Research Spotlight: Effects of Vitamin D3 supplementation on strength and body composition

Research Spotlight articles share concise breakdowns of interesting studies. The study reviewed is "Effects of Seasonal Vitamin D3 Supplementation on Strength, Power, and Body Composition in College Swimmers" by Rockwell et al.
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Vitamin D plays an important role in an athlete’s health, training and performance. However, blood vitamin D levels can get low, particularly during the winter months at high latitudes. A recent study evaluated the impact of fall season vitamin D3 supplementation on strength/power, body composition, and anabolic hormones in swimmers.

Male and female collegiate D-I swimmers with optimal vitamin D levels at the end of summer were randomly given 5000 IU of vitamin D3 (VITD; n=10) or a placebo (PLA; n=9) to take daily for 12 weeks. They resumed their typical diet and prescribed training, which included indoor swimming workouts, strength and conditioning sessions, and other non-swimming exercises such as running and plyometrics.

VITD levels increased by 8% and decreased by 44% in VITD and PLA, respectively. Strength and power was assessed via 7 exercises (1-RM bench press, squat, deadlift, standing broad jump, vertical jump, AMRAP dips, and pull-ups) and were administered in the same sequence over the course of 3 days. Squat, deadlift, and vertical jump improved more for the VITD than the PLA group (although the squat results weren’t quite statistically significant), whereas changes in bench press, standing broad jump, dips, and pull-ups were pretty similar for both groups. Body fat percentage decreased by 6.2 points in VITD and 4 points in PLA. Fat-free mass increased by 13.6% in VITD, but was pretty much unchanged in PLA. Free testosterone decreased and sex hormone-binding globulin increased significantly in PLA, while no substantial changes were seen in VITD.

The bottom line: These results suggest that seasonal vitamin D supplementation could have positive effects on strength & power performance, body composition, and anabolic hormones in trained athletes. However, it’s important to note that the dosing in this study is above the recommended daily upper limit. Ideally, supplementation with fat-soluble vitamins should be done under the direction and supervision of a medical professional.

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