Low vitamin D prevents insulin response; sunlight may not be enough

A recent study from the University of Wisconsin Osteoporosis Clinical Research Program in Madison, showed that some people have very low levels of vitamin D in spite of getting a lot of sunlight where they live in sun-drenched Hawaii (Journal of Clinical Endocrinology & Metabolism, June 2007).
The 93 participants in the study averaged 22.4 hours per week outside without sun screen. In spite of abundant sun exposure, 51 percent had low vitamin D levels.

Many people think of vitamin D as the vitamin that helps to prevent rickets, a disease characterized by weak bones that break easily. However, vitamin D does much more than that. It is necessary for your immune system to search out and destroy invading bacteria, viruses and even cancer cells. Recent studies show that lack of vitamin D prevents your body from responding to insulin adequately. The Third National Health and Nutrition Examination Survey shows that having low levels of vitamin D increases risk for high blood pressure, obesity, diabetes and having high blood levels of triglycerides (Archives of Internal Medicine, Volume 167, 2007). Researchers at Tufts University in Massachusetts showed that obesity increases a person's chances of having low vitamin D levels (Journal of Clinical Endocrinology & Metabolism, May 2007). A possible explanation is that fat sops up the available vitamin D so that it is not readily available to be used by the body.

Most people do not get adequate amounts of vitamin D from the food that they eat. They must depend on sunlight. People with dark skin require more sunlight to meet their needs for that vitamin, which could explain the increased risk for diabetes in dark-skinned people who live far from the equator. Calcium blocks the activation of vitamin D so people who take calcium supplements need to get extra sunlight or take vitamin D supplements. The large amount of calcium in milk can lower vitamin D levels even when the milk is fortified with the vitamin.

Vitamin D can be made in the skin when it is exposed to ultraviolet light from the sun. Then the liver converts vitamin D to an active hormone form called 25 hydroxy vitamin D. Then your kidneys convert this hormone to a more active di-hydroxy vitamin D. So you get that vitamin from your skin, liver and kidneys, and lack of sunlight and liver or kidney diseases can cause a deficiency. Now we learn that some people can be deficient even with adequate sunlight exposure and normal kidneys and livers. The only way to find if you are deficient of vitamin D is to get a blood test called 1,25 diyhxdroxy-vitamin D.