Two recent studies have shown that vitamin D deficiency may be a contributing factor in some cases of diabetes and/or metabolic syndrome (insulin resistance). In a study of 126 healthy volunteers (mean age 26 years) with normal glucose tolerance, there was a significant positive correlation (p < 0.0001) between the serum 25-hydroxyvitamin D concentration (a measure of vitamin D nutritional status) insulin sensitivity, and a significant negative correlation between serum 25-hydroxyvitamin D and measures of pancreatic beta-cell function. People with subnormal concentrations of 25-hydroxyvitamin D had a greater prevalence of components of the metabolic syndrome than did those with normal levels (30% vs. 11%; p < 0.01). In the other study, 7 of 10 women with type 2 diabetes had low serum concentrations of 25-hydroxyvitamin D. After supplementation with 1,332 IU/day of vitamin D for one month, a significant 34% increase was seen in insulin secretion, although it still remained significantly lower than that in nondiabetic controls. Insulin resistance also decreased by a mean of 21%, although that change was not statistically significant.

Comment: The results of these and other studies indicate that vitamin D plays a role in the secretion, and possibly the action, of insulin. Moreover, vitamin D deficiency appears to be common among people with type 2 diabetes, and may also be a risk factor for the metabolic syndrome. Preventing these common disorders, therefore, requires more than just turning off the television and getting some exercise; we need to do some of that exercise outdoors, under the vitamin D-producing sun.

Chiu KC, et al. Hypovitaminosis D is associated with insulin resistance and beta cell dysfunction. Am J Clin Nutr 2004;79:820-825.

Borissova AM, et al. The effect of vitamin D3 on insulin secretion and peripheral insulin sensitivity in type 2 diabetic patients. Int J Clin Pract 2003;57:258-261.