Winter has settled in. The days are short. And medical experts worry that many of us may be falling short on a vitamin that’s far more important than anyone realized.

The nutrient in question, vitamin D, is sometimes called the “sunshine” vitamin, because the body produces it when skin is exposed to sunlight. Most of the vitamin D we need, in fact, is generated that way.

But during the winter months in many parts of the world, sunlight levels are too low to stimulate vitamin D production. And though the body can store the vitamin for several months, those stores may begin to run out just about now.

Why worry? Doctors have known for more than a century and a half that D is essential for normal bone development. Children who don’t get enough develop rickets, a serious disease marked by joint and bone abnormalities. Rickets was once considered a disease of the past. But lately, cases have been showing up in kids.

And there are other more widespread risks associated with vitamin D deficiency. Falling short increases the danger of thinning bones and osteoporosis. Vitamin D deficiency has been linked to muscle pain and weakness, high blood pressure, multiple sclerosis, and even some forms of cancer. “Many cells in the body have receptors for vitamin D, which tells us that it plays many roles,” said Michael Holick, PhD, MD, professor of medicine at Boston University School of Medicine and one of the leading experts on the nutrient. “So it’s not surprising that deficiencies cause a variety of problems.” Dr. Holick is author of The UV Advantage: The Medical Breakthrough that Shows How To Harnass the Power of the Sun for Your Health (IBooks, Inc.).

Plenty of evidence underscores D’s critical importance. A small study reported this month by researchers at the University of Alberta showed that many cases of chronic back pain may be caused or at least worsened by vitamin D deficiency. Other findings suggest that low levels of D impair the working ability of muscles. A study by Dr. Holick and colleagues, published in December, showed that women deficient in D were more likely to undergo Caesarean sections. The latest evidence shows that a shortfall in D could increase risk of heart disease.

Unfortunately, many of us aren’t getting even the minimal recommended levels of vitamin D, especially at this time of year. More than 93 percent of young adults tested in Canada fell short of the current recommended blood level of vitamin D, according to a study published in 2008 by researchers at the University of Toronto. A survey of African American women between the ages of 15 and 49 found that almost half were deficient not just in the winter but all year round.

Not long ago, scientists at the Joslin Diabetes Center in Boston tested young people with type 1 diabetes. “To our suprise, we found extremely high rates of vitamin D inadequacy,” said physician Lori Laffel, MD, MPH, who directed the study. Twenty-four percent of the youths examined had sufficient levels, 61 percent insufficient levels, and 15 percent were at the lowest end of the scale. The findings are particularly worrisome, since diabetes itself can compromise bone health. Adding vitamin D deficiency can be a recipe for trouble down the road.

The farther north people live, in general, the greater the risk of not being exposed to enough sunlight to generate adequate levels of vitamin D. The darker your skin, the less efficient it is at generating vitamin D from sunlight. The skin’s ability to convert sunlight to D diminishes with age, putting older people at risk, as well. No matter where you live, of course, if you spend most of your daylight hours indoors, you may be coming up short.

How to make sure you’re getting enough? Although some foods contain vitamin D (salmon, mackerel and other fatty fish, for examples) and foods like orange juice, milk and breakfast cereal are fortified with D, “it’s almost impossible to get enough vitamin D from your diet,” according to Robert Heaney, MD, professor of medicine at Creighton College in Omaha, Nebraska.

That leaves vitamin D supplements. And a growing number of experts say most of us should be taking a pill, especially this time of year. But what dose remains an open question. In 1997, federal health officials in the U.S. revised the recommended level upward to 400 international units (IU) a day for adults and 600 IU for people over 70. In 2008, the American Pediatric Association began recommending 400 IU a day from birth onward.

But many leading authorities think most of us need more. “The current recommendations are hopelessly out of date,” says Heaney. “The optimal level is probably much higher.” Indeed, a team of international authorities from Canada, the U.S., Australia, and Finland, writing in the American Journal of Clinical Nutrition, recently suggested that the recommended intake should be 1,700 IU a day or more during the winter months.

One of the leading multivitamins contains 400 IU. For more than that, you’ll need to take an extra vitamin D pill. When choosing a supplement, look for pills that contain a form of the nutrient called vitamin D3, which is especially potent. New guidelines for D are likely to come along, perhaps even this year. So it’s wise to check back for the latest. For now, experts generally agree hat most of us could use a lot more than we currently get.

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Tags: bone loss, Caesarean, heart disease, Michael Holick, muscle weakness, osteoporosis, vitamin D deficiency, vitamin D supplements