Currently, therefore, it cannot be definitively said that vitamin D deficiency causes diabetes or that raising vitamin D levels helps manage it, but more clinical trials are under way, so the relationship may become clearer over time.
Vitamin D in food and nature
Vitamin D is found naturally in very few foods. When it is present, it is primarily found in its active form, vitamin D3, and only mushrooms have small amounts of D2, which can be converted to D3 in the body.
The best natural sources of vitamin D are fatty fish like salmon, trout, and sardines, which have between 46 IU and 794 IU per 3-ounce serving, or 12% to 199% of the Daily Value (DV), which is 400 IU (10 micrograms). One egg yolk has about 25 IU (6% DV), and 3.5 ounces of beef liver contain 46 IU (12% DV), but many people avoid these foods due to their high fat and cholesterol content. Fortified milk is the primary source of vitamin D in the US diet, with 100 IU (25% DV) in each 8-ounce serving. Other foods such as breakfast cereals and energy bars may also be fortified with vitamin D during processing.
Vitamin D is called the sunshine vitamin because even modest exposure to the sun can produce up to 20,000 IU in a single day. When produced via sun exposure, this amount of vitamin D poses no risk for toxicity. Interestingly, the indigenous populations of far northern regions such as Alaska cannot synthesize adequate vitamin D from sun exposure — it is too cold and/or dark most of the time to expose enough skin, and the sun reaches them at a steep angle, which weakens its rays. But they get ample amounts from their traditional diets, which are primarily composed of fatty fish.
Since most people are unlikely to eat like an Eskimo, what is the best way to get vitamin D? Dr. Michael Holick, director of the General Clinical Research Unit and professor of medicine, physiology, and biophysics at Boston University Medical Center, has done extensive research on the topic and suggests several methods. One is “sensible sun exposure,” which means exposing skin to the sun while taking caution never to burn. Dr. Holick points out that sunscreen with an SPF of 30 reduces vitamin D production by 99%, so he recommends that those who live in a northern latitude such as Boston expose their arms and legs to the sun without sunscreen for 5–15 minutes, two to three times each week in the summer months to encourage natural vitamin D production. He also supports the MyPyramid guideline to consume 2–3 servings of milk daily and strongly believes that supplements are also necessary for most people (although not everyone agrees with him on this point).
How much vitamin D do you need?
The amount of dietary vitamin D needed varies from person to person. Accounting for these differences are both the varied amount of sun exposure and the wide variations in internal production among individuals. One small study of young people living in Hawaii showed that approximately half had levels under 30 ng/ml (nanograms per milliliter), which is considered insufficient by many doctors. This was the case even though they averaged almost 24 hours of sun exposure weekly with no sunscreen. It appears, therefore, that sun exposure is not sufficient for everyone, which is why some experts consider dietary D a more reliable source.
The dietary recommendations for vitamin D were set in 1997 (see “‘D’ by the Numbers”) and have recently been criticized as being far too low. They are currently under scrutiny by the Institute of Medicine’s Food and Nutrition Board, and may be increased soon.
Until then, it is agreed that there are several populations at particular risk for vitamin D deficiency who likely need a vitamin D supplement. Among them are breast-fed infants (breast milk is very low in D), the elderly (who do not synthesize it as efficiently and often spend little time outside), dark-skinned individuals, and those who never expose their skin to the sun (for religious or other reasons). Obesity is also a risk factor. Since vitamin D is stored in body fat, a person with more body fat will tend to store more vitamin D rather than having it circulating in the bloodstream doing its many jobs. Because of this, it may be that obese individuals need two or three times the amount of vitamin D as people with less body fat.