Abstract
BACKGROUND & AIMS:
The impact of vitamin D supplementation in overweight and obese adults during resistance training on body composition, muscle function, and glucose tolerance was investigated.

METHODS:
Twenty-three overweight and obese (age: 26.1±4.7y; BMI: 31.3±3.2kg/m(2); 25-hydroxyvitamin D: 19.3±7.2ng/mL) adults were recruited for participation in a double-blind, placebo-controlled trial. Participants were randomly divided into vitamin D (VitD, 4000IU/d; 5 females, 5 males) and placebo (PL; 7 females, 6 males) groups. Both groups completed 12weeks of resistance training. 25-hydroxyvitamin D, parathyroid hormone, body composition, and glucose tolerance were assessed at baseline and 12weeks. Muscle function (strength and power) was assessed at baseline, 4, 8, and 12weeks.

RESULTS:
During the intervention, 25-hydroxyvitamin D increased and parathyroid hormone decreased in the VitD group (P<0.05). Peak power was significantly increased at 4weeks in the VitD group only (P<0.05). Regression analysis revealed an inverse association between the change in 25-hydroxyvitamin D with the change in waist-to-hip ratio (R(2)=0.205, P=0.02). No other improvements were observed with supplementation.

CONCLUSIONS:
Vitamin D supplementation in overweight and obese adults during resistance training induced an early improvement in peak power, and elevated vitamin D status was associated with reduced waist-to-hip ratio. Clinical trial registration number: NCT01199926.