
EATING THE OKINAWA WAY: VITAMIN D
Vitamin D has been around for a long time. Ocean-dwelling phytoplankton began producing it over 500 million years ago, after exposure to sunlight. Vertebrates during evolution became dependent on it for the health of their skeletons, the health of their nervous systems, and cellular functions (in close conjunction with calcium and phosphorus). Vitamin D deficiency (rickets) has plagued humans for millennia, causing deformation of long bones and the rib cage, bowed legs, bent spines, and weak muscles in those affected (e.g., up to 90 percent of children in autopsy studies in nineteenth-century Holland). Since it rarely occurs with adequate sunlight exposure, it did not become a major public health concern until the industrial revolution, when smog-filled crowded cities arose in northern Europe and America, and people (including small children) spent extended periods indoors. This led to a 270-year search for a cure, and eventual isolation of vitamin D in the early 1900s. Supplementation of dairy products with vitamin D since the 1930s has virtually eliminated the problem in children.
Getting the right amount of vitamin D can be tricky for adults. Dermatologists tell us to use sunscreen at all times, since the sun causes premature aging of the skin and skin cancer. Yet, this virtually eliminates vitamin D production in the skin; an SPF of 8 cuts vitamin D production by 95 percent. Moreover, if you live much north of Los Angeles (e.g., in New York, Boston, or Toronto) you will have a tough time making vitamin D from November to March because the sun is too weak. To further complicate matters, healthy elderly produce only 30 percent of the vitamin D that younger subjects do with the same sun exposure. You can see the problem here.
What it does. Vitamin D is a hormone. It is essential for packing calcium into your bones. Without it you cannot absorb adequate calcium for bone health. It also helps with the function of your heart and recent evidence has shown a potential role in reduction of cancers of the breast, prostate, and colon.
Top sources. Seafood—cod liver oil, tuna, salmon, and sardines—dairy products (fortified), margarine (fortified), mushrooms, and sunshine.
Average intake. Top vitamin D researchers believe 9 to 40 percent of the U.S. population is vitamin D deficient, depending on age, ethnicity, and locale.
Optimum intake. If you wear sunscreen outdoors and eschew fish or dairy products, you had better take a supplement, especially if you have darker skin. Adults up to age fifty require a minimum of 200 IU a day, fifty-one- to seventy-year-olds need 400 IU a day, and those over seventy require a minimum of 600 IU a day with minimal sun exposure.
Some researchers believe those with darker skin (brown or black) who are sunlight deprived require at least 1,000 IU a day, despite a diet that contains vitamin D foods. A prudent recommendation is for five to fifteen minutes of sun exposure of hands, forearms, and face two to three times per week (not to the point of sunburn) and application of sunscreen (SPF at least 8) thereafter. Then take a daily supplement of 200 to 600 IU according to your age. With no sun exposure and little dietary intake, a supplement of 800 IU is advisable.
Cautions. Toxicity can result from prolonged high doses because of overabsorption of calcium. Signs and symptoms include headaches, high blood pressure, nausea, constipation, growth retardation, seizures, and calcium deposits in the blood vessels, kidneys, or heart.
Best Evidence
A one-year, double-blind, placebo-controlled interventional study of 249 healthy postmenopausal women found those who received a 400 IU supplement of vitamin D significantly reduced wintertime bone loss and improved net bone density versus the placebo group.
In a study of sunscreen users, almost 50 percent of subjects in Springfield, Illinois, who always wore a sunscreen before going outdoors and did not take vitamin D supplements were vitamin D deficient, some of them severely so.
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General health
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