Vitamin D isn't just for bones anymore

Feb 21 2008

Researchers at The Rockefeller University Hospital have begun a clinical study to explore a possible connection between vitamin D deficiency and insulin resistance.

The hypothesis, that raising blood vitamin D levels in an obese, insulin-resistant population will improve the subjects? ability to metabolize sugar, could have eventual implications for patients with type 2 diabetes and their doctors.

Vitamin D is produced through a photochemical reaction that occurs when the skin is exposed to ultraviolet B radiation. Because few food sources are naturally rich in the nutrient, the human body depends on sunlight and/or supplements as its main sources. The connection between vitamin D and calcium absorption ? and thus the necessity of vitamin D for bone formation ? is well established in the medical community. But researchers have begun to uncover other links as well, to diseases as significant and as varied as ? among many others ? rheumatoid arthritis, cardiovascular disease, schizophrenia, cancer and diabetes.

The National Health and Nutrition Examination Survey ? a report of statistics kept by the National Center for Health Statistics, part of the Centers for Disease Control and Prevention ? and its report on vitamin D and diabetes supported for the study?s lead investigator and Instructor in Clinical Investigation Allegra Grossman the connection between vitamin D and insulin.

Vitamin D receptors exist on various kinds of cells throughout the body, including those responsible for the action of insulin: Sugar is deposited on muscle cells, where insulin, produced by pancreatic beta cells, processes it. The question posited by Grossman is double-edged: Does vitamin D sensitize muscle cells to insulin? And does it improve insulin secretion in pancreatic beta cells?

Grossman and her colleagues at Rockefeller are recruiting 10 insulin-resistant subjects between the ages of 18 and 65 with a minimum body mass index of 28 kilograms/meter 2 (within 10 percent of maximum weight) and a low blood vitamin D level as determined by screening tests conducted at the hospital. Subjects will take capsules of vitamin D3 ? cholecalciferol, the form produced by the body and one of two forms currently on the market ? three times a week at a dosage of 30,000 international units per week, for eight weeks. The dosage is roughly 20 times the minimum daily intake recommended by the United States Department of Agriculture (whose figures assume a lack of adequate sunlight exposure).

Following vitamin D repletion, the researchers will test for insulin sensitivity, using the euglycemic hyperinsulemic glucose clamp. They will also test for markers of inflammation, which has been correlated to both vitamin D level and insulin resistance.

?If we?re right, then the results of this study may one day help support the use of vitamin D in a clinical setting,? says Grossman. ?Practitioners could implement this at very low cost, because vitamin D supplements are cheap. Some of the more common treatments currently available for insulin resistance are quite expensive.? Investigators at The Rockefeller University Hospital began screening applicants in January and expect a conclusion to the study in one year.