Glucose tolerance and vitamin D: effects of treating vitamin D deficiency

We investigated the effects of vitamin D treatment on plasma glucose, serum insulin, and insulin sensitivity in vitamin D-deficient individuals without diabetes mellitus.

Thirty-three adults with vitamin D insufficiency (serum 25-hydroxyvitamin D concentration < or = 50 nmol/L) and without diabetes (12 with impaired glucose tolerance) were given two oral doses of 100 000 IU of cholecalciferol, 2 wk apart. Before the first dose and 2 wk after the second dose, a 75-g oral glucose tolerance test was performed. Plasma glucose, serum insulin, 25-hydroxyvitamin D, and parathyroid hormone concentrations were measured and insulin sensitivity was calculated from the oral glucose tolerance test.

Mean serum 25-hydroxyvitamin D increased from 39.9 +/- 1.5 (SEM) to 90.3 +/- 4.3 nmol/L (P < 0.0001) and mean serum parathyroid hormone decreased from 6.7 +/- 1.2 to 4.5 +/- 0.6 pmol/L (P = 0.055). There was no change in blood glucose mean of 0-120 min (6.1 +/- 0.3 before versus 6.2 +/- 0.3 mmol/L, P = 0.63) or insulin mean of 0-120 min (47.8 +/- 5.35 versus 48.9 +/- 5.22 mU/L, P = 0.67) concentrations, and no change in insulin sensitivity (Avignon's insulin sensitivity index [SiM], P = 0.97; insulin sensitivity index at 0 and 120 min [ISI(0,120)], P = 0.74; Quantitative Insulin Sensitivity Check Index [QUICKI], P = 0.88; homeostasis model assessment [HOMA], P = 0.99) after vitamin D treatment. Results did not differ between subjects, with and without, impaired glucose tolerance.

In adults without diabetes, correction of vitamin D deficiency is not associated with any effect on blood glucose or insulin concentrations or insulin sensitivity as assessed during an oral glucose tolerance test. These observations do not support an association between glucose/insulin homeostasis and vitamin D, at least in the short term.