist at Children’s Hospital in Boston, showed that 42 percent were vitamin D deficient.

Many doctors assume that because vitamin D is important for calcium metabolism, if blood levels of calcium are normal, vitamin D must be sufficient, too, said Holick. But this is not true.

Usually, Holick said, it’s only when doctors begin testing themselves for vitamin D deficiency that they “get religion” and begin taking vitamin D testing seriously. A growing number of researchers now recommend that everyone take at least 800 to 1,000 international units of vitamin D a day. It takes 10,000 IU’s a day for vitamin D to become toxic.

For what it’s worth, I take at least 1,400 IU’s a day.

Several recent studies presented at meetings of the American Association for Cancer Research and the American Society of Clinical Oncology suggest that adequate consumption of vitamin D- which most Americans do not get – is linked to lower risks of breast cancer.

One study, from researchers at the University of California, San Diego, looked at pooled data on 1,760 women and found that the highest level of vitamin D consumption was correlated with a 50 percent lower risk of breast cancer. To achieve this level of vitamin D, a person would have to consume at least 1,000 International Units of vitamin D a day, well beyond the current recommended levels of 200 IUs a day for children and people up to age 50, 400 IUs for people aged 51 to 70, and 600 IUs for people over 70.

In another study, researchers at Mount Sinai Hospital in Toronto interviewed 576 patients with breast cancer and 1,135 people without and found that those who had had frequent sun exposure as teenagers or young adults had a 35 to 40 percent reduced risk of breast cancer. (Sunlight helps the skin produce vitamin D.) Yet another study found vitamin D deficiency is highly prevalent among pre-menopausal women with early stage breast cancer. On the other hand, a study that was part of the government-sponsored Women’s Health Initiative found that calcium and vitamin D supplements did not reduce breast cancer risk in post-menopausal women, although when women randomized to get the supplements did get breast cancer, their tumors were smaller than those of women not taking supplements.

Dr. Michael F. Holick, a vitamin D expert and professor of medicine, physiology and biophysics at Boston University Medical Center, said that other studies suggest that vitamin D can also help prevent colon, ovarian and prostate cancer.

People can get vitamin D by “sensible” sun exposure (5 to 10 minutes several times a week without sunscreen), supplements and a diet rich in fortified cereals, milk and oily fish. Holick recommends at least 1,000 International Units of vitamin D a day, preferably the strong form, called D3, not D2, the kind that is in most supplements.

“While these studies certainly suggest that getting adequate vitamin D is important,” said Dr. Kala Visvanathan, a cancer specialist and epidemiologist at the Johns Hopkins Bloomberg School of Public Health, “it is not clear that taking extra is beneficial or whether dose should be altered, depending on age.”

Vitamin D in very high doses can be toxic. The Institute of Medicine, an arm of the National Academy of Sciences, is expected to decide within the next several years whether to recommend increased intakes of vitamin D. Currently, the institute sets the upper limit for consumption at 2,000 IUs a day.