Vitamin D3 supplementation improves insulin sensitivity in subjects with impaired fasting glucose

Abstract

Vitamin D has in vitro and in vivo effects on β cells and insulin sensitivity. Vitamin D deficiency (VDD) has been associated with the onset and progression of type 2 diabetes mellitus (DM-2). However, studies involving supplementation of vitamin D in subjects with previously established diabetes have demonstrated inconsistent effects on insulin sensitivity. The aim of this open-label study was to assess the effects of high-dose vitamin D3 supplementation on insulin sensitivity in subjects with VDD and impaired fasting glucose. We studied 8 subjects with VDD and prediabetes with the modified, frequently sampled intravenous glucose tolerance (mFSIGT) test before and after vitamin D supplementation. Vitamin D3 was administered as 10,000 IU daily for 4 weeks. The mFSIGT was analyzed with MinMod Millennium (purchased from Dr. Richard Bergman, Keck School of Medicine of USC, Los Angeles, Calif) to obtain estimates of acute insulin response to glucose (AIRg), insulin sensitivity (SI), and disposition index (DI). We found that AIRg decreased (P = 0.011) and SI increased (P = 0.012) after a intervention with vitamin D. If these findings are repeated in a randomized, double-blind study, the results indicate that orally administered high-dose vitamin D3 supplementation improves insulin sensitivity in subjects with impaired fasting glucose and suggests that high-dose vitamin D3 supplementation might provide an inexpensive public health measure in preventing, or at least delaying, the progression from impaired fasting glucose to diabetes.

Figure 1

Corrected Calcium and PTH Before and After Vitamin D Supplementation. Corrected calcium remained unchanged after vitamin D supplementation. PTH decreased significantly (p < 0.05) after vitamin D supplementation. Each value is the mean ± SEM of 8 subjects.

Figure 2

Acute insulin response to glucose (AIRg) and Insulin sensitivity (SI) before and after Vitamin D supplementation. Each point represents an individual subject. The line and bars are the median and interquartile range.

Figure 3

Relationship between the initial insulin sensitivity and the increment in insulin sensitivity. Each point represents an individual subject. The line is the least squares regression equation with the correlation coefficient = 0.619 and p=0.09.