Vitamin D supplementation increases calcium absorption without a threshold effect.

Randomized interventional study in healthy postmenopausal women. Active intervention arm sizes were 800 IU n=19, 2000 IU n=20, and 4000 IU n=18; 76 participants were randomized and 71 completed the trial.

Active intervention arms received vitamin D3 orally once daily for 8 weeks at 800 IU/day, 2000 IU/day, or 4000 IU/day. The study compared these doses with placebo.

Vitamin D supplementation increased calcium absorption in a dose-related manner, with no evidence of a threshold within the studied range. The linear dose effect on calcium absorption was significant (p=0.03), and serum 25(OH)D also rose with higher vitamin D exposure (linear p=0.05; quadratic p=0.35), supporting a roughly linear response. Reported changes in calcium absorption from baseline were 3.9% with 800 IU, 5.0% with 2000 IU, and 6.7% with 4000 IU, while the placebo group changed by -2.6%. Effects were small, and there were no serious adverse events, hypercalcemia, or hypercalciuria.

Small active-arm sample sizes, short 8-week duration, and a homogeneous population of healthy postmenopausal women limit generalizability. The outcome was a surrogate marker of calcium absorption, and the observed improvements were modest. The trial also provides limited information on ethnicity, physical activity, and other potential modifiers at the arm level.