Does vitamin D prevent the swine (H1N1) and seasonal flu, or the common cold?

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.”