Vitamin D is often called the "sunshine vitamin" because your skin manufactures it naturally when you are outside in the sun. Until recently, scientists thought that the primary biological function of vitamin D was to help your body in absorbing calcium, which helps you to build strong bones, and avoid osteoporosis. However, according to the "Nutrition Journal," low levels of vitamin D3 are now linked to people who are overweight and obese.
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There are two natural forms of vitamin D related to humans: Vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is obtained through certain foods in your diet such as cod liver oil, eggs and fortified milk. Vitamin D3 is synthesized when your skin is exposed to ultraviolet light by the sun or in indoor tanning booths. You can also get D3 through supplementation.
According to a 2009 study, Vitamin D levels measured at the beginning of a weight loss program can accurately predict the amount of weight loss in the participants. Thirty-eight overweight men and women were placed on a diet consisting of 750 calories per day more than needed to maintain their current weight for 11 weeks. Those participants with the lowest vitamin D levels lost less weight and less abdominal fat than those with higher levels of vitamin D.
"Our results suggest the possibility that the addition of vitamin D to a reduced-calorie diet will lead to better weight loss," said the study's lead author, Shalamar Sibley, MD, MPH, an assistant professor of medicine at the University of Minnesota.
Vitamin D3 More Effective
Many physicians believe that vitamin D2 and D3 are equivalent in humans. According to a 2004 study published in the "Journal of Clinical Endocrinology and Metabolism," however, vitamin D2 is much less effective than vitamin D3. Vitamin D3 is three times more potent than vitamin D2 as measured in the serum levels after a 28-day trial with 20 healthy male participants. The researchers wrote; "Physicians resorting to use of vitamin D(2) should be aware of its markedly lower potency and shorter duration of action relative to vitamin D(3)."
According to the Vitamin D Council, "If well adults and adolescents regularly avoid sunlight exposure, research indicates a necessity to supplement with at least 5,000 units (IU) of vitamin D daily." That's the equivalent of 50 glasses of milk or 10 doses of a multitvitamin, the council says. On the other hand, spending just 20 to 30 minutes in the sun induces the skin to produce approximately 10,000 IU of vitamin D, which is 50 times more than the U.S. government's recommendation of 200 IU per day.
Since the sun only manufactures the amount of vitamin D3 that you need, and that it is almost impossible to digest too much D2 from the foods that you eat, about the only way you can get too much vitamin D3 is through excessive supplementation. But, vitamin D is a fat-soluble vitamin, so the body can store up quite a bit in the liver. If you have any concerns, check with your doctor and suggest that he order a 25-hydroxy vitamin D test, which will give you an accurate measure of the amount of vitamin D in your body.