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Vitamin D: Is It Important For Athletic Performance? A Review

By Natalia Prato, student of Clinical Nutrition at the Francisco Marroquin University.

Resumen

Debido a su importancia en varias vías metabólicas y funciones en el cuerpo, la vitamina D se ha convertido en una de las vitaminas más investigadas. Los hallazgos en la función musculoesquelética, la salud ósea y la inflamación han hecho que se estudie en los deportes. Debido a estudios que han demostrado que tener niveles bajos de esta vitamina puede ser perjudicial para la recuperación y el rendimiento deportivo, ha ganado popularidad en la arena. El propósito de esta revisión es recopilar evidencia científica y determinar si el nivel sérico y la suplementación con vitamina D tienen un efecto sobre el rendimiento deportivo. Existe evidencia científica de que la mayoría de los atletas tienen niveles séricos inadecuados de 25-hidroxi vitamina D. La suplementación de vitamina D en los atletas puede ayudar a restaurar los niveles y mejorar la fuerza, la potencia, los tiempos de referencia y la salud ósea. Esta revisión concluyó que se necesita hacer más evidencia científica en atletas, con muestras más grandes, para tener una recomendación exacta de los niveles séricos y la suplementación adecuada en esta población. Asimismo, la disminución de los marcadores inflamatorios aún no está claramente demostrada. No obstante, se sabe que siempre se recomiendan niveles suficientes de vitamina D, seas o no deportista.

Abstract

Due to its importance in various metabolic pathways and body functions, vitamin D has become one of the most researched vitamins. The findings in musculoskeletal function, bone health, and inflammation have caused it to be studied in sports. Due to studies that have proven that having low levels of this vitamin can be detrimental to recovery and performance in sports, it has gained popularity in the arena. The purpose of this review is to gather scientific evidence and find if serum level and supplementation of vitamin D have an effect on athletic performance. There is scientific evidence that most athletes have inadequate serum levels of 25-hydroxy vitamin D. Supplementation of vitamin D in athletes can help restore levels and improve strength, power, benchmark times, and bone health. This review concluded that more scientific evidence in athletes, with larger samples, needs to be done, to have an exact recommendation of serum levels and adequate supplementation in this population. Likewise, the decrease of inflammatory markers is still not clearly demonstrated. Nonetheless, it is known that sufficient vitamin D levels are always recommended whether you are or not an athlete. 

Keywords

Vitamin D, Athletic Performance, Supplementation, Vitamin D levels, Serum 25-hydroxy vitamin D, Deficiency

Introduction

It is no wonder that sports performance is always a top priority for athletes, which is why in recent years there has been a noticeable interest in vitamin D and its effects on sports health. Not only because of its importance in bone mineral metabolism, as it has been known for years, but also because of its effects on the skeletal muscle, immune system, and function in injury prevention; that is now more well-known and studied (1,2). Vitamins are essential substances for cellular function, growth, and development. Vitamin D is part of the fat-soluble vitamins, these have an important role in body mineral equilibrium and are part of the essential micronutrients for the body and metabolism function (1).

The main source of vitamin D is the sun, this is the endogenous way of producing it, but it can also be found in food such as oily fish, red meat, egg yolks, and fortified foods, and also as a supplement (1,2). Vit D is important for calcium absorption and regulation, body immunity, modulating inflammatory responses, and maintaining skeletal muscle function. The functions listed above contribute to its relevance to athletic performance and health (2,3). There are two types of vitamin D; cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2) (4-6).  Both forms are absorbed in the small intestine, this occurs by simple passive diffusion (6). Cholecalciferol has a better potential for storage and for increasing serum 25-hydroxy vitamin D levels, the test that is majorly used to evaluate vitamin D levels (Table 1) (4,5).  Serum concentrations of 25-hydroxy vitamin D are currently the best indicator for vitamin D body status because it reflects both vitamin D produced endogenously and the one obtained from food or supplements (6).

Vitamin D in athletes

Vitamin D is an essential nutrient in sports nutrition, its deficiency has been associated with poor performance in sports, and it has been considered that adequate levels and supplementation can be an enhancer of muscular and bone system functions, among others (7). A number of physiological functions are carried out by this vitamin, especially in regard to the metabolism of calcium, phosphorus, and hormones, for which it has been determined that, in order for the body to function well and especially for athletes to perform well, athletes should maintain their optimal levels (8). Vitamin D sufficiency correlates strongly with optimal muscle function in athletes. Because it plays an important role in reducing inflammation, pain, and myopathy while increasing muscle protein synthesis, ATP concentrations, strength, cardiopulmonary capacity, exercise capacity, and physical performance (9).

