Eight weeks of supplementation with Vitamin D3 improves serum concentrations but does not affect strength or body composition in young-adult resistance training females

Abstract

Various lines of evidence suggest vitamin D exerts a range of beneficial effects in skeletal muscle. Nonetheless, there are limited and mixed data regarding the efficacy of vitamin D supplementation on body composition and strength outcomes in females with low serum 25(OH)D levels. Thus, the purpose of this study was to investigate the effects of vitamin D3 supplementation on strength and body composition in vitamin D deficient females who recreationally resistance train (RT). This quasi-experimental, single-blinded, and placebo-controlled trial involved women that regularly engaged in RT (n=30, 27 ± 8 years old, 24.7 ± 3.0 kg/m2). Briefly, all participants completed pre-intervention testing (PRE), which included a venipuncture for blood vitamin D testing, a dual energy x-ray absorptiometry (DEXA) for body composition analysis, and peak torque evaluations of the right knee extensors and flexors using an isokinetic dynamometer. Participants were then allocated to either the treatment group (VD; n=20) or placebo group (PLC; n=10) based on PRE serum vitamin D status (serum 25(OH)D < 30 ng/mL allocated to VD and > 30 ng/mL allocated to PLC). VD participants received 60 soft-gel pills, each containing 5,000 IU (125 mcg) of vitamin D3 and were instructed to take one pill per day for a duration of 8 weeks. PLC participants received 60 soft-gel pills, each containing 1,500 mg/day of Oil of Oregano and consumed one pill per day during this duration. All participants were instructed to continue their RT regimens for the 8-week duration and post-intervention testing (which included all PRE assessments) occurred 72 hours following the last workout bout. A significant group-by-time interaction existed between VD and PLC for serum vitamin D (p<0.001), and values significantly increased in the VD group (+99.3%, p<0.001) but were not significantly altered in the PLC group (-13.6%, p=0.149). There were no significant group-by-time interactions (p<0.05) for DEXA-derived fat mass, DEXA-derived lean body mass, or knee extensor/flexion outcomes. In conclusion, while eight weeks of daily vitamin D3 supplementation effectively elevated serum vitamin D levels in recreationally-trained females that presented low pre-intervention levels, this supplementation strategy did not significantly alter body composition or select lower body strength outcomes.

Key words: Vitamin D, strength, muscle, females