Vitamin D - Official state vitamin of Florida
July 6, 2011
In last week's article, we discussed how vitamin D was discovered and its importance in the health of bones. In this week's column, we will discuss the various sources of vitamin D through food, sunshine, and supplements.
Sources of Vitamin D
Animal products constitute the main food source of vitamin D that occurs naturally in unfortified foods. Fatty salt water fish such as mackerel, herring, salmon, and sardines are good sources of vitamin D, as are fish liver oils such as cod liver oil. Small quantities of vitamin D are derived from butter, cream, egg yolk, and shrimp.
Vitamin D is also found in the skin of poultry.
However, the most important source of vitamin D is sunlight. According to Michael F. Holick, director of the Vitamin D, Skin and Bone Research Laboratory at Boston University Medical Center, it is not often appreciated that casual exposure to sunlight during everyday activities provides most humans with their vitamin D requirement. Your skin can make vitamin D when it is exposed to sunlight. When ultra-violet light penetrates the skin, it converts a precursor in the skin to vitamin D. Floridians can make vitamin D all year round. But if you live north of Los Angeles on the West Coast or north of Atlanta on the East Coast, you don’t get enough ultra-violet light from the sun in the winter to make adequate vitamin D. North of that latitude - above 35 degrees - you can’t make any vitamin D in your skin in the winter, even at noon. Canadians can’t make vitamin D in their skin for four to seven months of the year. They are at high risk of vitamin D deficiency.
The Sunshine vitamin
According to Holick, who is also Professor of Medicine, Dermatology, Physiology, and Biophysics at Boston University Medical Center, there are clear benefits to sensible exposure to sunlight and essentially no evidence that it will increase risk of skin cancer. By sensible exposure he means five to ten minutes a week between 11 a.m. and 2 p.m. for a light-skinned Caucasian living in Boston in June. In Florida, it may be only two or three minutes.
Aging has no impact on how much vitamin D you absorb from food. How much you make from the sun is another matter. If you are 70, your skin can make only a quarter of the vitamin D that a 20-year-old can make when exposed to the same amount of sun. But a 70-year-old can make enough. According to Holick, if elders are out in the sunlight for 15 to 20 minutes a couple of times a week, they will maintain their vitamin D levels. It elderly persons do not take advantage of the beneficial effect of sunlight, they can develop vitamin D deficiency, which can result in secondary hyperparathyroidism. This condition accelerates osteoporosis and can cause a mineralization defect in bones, resulting in adult rickets or osteomalacia. The net effect is to weaken bones and increase the risk of fracture.
For older people living in Boston, Holick recommends exposure to sunlight in the morning or late afternoon for 5 to 30 minutes in the spring, summer, and fall (depending on skin sensitivity to sunlight). Elderly people need not be exposed to prolonged periods of sunlight because the amount of vitamin D they can produce in this period of time should satisfy the body’s requirements. Although excessive sun can damage the skin, there has never been a case of vitamin D toxicity because of too much sun. Nature has programmed into the system that any excess from the sun is destroyed.
Excessive amounts of vitamin D are not normally available from usual dietary sources and thus reports of vitamin D toxicity are rare. However, there is always the possibility that vitamin D intoxication may occur in individuals who are taking excessive amounts of supplemental vitamin D. The National Academy of Science recommends a safe upper limit of 2000 International Units. But if you live in Florida, a vitamin D supplement is as redundant as a tanning salon.
Mary Lou Williams, M. Ed., is a writer and lecturer in the field of nutrition. She welcomes inquiries. She can be reached at 267-6480.