Vitamin D – Simone do Carmo

The last decade has seen an enormous amount of research on vitamin D, particularly with regard to immune function and skeletal and cardiac health. Simone do Carmo expands on the role of vitamin D in athletic performance. 

Vitamin D plays a pivotal role in bone health by regulating calcium and phosphorus levels, but nearly every tissue in the human body has receptors for vitamin D (VDRs), which only highlights its importance for other bodily functions. Research shows that vitamin D is needed to support the immune system, regulate cell growth and development, and prevent cardiovascular disease. Additionally, it may be essential in athletic populations to maintain the optimal musculoskeletal and cardiac health needed for performance and to reduce injury risk. Let’s take a look at the evidence:

Prevalence of vitamin D inadequacy

The most recent meta-analysis to date (1) revealed a concerning 56 per cent prevalence of vitamin D inadequacy in 2,313 athletes. Inadequacy refers to both deficient (≤50nmol/l) and insufficient (~50-75nmol/l) status. In this study, the prevalence of inadequacy increased sharply during the winter and spring months, in indoor sports, and at higher latitudes. The Middle East was eliminated as an outlier, but it surprisingly had the highest prevalence of inadequacy, at 84 per cent. This could possibly be explained by less exposure to sunlight as a result of body concealment and by UVB insulation due to dust particles in the atmosphere. Although the meta-analysis revealed a 43 per cent prevalence of musculoskeletal injuries, the association with vitamin D status was not clearly established due to inadequate data. Only a well-designed longitudinal study will help confirm if any such association exists.

Vitamin D in kseletal health and function

Few studies have investigated the effect of vitamin D on athletes’ skeletal health. A two-year prospective cohort study (2) showed that young female cross-country runners who consumed more dairy products high in calcium and vitamin D had a 68 per cent reduced risk of developing stress fractures, and they had significant gains in bone mineral density and content, especially in the hip region. However, causality between vitamin D levels and the incidence of stress fractures in athletes can be determined only with randomised controlled trials (RCTs), which are scarce.

Lewis et al (3) randomised male and female collegiate swimmers and divers to 4,000IU/day of vitamin D or a placebo for six months. All 32 athletes included in the analysis were considered to have sufficient 25(OH)D₃ levels, and although no correlations were found between 25(OH)D3 levels and inflammatory markers or injury, 77 per cent of the recorded injuries coincided with decreased levels of 25(OH)D3. Positive correlations were found between 25(OH)D3 levels and increased total mineral-free lean mass in men, who generally gain muscle mass more easily than women.

The authors hypothesised that this could be due to increased protein synthesis, specifically the association between vitamin D and fast-twitch fibres. Contrary to the previous study, no correlations were found between total bone mass and content. The non-weight-bearing aspects of swimming and diving may have had an effect, as it is not uncommon for these athletes to lose bone mass and content throughout a training season. Since all athletes in this study maintained normal bone mass density, supplementation could be an easy and cost-effective way to preserve bone mass preservation and decrease injury risk.

Skeletal muscle tissue health and function

Vitamin D inadequacy may also adversely affect muscle tissue, such as muscle fibre atrophy, more specifically, fast-twitch fibres, and the discomposure of the intermyofibrillar network. Genomic and non-genomic mechanisms by which vitamin D may affect skeletal muscle tissue have been proposed. Genomic mechanisms are those by which vitamin D directly affects the receptor, promoting gradual epigenetic changes, while non-genomic mechanisms are indirect and rapid, triggering secondary messengers to improve calcium kinetics, such as the uptake of calcium by the sarcoplasmic reticulum (4).

A few trials with conflicting results in athletic populations have assessed vitamin D supplementation on muscle function and performance. In a placebo-controlled and double-blinded trial, Close, et al (5) randomised 10 professional soccer players equally to 5,000IU of vitamin D or a cellulose placebo for eight weeks. At baseline, 7 out of the 10 athletes revealed deficient serum 25(OH)D₃ and 2 out of 10 showed low enough levels to be associated with osteomalacia. After the eight-week intervention, a significant increase in serum 25(OH)D3 was observed in the supplemented group, while no significant increase was seen in the placebo group.

In addition, 60% of the supplemented group showed optimal levels consistent with a greater vertical jump and faster 10m sprint time. There were also mean increases in the supplemented and placebo groups for the 1-RM back squat (6.5kg vs. 2.5kg) and bench press (9kg vs. 3kg). No significant differences were found in agility or 30m sprint performance. Despite the small sample size and male-only assessment, this study provided preliminary evidence that 5,000IU/day of vitamin D₃ supplementation for 8 weeks may improve musculoskeletal performance in athletes.

In a similar study, Close, et al (6) randomised 30 club-level athletes to a placebo, 20,000IU (~2857IU/day), or 40,000IU vitamin D (~5714IU/day) for 12 weeks. At baseline, no significant difference was found in serum 25(OH)D₃ between groups (mean = 50nmol/l). Additionally, 17 out of 30 recreational athletes showed deficient serum 25(OH)D₃ levels and 6 out of 10 showed low enough levels to be severely deficient. Although the group supplemented with 40,000IU showed a greater increase (mean = 90nmol/l) than the group supplemented with 20,000IU (mean = 85nmol/l) at six weeks, this difference did not persist at 12 weeks. Conversely, the placebo group showed a decline over time. Negative correlations were found between baseline serum 25(OH)D₃ and the magnitude of increase in both the 20,000IU and 40,000IU groups, suggesting those with lower baseline levels are likely to benefit more from supplementation.

