Vitamin D, Insulin Secretion, Sensitivity, and Lipids

Randomized, double-blind, placebo-controlled trial nested in the population-based Tromsø Study in Norway. Adults aged 30 to 75 years with low serum 25(OH)D and without established diabetes or other major exclusions were enrolled; for the vitamin D3 arm, 51 participants were randomized and 49 completed the 6-month study.

Vitamin D3 was given as 20,000 IU per capsule by mouth twice per week for 6 months, using a placebo-controlled double-blind design.

Vitamin D3 supplementation raised serum 25(OH)D and lowered PTH, but it did not improve insulin secretion, insulin sensitivity, or lipid profile versus placebo after 6 months. Mean serum 25(OH)D reached 142.7 nmol/L in the vitamin D group versus 42.9 nmol/L in placebo, with no statistically significant between-group differences in the metabolic outcomes. Compliance was high at 98% in the vitamin D group and 97% in placebo. No cases of hypercalcemia or kidney stones were observed.

The intervention arm was small, with only 49 completers, and follow-up lasted just 6 months, limiting power to detect modest metabolic effects. Generalizability is limited because participants were almost exclusively Caucasian adults from one Norwegian cohort, and detailed per-arm demographic data were sparse. The absence of significant metabolic benefit may also reflect that the intervention primarily corrected vitamin D status rather than changing downstream cardiometabolic physiology over a short period.