Vitamin D supplementation and calcium absorption during caloric restriction: a randomized double-blind trial.

Randomized double-blind 6-week trial in postmenopausal women with overweight or obesity and low-to-moderate vitamin D status. Active intervention arms included weight loss plus vitamin D3 and weight maintenance plus vitamin D3; all participants were women aged 50 to 70 years with BMI 25 to 40 and no diseases affecting bone metabolism.

Oral vitamin D3 was given in the active arms as 3 tablets weekly for 5 weeks, equivalent to about 2500 IU/day, taken with the largest meal of the day. Matching placebo tablets were used in the control condition; calcium intake was standardized to approximately 1.2 g/day with background diet and a multivitamin/mineral supplement.

Vitamin D supplementation increased true fractional calcium absorption (TFCA), whereas caloric restriction and weight loss decreased TFCA. TFCA changed in the direction of higher absorption with vitamin D treatment (P<0.001) and lower absorption during weight loss (P<0.001). The authors concluded that when calcium intake was about 1.2 g/day, either a higher or lower vitamin D dose appeared sufficient to maintain calcium balance during caloric restriction. Serum 25(OH)D and 1,25(OH)2D3 did not predict TFCA, and no serious adverse events were reported.

The active arms were small (WL-D n=19; WM-D n=20), and the intervention lasted only 6 weeks, limiting precision and long-term inference. Generalizability is restricted to postmenopausal women with overweight or obesity and similar baseline vitamin D status. The biologic mechanism for the TFCA response remained unclear, and key covariates such as ethnicity and physical activity were not reported separately by active arm.