In contrast to Close et al (5), the changes in 25(OH)D3 levels did not translate into a significant improvement in any of the performance measures and the authors hypothesised that a higher serum level of ≥120nmol/l might be required for an improvement. As they pointed out, although values of ≥50nmol/l have improved physiological function, different tissues may respond differently to diverse serum concentrations; therefore, a higher concentration might be needed for skeletal muscle tissue.

Most recently, Wyon et al (7), in a double-blinded manner, randomised 22 male Caucasian national-level judoka athletes to either an acute bolus of vitamin D (150,000IU) or a placebo. Only the supplemented group revealed a 34 per cent increase in serum 25(OH)D3 levels and a 13 per cent increase in muscle strength after a week, with no differences found in the placebo group. The findings suggest that, within a week, acute supplementation may be beneficial in increasing serum 25(OH)D3 levels and muscle function in indoor athletes prone to inadequacy. Conversely, an RCT showed that 5,000IU/day of supplemental for eight weeks did not produce any benefits with regard to high-intensity interval training adaptations in well-trained soccer players (8). 

Another recent study (9) revealed that approximately 68 per cent of the 103 NCAA collegiate athletes tested had an adequate vitamin D status, while 23 per cent were insufficient, and 9 per cent, who were primarily non-Caucasian athletes, were considered to be deficient. The main findings were that those athletes with lower levels also had reduced muscular strength and power-related measures.

In the context of recovery, Barker et al (10) investigated the effect of supplementation on muscle function after exercise-induced muscle damage (EIMD). In a placebo-controlled and double-blinded trial, 25 healthy active males were randomised to either a placebo or 4,000IU of vitamin D for 35 days. At baseline, the mean serum 25(OH)D₃ levels were considered sufficient; however, individual assessments in the placebo group revealed that one participant was classed deficient, five insufficient, and seven sufficient. Similarly, in the supplemented group, two participants were classed deficient, five insufficient and eight sufficient. After 35 days, individual numbers remained the same in the placebo group, while serum 25(OH)D₃ increased significantly in the supplemented group.

This sounds promising for vitamin D because a speedy recovery is particularly important for athletes, yet it is questionable whether these findings can be extrapolated to an athletic population because the subjects, in this case, were simply healthy men doing 30 minutes of moderate physical activity at least three times per week.

Vitamin D in cardiac muscle tissue health and function

Cardiac muscle and vascular tissue also contain VDRs, indicating that vitamin D might influence an athlete’s maximal oxygen uptake (VO₂max), a measure of aerobic capacity. Additionally, hyperparathyroidism, which may result from low levels, has been shown to affect hypertrophy of the left ventricle. This may concern endurance athletes more prone to cardiac enlargement, specifically the thickening of the left ventricle wall, due to exercise stress. It is unclear whether low vitamin D levels put these athletes at greater risk of developing pathological hypertrophy of the left ventricle.

Nonetheless, an observational study of 506 national-level team sport athletes revealed that those considered severely deficient (<25nmol/l) had a significantly smaller aortic root, intraventricular septum diameter, left atria diameter, right atria area, and left ventricular diameter and mass than those considered insufficient and sufficient (11). Although these findings are correlational, they suggest that vitamin D may play a role in cardiac function.

While there are many associational studies investigating the effect of vitamin D on an athlete’s aerobic capacity, the findings are conflicting. Koundourakis et al (12) found strong correlations, not only between vitamin D intake and strength measures, jumping and sprinting performance, but also with aerobic capacity in 67 professional footballers. In contrast, a cross-sectional study (13) showed no correlation between vitamin D status and aerobic capacity in 52 competitive ice hockey players.

Despite the conflicting findings of correlative studies, there is one RCT in elite rowers with positive preliminary results. Jastrzebski et al (14) randomised 15 elite rowers to a placebo or 6,000IU/day of vitamin D supplementation, single-blinded, during an eight-week high-intensity training phase. All athletes presented sufficient serum 25(OH)D3 levels at baseline.

After eight weeks, serum 25(OH)D3 increased significantly only in the supplemented group, and all athletes in both groups improved their aerobic capacity, suggesting a training effect. However, the supplemented group showed a 12.1 per cent improvement in aerobic capacity compared to 10.3 per cent in the placebo group, leading the authors to suggest that vitamin D may have had an effect. Although the specific physiological mechanism remained unclear, the authors suggested that it may be due to the increased binding affinity of oxygen to haemoglobin. Whether that small difference in aerobic capacity translates into a meaningful ergogenic effect, however, remains an open question.

Conclusions and recommendations

The 56 per cent prevalence of inadequacy in athletic populations is worrying. Despite conflicting results from studies, there are some promising findings that supplemental vitamin D may benefit athletic performance and recovery, especially in athletes who present on the lower end. However, there is a need to determine optimal doses for athletes in order to design proper guidelines, and supplementation should, therefore, be addressed carefully. If deficiency is suspected, testing for vitamin D status and continual monitoring would be the first recommended step. Genetic testing is also available and could provide further insight into an individual’s response to supplementation. This can better distinguish those athletes who need supplements from those who might suffice with added sunlight exposure and foods high in vitamin D, such as oily fish, dairy, eggs and mushrooms.