Effects of Combined Calcium and Vitamin D Supplementation on Insulin Secretion, Insulin Sensitivity and β-Cell Function in Multi-Ethnic Vitamin D-Deficient Adults at Risk for Type 2 Diabetes: A Pilot Randomized, Placebo-Controlled Trial

Pilot randomized, placebo-controlled trial in multi-ethnic vitamin D-deficient adults at risk for type 2 diabetes in Melbourne, Australia. Eligibility included overweight or obesity, with either prediabetes or a high AUSDRISK score; 35 participants in the active calcium plus vitamin D arm completed baseline and final visits.

Participants in the active arm received oral vitamin D3 2,000 IU daily (two OstevitD capsules) plus elemental calcium 1,200 mg daily as calcium carbonate, taken with breakfast for 6 months. If needed at 2 and 4 months, the vitamin D3 dose was increased by 2,000 IU every 2 months to achieve serum 25(OH)D at least 75 nmol/L; placebo capsules were used in the control arm for blinding.

Overall, 6 months of calcium plus vitamin D did not improve insulin sensitivity, insulin secretion, or beta-cell function versus placebo in this vitamin D-deficient, diabetes-risk population. For HOMA2%S, the treatment group changed from 63.9 (29.0) at baseline to 61.1 (25.6) at 6 months, while placebo changed from 55.1 (30.4) to 52.5 (27.7); the treatment-by-time interaction was not significant (0.96, 0.10). Matsuda index, insulinogenic index, AUC for C-peptide, and disposition index were also not significantly different between groups. In post hoc analysis among participants with prediabetes, insulin sensitivity appeared to improve, but this subgroup finding was exploratory.

This was a small pilot study with only 35 completers in the active arm, limiting power to detect modest effects. The trial lasted 6 months and was single-site, which limits generalizability. The apparently favorable prediabetes finding was post hoc and should be considered hypothesis-generating rather than confirmatory.