Effects of vitamin D supplementation in physically active adults

Participants completed two testing sessions (one pre-supplementation and another post-supplementation) over approximately 13 weeks. First, participants completed initial assessments, including an assessment of RMR, body composition, muscular strength and power, and hematological markers [total and free plasma 25(OH)VitD, Ca2+ and parathyroid hormone] between week 0 and 1. Data collection started after the summer season (March/April/May, Western Australia) to increase the chance of individuals being VitD sufficient at the baseline and to optimize the chance of participants reaching higher serum total concentration (= 120 nmol·L-1) at the end of the supplementation period. Following pre-testing, participants were matched for sunlight exposure and randomly allocated in a double-blind and counterbalanced manner to the VitD3 group (total n = 17, female n = 11; 50 IU·kg-1 body-mass [BM]·day-1 Elite Vitamin D3, Healthspan Ltd®, United Kingdom [UK], batch tested by Informed-Sport, LGC Limited, UK) or placebo (total n = 14, female n = 8; dextrose, Glucodin, iNova Pharmaceuticals, Australia) supplement group for 12 weeks. All doses were concealed in opaque gelatin capsules (oral supplementation) and organized in sequentially numbered envelopes to ensure that participants and the testing researcher were blinded to group allocations. This dosing strategy was selected because it has been associated with a positive effect on strength performance in previous research. In the middle of the study (week 7) hematological markers were again assessed to measure VitD concentration and check for any possible adverse effects of supplementation. The frequency of the intervention administration was once a day for 12 weeks. Then, following 12 weeks of supplementation, participants repeated the pre-supplementation testing at the same time of day and referring back to their three-day food diary to ensure that they were similarly prepared to perform. Adequate and optimal VitD status were defined as 25(OH)D between 50-100nmol·L-1 and > 100 nmol·L-1, respectively, based on previous research that suggests that these concentrations may be related to optimal skeletal muscle outcomes. Participants were in contact with the main researcher weekly by personal text messages and email, to ensure training and supplementation adherence and report any perceived side-effects from the supplementation (self-report). Vitamin D3 supplementation was distributed to participants fortnightly and the capsules left were counted to determine compliance. A three-day food diary will be kept to measure dietary intake through the whole study (pre, middle and post supplementation period). First, participants recorded the amount of food, fluid and supplements consumed during 3 consecutive days (2 weekdays and 1 day of the weekend), after having received detailed instructions about how to complete their dietary intake. This diet monitoring period is considered adequate for the estimation of habitual energy and macronutrient consumption. Then each food record was briefly reviewed by a nutritionist together with each participant to ensure that sufficient detail is captured. Food records were analysed using Foodworks® (V9, Xyris Australia). Each individual will be encouraged to follow similar eating patterns throughout the study to minimise deviations in macronutrients, and vitamin and mineral intake.