Effects of Vitamin D on Skeletal Muscle and Athletic Performance
Geoffrey D. Abrams, MD; David Feldman, MD; Marc R. Safran, MD
J Am Acad Orthop Surg. 2018;26(8):278-285.
EFFECT ON SPORTS PERFORMANCE:
Vitamin D is not currently listed as a banned substance by the World Anti-Doping Agency. This fact has led to interest among investigators in examining the effect of vitamin D on sport-specific performance measures. In a recent study, Maroon et al investigated the correlation of vitamin D levels and the ability to obtain a professional contract in a cohort of 80 American professional football players. Although 77% of the athletes studied were characterized as vitamin D «deficient» or «insufficient,» a statistically significant correlation was found between lower vitamin D levels and release from the team (because of either poor performance or injury) before the start of the regular season.
In a randomized placebo-controlled trial, the effect of vitamin D (5,000 IU per day over an 8-week period) on sprint times and vertical jumps was assessed in a cohort of athletes, with nearly 70% having baseline serum 25(OH)D levels of <20 ng/mL. The group receiving vitamin D supplementation recorded substantially increased vertical jump heights from the beginning to the end of the study period, whereas no change was observed in the placebo-controlled group. A different randomized controlled trial of postmenarchal women examined the effect of vitamin D supplementation on lower limb muscle force, power, velocity, jumping height, and the Esslinger Fitness Index. The authors of the study found that jumping and movement efficiency was substantially increased in the vitamin D group, compared with the control group, a result potentially stemming from improvements in jumping velocity and height.
EFFECT ON ATHLETIC INJURY RISK:
Because of the important role of vitamin D in bone health, much of the current literature has focused on the association between vitamin D levels and the risk of stress fractures in the athletically active population. Davey et al performed a prospective age-matched cohort study of >1,000 Royal Marine recruits in the United Kingdom to evaluate the association between vitamin D levels and stress fracture risk. They identified 92 stress fractures and found that recruits with a baseline serum vitamin D concentration of <50 nmol/L (20 ng/mL) had a 60% higher incidence of stress fracture than recruits with vitamin D concentrations above this threshold had. As would be expected, serum vitamin D levels were found to peak in the summer months, but no association between occurrence of fracture and the time of year could be found. The results of this study were supported by a recent systematic review of eight investigations comprising >2,600 military recruits, which found an association between lower vitamin D levels and increased incidence of stress fracture.
Similar results have been found in a nonmilitary population. In a retrospective review of 124 patients with imaging-confirmed stress fractures at a single center over a 3-year period, Miller et al obtained vitamin D levels near the time of evaluation and reported deficient or insufficient vitamin D levels in 83% of patients. Another investigation found that vitamin D levels were considerably lower in American professional football players with at least one bone fracture, compared with players without a history of fracture.
In contrast, endurance athletes who participated exclusively in an outdoor environment in the southern United States were evaluated for serum 25(OH)D levels, history of stress fracture, bone mineral density, and PTH levels. The authors of that study found low levels of vitamin D deficiency and insufficiency in these athletes and no difference in any outcome measure between the athletes with insufficiency or deficiency and those with adequate levels of serum 25(OH)D.
EFFECT ON MUSCLE STRENGTH:
Although vitamin D has been known to affect muscle histochemistry, data have also become available to demonstrate that increased vitamin D levels and supplementation can have a positive effect in otherwise healthy people. One of the first large studies to investigate this association examined nearly 1,000 patients and correlated serum 25(OH)D levels with physical activity and muscle strength. Although the authors found that many study participants were vitamin D deficient, increased 25(OH)D levels were substantially correlated with a higher physical activity metric (short performance physical battery) and with greater hand grip strength. This result was found in participants above and below both the lower (50 nmol/L) and proposed higher (75 nmol/L) thresholds for serum 25(OH)D insufficiency.
These findings were corroborated in a level I meta-analysis of randomized controlled trials investigating the effect of vitamin D supplementation on muscle strength in a young and active cohort. The study examined 310 participants who received either vitamin D3 or placebo. The participants had a mean baseline serum 25(OH)D of 12.3 ng/mL and an average age of 24 years. Strength metrics for the upper extremity included handheld dynamometer grip strength, one repetition maximum bench press, and assessment with isokinetic dynamometers. Lower extremity strength testing consisted of single-repetition maximum leg press, free weight squats, gastrocnemius-soleus strength isokinetic dynamometer testing, and isometric quadriceps contraction. The authors found that vitamin D supplementation demonstrated a statistically significant positive effect on both upper and lower limb strength indices. Consistent with prior literature, Tomlinson et al also suggested that daily vitamin D3 administration would be more effective than weekly or monthly doses at improving muscle strength.
Vitamin D is a secosteroid hormone that has many important functions, including the regulation of calcium homeostasis and bone metabolism. In addition, VDR has been found in muscle cells, where activation of the receptor has multiple direct effects, including enhanced movement of myosin over the actin filaments through augmentation of calcium release from the sarcoplasmic reticulum. These molecular and cellular changes, as well as other actions in muscle, may be responsible for the findings of decreased fall risk in the elderly, improved muscle strength, lower injury rates, and enhanced athletic performance associated with sufficient 25(OH)D levels. These molecular and cellular changes may even benefit people with adequate vitamin D levels, but particularly improve function and decrease fracture risk in those who are vitamin D deficient.
* These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease.
