The Role of Vitamin D in Athletic Performance
By Kelly P. Massey, Ph.D. EP-C, CMES, AASDN-NS and Anne R. Loughren, M.S.
Introduction
Supplementation, hydration techniques, and training methods are practices used by athletes to enhance their athletic performance. Vitamin D supplementation may have benefits as well to enhancing athletic performance. Is vitamin D a vitamin or a hormone? Vitamin D is a fat-soluble vitamin produced in the skin.7 Calcitriol is the predominant source of circulating vitamin D in humans.8 When ultraviolet B (UVB) radiation hits the skin surface the process of converting vitamin D into its active form calcitriol begins.3,6,7 Calcitriol, a secosteroid hormone, is transported in the blood by binding to a vitamin D protein with the purpose to signal gene transcription.7 Vitamin D’s role is to maintain serum calcium and phosphorus levels by increasing absorption efficiency of calcium and phosphorus in the intestines and reabsorption of calcium in the kidneys.6 Vitamin D may be beneficial in injury prevention as adequate levels have been found to be important to musculoskeletal health.2,3,5-7,9
Vitamin D is measured by the 25(OH)D concentration.5-7,9 This concentration is formed from the hydroxylation of vitamin D2 and D3 in the liver.9 From the liver, 25(OH)D undergoes further hydroxylation in the kidneys forming the active form of vitamin D (1,25(OH)2D).9 Vitamin D classifications are controversial; however the general consensus lists sufficient levels >30 ng/ml and deficient levels between 10 to 20 ng/ml, with severe deficiency > 10 ng/ml.5-7 Vitamin D deficiency is considered an epidemic for all ages worldwide and athletes are not immune from this deficiency.5,6,8,10 Athletes who train or compete during cold or winter months, indoors, and early or late times of day are usually more susceptible to vitamin D deficiency.2,10
Previous research has suggested that vitamin D supplementation can possibly improve athletic performance; however, many athletes are inadequate in vitamin D.5,6,8,10 The meta-analysis by Farrokhyar et al.2 examined more than 2,000 athletes and found that 56% of the athletes had vitamin D levels classified as inadequate. Dubnov-Raz et al.1 found that 66% of their participants, who were competitive adolescent swimmers, had insufficient levels; 14% of which were deficient. A study by Willis et al.10 examining young Finnish runners, gymnasts, and nonathletes ages 9 to 15 years, found insufficient levels of vitamin D in nearly 68% of the participants. Examining how inadequate levels of vitamin D affect skeletal muscle can be important to understanding how to possibly prevent athletic injuries.
Vitamin D: Skeletal Muscle
Vitamin D functions through a vitamin D receptor (VDR) which is selective for 1,25 dihydroxyvitamin D (1,25(OH)2D).5 VDR was discovered in cultured rat myoblast cells in 1985, which showed muscle as a direct target organ for 1,25(OH)2D.5 The mechanisms reviewed by previous studies classify the effects of vitamin D as genomic (direct) and non-genomic (indirect) effects of 1,25(OH)2D on skeletal muscle.9 Hamilton points to a previous study showing VDR null mice having fiber size of the quadriceps and other muscles 20% smaller than that of the VDR-replete mice.5 This finding helped lead to 1,25(OH)2D and VDR having a direct role in the metabolic processes and the transcriptional regulation of skeletal muscle.5
Calcium acts as a regulator of skeletal muscle function.6 The maintenance of the physiologically acceptable range of intracellular and extracellular calcium concentrations is achieved by the function of 1,25(OH)2D and the parathyroid hormone (PTH).10 If vitamin D levels are inadequate, the intestinal absorption of calcium is reduced creating a decreased level of serum calcium concentrations. This decreased level of serum calcium concentrations stimulates the secretion of PTH, pulling calcium and phosphorus from bone in order to restore serum calcium levels to normal.10 This process, known as bone remodeling, may result in cortisol bone loss, an increase in fragility, and a greater risk of fractures.7 In a study conducted using military recruits, the risk of bone fracture was found to be significantly associated with reduced serum 25(OH)D and increased PTH among military recruits.7 Furthermore, a decrease in bone fractures was found with a supplementation of 800 IU/day vitamin D plus calcium.7
Vitamin D levels and injuries
Vitamin D deficiency can lead to an increased risk of injuries such as stress fractures.7 Stress fracture risk was significantly increased among young Finnish military recruits when their 25(OH)D levels fell below 30 ng.mL.7 Stress fracture incidence among female naval recruits was significantly reduced with the subjects that followed eight weeks of 800 IU of vitamin D plus 2000 mg of calcium supplementation compared to the placebo group.7,8 American professional football players with and without muscle injuries showed a significant difference in serum 25(OH)D levels.2 Whether inadequate vitamin D levels or supplementation with vitamin D affects injuries, is still a question. Health and athletic performance may be improved with vitamin D supplementation.8
The possibility of vitamin D affecting athletic performance has been around since the early part of the 20th century.3-6,7 Earlier studies believed athletes benefited from ultraviolet radiation exposure.7,8 To this day, many still believe that athletic performance can be enhanced with vitamin D even though there is limited research supporting vitamin D supplementation benefiting athletic performance.7,8 With that said, athletes with the lowest baseline vitamin D levels may benefit the most from vitamin D supplementation.7
A significant improvement in athletic performance is thought to occur in athletes that increase their vitamin D levels from 15 to 30 ng/mL, with little to no improvement occurring from 30 to 50 ng/mL.7 Todd et a1.9 mentions that athletes with high 25(OH)D concentrations may not benefit from vitamin D supplementation, while those with low 25(OH)D concentrations may have hindered their performance. Powers et al.8 found a significant positive correlation from two previous studies between serum 25(OH)D levels and muscle force, muscle power, hand grip strength, and jumping height and velocity of the subjects. Furthermore, a study conducted in the UK found a significant increase in 10 m sprint times and vertical jump (p < .05) in the subjects participating in vitamin D3 supplementation of 5,000 IU/day compared to the placebo group.3 Conclusion Vitamin D does play an important role in the human body. Research has shown 25(OH)D to have a significant impact on injuries, specifically due to bone loss or weakness. Many studies have found a benefit in athletic performance with vitamin D supplementation; however, these findings are not conclusive. It does appear for athletes benefiting from vitamin D that the condition of the athlete, baseline concentrations of vitamin D, and the time of day/year of training/competition is meaningful. All athletes are at risk of injury; however, vitamin D and its relationship with the musculoskeletal system appear to have merit. Athletes, coaches, and trainers need to be aware of their athlete's vitamin D levels. Even though levels of vitamin D per day an athlete should have is debatable, having athletes maintain sufficient levels of vitamin D (>30 ng/ml) should be of importance.