Lately, sports specialists have noticed and are more aware of vitamin deficiencies in high-performance athletes or elite athletes, specifically vitamin D (4,10). The exact level of vitamin D required for optimal performance is still unknown. However maximal neuromuscular performance is associated with 50 ng/mL of serum 25-hydroxy vitamin D test level, which is above the considered sufficient level (Table 1) (11,12). Deficient levels of this vitamin have become increasingly common among general populations. In athletes, deficiencies and insufficiencies may vary based on several factors such as the season, place of training, sport, or skin color. Athletes who train indoors in winter months or at higher latitudes generally have the lowest levels (4,13).

In different studies, it has been determined that many athletes do not have sufficient vitamin D levels. Thus, many athletes might need a supplement to reach optimal levels and avoid deficiency-related issues (Figure 1) (4,10). Some may need up to 5,000 IU of vitamin D/day for eight days to get to optimal serum 25-hydroxy vitamin D test level which is 40 ng/ml, and then between 1,000 and 2,000 UI/day for maintenance. But the exact dosage related to health and performance still needs to be studied (4). When athletes have a deficiency of less than 30 ng/mL of serum 25-hydroxy vitamin D supplementation protocols call for 50,000 IU of vitamin D3 per week for 8 weeks until the levels are stable, studies have shown that it may take up to 90 days of supplementation for the levels go get stable (10). There is a need for more studies on vitamin D in athletes since it is one of the most commonly used supplements (5).

In one study, researchers evaluated professional baseball players after wintertime, most of them had low levels of serum vitamin D. They found that serum 25-hydroxy vitamin D levels correlate with the daily intake of vitamin D (r=0.65; P=0.001). They concluded that professional players have a higher risk of hypovitaminosis after wintertime and that insufficient dietary intake and exposure to sunlight are the cause (14). Because of this, supplementation is sometimes necessary, especially for athletes who don’t have enough sunlight exposure, like those in indoor sports or those in countries where sunshine is scarce during certain seasons.

Description of Figure No.1: The different physiological consequences related to performance that athletes can develop when vitamin D levels are below expected. This and other consequences can happen in non-athletic persons too but the figure puts emphasis on performance-related conditions.

The musculoskeletal function of vitamin D

Vitamin D has different functions in the human body. Lately, it has been studied due to its function in skeletal muscle, several studies have shown the effectiveness of the supplementation in the improvement of muscle quality and morphology (8).  The function of vitamin D in muscle tissue is related to the quantity of vitamin D receptors (VDR) that are found there. These receptors are crucial for the absorption of calcitriol, which is needed for the synthesis of de novo protein, muscle contraction, and strength among other functions. It’s been demonstrated that its deficiency can lead to a decrease in the concentration of VDR and VDR gene expression, leading to myopathy and muscle fiber atrophy (15). Also, low levels negatively affect muscle function and contribute to proximal muscle weakness with a reduction in type II muscle fibers. These muscle fibers are particularly sensitive to the effects of vitamin D deficiency (8,16). Athletes are always looking for muscle health to prevent injuries and perform at their best, vitamin D control can be part of the assessment.

Scientific evidence related to the musculoskeletal function

Vitamin D plays an important role in musculoskeletal health, as we already mentioned. There has been evidence that supplementation can benefit recovery, regeneration, and hypertrophy of skeletal muscles (4).  Additionally, calcifediol supplementation has positive effects on increasing mitochondrial function and inhibiting muscle atrophy (7). The administration of vitamin D during muscle injury has been shown to reduce the production of stress-related proteins, inflammatory cytokines, and oxidative stress (17).

Most studies investigate the relationship between vitamin D and different musculoskeletal functions, evaluating the vitamin D levels in the population, and then providing vitamin D supplementation to see if it has a positive effect. Antoniak. A et al in their meta-analysis looked for the effect of vitamin D supplementation combined with resistance training, they observed its role in musculoskeletal mass in older adults. They found out that there was a significant effect on the combination of both, supplementation and resistance training (0.98, 95% CI 0.73 to 1.24, p<0.001). They found a greater increase in muscular strength and muscular power of the lower extremities during the evaluation of the 30 seconds sitting on one foot test, four square steps test, body balance test, and reverse walk test (18). The obtained information from this study can be transferred to athletes and could be useful in relation to musculoskeletal health and maintenance because positive effects were found.

In another study, they evaluated the serum 25-hydroxy vitamin D levels and their relationship to human skeletal muscle regeneration. 20 males with insufficient levels of vitamin D were tested before and after the supplementation of 4,000 IU/day of vitamin D, they evaluated the eccentric contraction of the knee extensors and the level of vitamin D. They found significant evidence that those with lower levels of vitamin D had a significant loss of peak torque postexercise (p<0.005) and also that after supplementation they had a significant improvement in torque recovery after 48 hours of supplementation  (p= 0.042). They found improvement in function and hypertrophy of the myotubes (4).

