HONG KONG, China  Since the 17th century, children have been suffering from rickets, a disease causing soft and bent bones. By the 19th century, physicians realized that it could be prevented with exposure to sunlight and by consuming cod liver oil. In the 20th century, scientists established that deficiency of an essential nutrient, which they labeled as vitamin D, causes rickets and this nutrient is produced in the skin when it is exposed to sunlight.

Vitamin D has thence been well known for its importance in maintaining good bone health. Recent studies suggest, however, that there is also a positive relation between vitamin D status and not just bone diseases, but also risks of several types of cancers, autoimmune diseases, hypertension, heart diseases and even diabetes.

"Rickets can be considered just the tip of the vitamin-D deficiency iceberg," says Dr. Michael Holick, professor of medicine, physiology and biophysics and director of the General Clinical Research Center and Bone Health Care Clinic at the Boston University Health Center, in a review article published in the New England Journal of Medicine last July.

Consider the clinical trial conducted by Dr. Joan Lappe and her group from the Osteoporosis Research Center at Creighton University in Omaha, published in the American Journal of Clinical Nutrition in June last year.

The study, which lasted four years and included 1,180 menopausal women, noted that incidences of cancers of the breast, colon, lung, uterus and leukemia significantly decreased in women receiving a combination of vitamin D and calcium.

Another study conducted by a group led by Dr. Thomas Wang at Massachusetts General Hospital in Boston, published this month in the journal Circulation, suggests that deficiency of the vitamin may be a risk factor for developing heart diseases, more pronounced in people with high blood pressure.

Yet another analysis by the National Health and Nutrition Examination Survey, which appeared in last year's June 14 issue of the Archives of Internal Medicine, shows that adults with low serum levels of vitamin D are more likely to have hypertension, diabetes, obesity and high triglycerides.

While speaking to journalists of heart.org, Dr. Keith Norris, senior author of the study, said, "There's been more and more evidence that vitamin D is involved in a variety of bodily functions, including heart health."

Unlike other vitamins that cannot be manufactured by the human body, vitamin D can be produced in the skin, with the help of sunlight, and hence technically speaking it is not a vitamin. In fact, its structure is that of a steroid and its behavior is like a hormone.

Considering that it can be produced in sunlight, one would imagine that most people in the world, especially those living in tropical countries, would have ample stores of vitamin D. But, in fact, there is a high prevalence of vitamin D deficiency in many parts of the world.

Surveys conducted in some large cities of India, a sunny country, show that more than 30 percent of school kids and pregnant women have subnormal levels and up to 90 percent of menopausal women have low levels of vitamin D. Similarly, deficiency is also common in populations of Korea, Japan, China, Turkey, Lebanon, Jordan, Italy, Spain, Australia and even North America.

The elderly who spend a lot of time indoors, as in residential care; people who cover their entire body when they are outdoors, for cultural or religious reasons or for protection from the sun; patients of skin conditions who avoid sunlight, people given to using sun block creams all the time, people with dark skin and people living in regions with poor sunlight are susceptible to this deficiency. Breast-fed babies are also at risk as breast milk has only small amounts of vitamin D.

With emerging evidence of the role vitamin D plays in normal functioning and the prevention of cancer and its spread in several organs and tissues besides bones, like the prostate, breast, intestine, kidney, muscles and immune cells, it is important to raise its levels in the blood.

While the exact role this sunshine vitamin plays is still being established, experts believe it is time to educate the masses about its significance and take steps to increase its levels in the blood. A blood level of at least 30 nanograms per milliliter has been proven to be most appropriate.

Sensible exposure to the sun, which includes exposing arms and legs for five to 30 minutes a day depending on the time of day, season, distance from the equator and skin pigmentation, is advised by Dr. Holick. The World Health Organization, however, warns against overdoing it, to protect against cancers due to overexposure to the sun.

Fortifying food, which is easy and cheap and is already practiced in North America, and nutritional supplements of vitamin D, in consultation with a doctor, is a good alternative.

Meanwhile, sipping a glass of milk while basking in the early morning sun might go a long way toward prolonging life.

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(Dr. Pradnya Kulkarni is a clinical pathologist from India. She worked as a consultant pathologist in a private laboratory in Pune before moving to Hong Kong with her husband and young daughter. She is also a freelance writer, focusing on medical issues of public concern. ©Copyright Pradnya Kulkarni.)