“The purpose of this study was to evaluate the impact of fall season vitamin D3 supplementation on strength/power, body composition, and anabolic hormones in swimmers with optimal vitamin D status at summer’s end.”

“Male and female NCAA Division I swimmers (N = 19) with optimal 25-hydroxyvitamin D [25(OH)D] randomly received 5,000 IU of vitamin D3 or placebo daily for 12 weeks while participating in swimming and strength and conditioning training (August–November).”
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Before and after the intervention, the participants underwent blood sampling for analysis of serum 25(OH)D, parathyroid hormone, total testosterone, free testosterone, sex hormone-binding globulin, and insulin-like growth factor 1, dual-energy X-ray absorptiometry, and strength/power testing (bench press, squat, dead lift, standing broad jump, vertical jump, and dips and pull-ups).”
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“The 25(OH)D was decreased by 44% in placebo and increased by 8% in 5,000 IU of vitamin D3 over the 12 weeks.”
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“The findings suggest that vitamin D supplementation is an efficacious strategy to maintain 25(OH)D during the fall season training and to enhance some aspects of strength/power and fat-free mass in swimmers.”
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M.S.Rockwell, et al
Effects of Seasonal Vitamin D3 Supplementation on Strength, Power, and Body Composition in College Swimmers
International Journal Of Sport Nutrition and Exercise Metabolism — Volume 30 #2 — 2020 — page 165