When the sun dips low in the winter months, it’s hard to get enough of the sunshine vitamin.

Your skin can make vitamin D from sunlight that falls on your skin. But from October through March, dim light stimulates hardly any vitamin D production if you live north of latitude 35, which passes near Los Angeles on the West Coast and Raleigh, N.C., on the East.

What’s a body to do? Here’s how to avoid a shortfall of vitamin D, and why it’s wise to restock your cold weather supplies.

Just about everyone is at risk, not just the elderly.

Studies indicate that vitamin D deficiency commonly occurs in healthy young women and men and even preteen girls during prime growth years. An estimated 20 percent to 60 percent of the general U.S. population is vitamin D-deficient.

Factors that boost risk are older age, dark skin, obesity, an indoor job, northern climes, being homebound, digestive or kidney diseases, and exclusive breastfeeding in infants.

The chief function of vitamin D is to maintain normal blood levels of calcium and phosphorus. But lack of sufficient vitamin D leads to health effects that are surprisingly wide-ranging. Early research has uncovered the following:

Bones. By promoting calcium absorption, vitamin D helps to form and maintain strong bones. Classic vitamin D deficiency results in bone diseases such as rickets in children and osteomalacia in adults. Long-term lack of vitamin D can also result in osteoporosis, a brittle-bone disease that causes broken hips, as well as chronic joint and muscle pain and weakness associated with falls in the elderly.

Cardiovascular system. High blood pressure and coronary heart disease are linked to D deficiency.

Autoimmune diseases. Vitamin D helps regulate your immune system. Lower blood levels have been linked to type 1 diabetes, rheumatoid arthritis, ulcerative colitis, Crohn’s disease and multiple sclerosis.

Cancer. Research suggests higher rates of cancer of the colon, breast, ovaries and prostate are associated with vitamin D insufficiency. More data is needed, however, before scientists know if getting more of the vitamin helps protect against cancer.

Advice has been changing on the forefront of vitamin D research. Current recommendations from the Institute of Medicine call for 200 international units per day for people from birth through age 50, 400 IU for those 51 to 70, and 600 IU for those older than 70.

Some experts say that optimal amounts are closer to 1,000 IU daily. Until more is known, however, it’s wise not to overdo it. Potential side effects include nausea, vomiting, constipation, confusion, high blood levels of calcium and irregular heartbeats. The Institute of Medicine has set the upper limit at 2,000 IU per day (or 1,000 IU for infants up to one year).

Best vitamin sources

During warm months, sun exposure is the simplest way to get vitamin D. (Infants are the exception since they must stay out of direct sunlight.) Minimum adult requirements are 10 to 15 minutes of sunlight on your face, arms or legs twice a week; dark-skinned people may need more time in the sun.

After your time’s up, apply sunscreen to prevent skin damage.

I’m concerned about the skin cancer, aging and wrinkling that’s associated with sun exposure.

Luckily, you can get vitamin D from food: Salmon, mackerel, tuna, sardines, fortified milk, soy milk, margarine, orange juice and some ready-to-eat cereals are all excellent sources. Consult your doctor before taking a supplement.

Contact Dr. Elizabeth Smoots, a board-certified family physician and fellow of the American Academy of Family Physicians, at doctor@ practicalprevention.com. Her columns are not intended as a substitute for medical advice or treatment. Before adhering to any recommendations in this column consult your health care provider.

2006 Elizabeth S. Smoots.