In a study where they evaluated marathon runners, they found out that the increase of serum 25-hydroxy vitamin D levels by supplementing vitamin D reduces muscular weakness after an exercise-induced injury. Thus reducing recovery times, however, they concluded that more studies need to be made to determine its effect in sports (11). Wyon et al. evaluated 24 elite ballet dancers and they found significant increases in isometric strength (18.7%, p<0.01), vertical jump (7.1%, p<0.01), and fewer injuries (p<0.01) in a supplemented group in comparison with the control group (19). In a randomized controlled trial, they found a significant increase in 10 m sprint times (p= 0.008) and vertical jump (p= 0.0028) after correcting vitamin D levels (serum 25-hydroxy vitamin D) in professional athletes (20).

Even though some of the studies mentioned before were not made specifically for athletes, the information is useful in analyzing the fact that insufficient levels and deficiency of vitamin D may result in consequences related to skeletal muscles, thus affecting athletes’ performance.

The function of vitamin D in the immune system and inflammation

Training and exercise with high intensity, like elite athletes, usually do, increase the production of inflammatory cytokines like IL-6 and TNF alpha (17,11). Vitamin D has an important role in the modulation of the immune and inflammatory systems. It regulates the production of inflammatory cytokines and inhibits the proliferation of pro-inflammatory cells. Several studies have associated low vitamin D levels with the increased risk and bad prognosis of acute infections (21). Vitamin D plays an important role in regulating adaptive immune responses and innate immune responses. Epidemiological studies have found an inverse association between serum 25-hydroxy vitamin D levels and inflammatory markers like CRP, TNF-alpha, and IL-6 (12,22). Vitamin D also increases the production of anti-inflammatory cytokines TGF, IL-4, IL-10, and IL-13 (23).

When athletes develop chronic inflammation and their immune systems are depressed due to overtraining or overload, adequate vitamin D levels that can be obtained by supplementation could be a complementary treatment (12).  There is still no exact recommendation for vitamin D supplementation as an inflammatory treatment, however supplemental vitamin D and calcium have been found to decrease inflammatory biomarkers (21). There is still little evidence directly linking this deficiency with an increased risk of inflammation severity or sports-related injury (24).

The effect of vitamin D supplementation on the inflammatory response

Vitamin D plays a role in the immune system, and studies are being conducted to determine the relationship between serum levels and inflammatory markers. It is also being evaluated whether supplementation reduces these markers. Mieszkowski. J et al analyzed the effect of a single high dose of vitamin D supplementation on inflammatory markers in 35 ultramarathon runners. Immediately after the ultramarathon, they found significant negative correlations between serum 25-hydroxy vitamin D levels and IL-6 (p<0.05), 24 hours later they also found a significant negative correlation between vitamin D levels and IL-15 (P<0.05). The levels went down in comparison with the controlled group, but it was not enough to reduce inflammation (25). So improving vitamin D levels before an event may be a better alternative than taking anti-inflammatory drugs.

In another study, researchers evaluated baseline vitamin D levels and the impact on post-exercise serum hepcidin, IL-6, and iron. After two weeks of supplementation, the intervention group had a higher level of serum 25-hydroxy vitamin D than the control group (27.82 ± 5.8 ng/mL vs. 20.41 ± 4.67 ng/mL; p < 0.05), but even after this there was no significant difference in the circulating levels of hepcidin and IL-6 after exercise. The only significant finding was the iron concentration, the decrease was smaller in the intervention group (p<0.05) (26). Shuler. et al found that increasing levels of vitamin D reduces inflammation, pain, and myopathy; also it reduces the incidence of respiratory-related illness (12).

In athletes, prolonged and exhausting training has a suppressive effect on innate immune function and increases the risk of upper respiratory tract infection. It has been suggested that vitamin D status influences susceptibility to influenza and cold virus, in a study where they evaluated men completing military training they found that those with sufficient levels of vitamin D had fewer respiratory tract infections (p<0.05) (27). Further research is needed into the use of vitamin D as a treatment of inflammation in athletes. It has a promising future.

Vitamin D and bone health

Vitamin D influences bone health by activating the expression of genes that improve calcium intestinal absorption, renal

reabsorption and bone resorption. When vitamin D serum concentration is less than 75 nmol/L calcium becomes 30% less bioavailable. Furthermore, research suggests that vitamin D levels are associated with bone mineral density and bone mineral content in the hip and lumbar spine (14,28). Several studies show that a sufficient level of vitamin D is important for bone health and the prevention of bone injuries in athletic populations. In a study, the risk of stress fracture was reported to be 3.6 times higher in army recruits who had blood concentrations of vitamin D less than 75nmol/L (29). Daily supplementation with 800IU of vitamin D for 8 weeks reduced the incidence of stress fractures by 20% in women recruited in the navy and Oral supplementation of 2,000IU of vitamin D/day reduced the incidence of injuries in professional dancers (30). Vitamin D plays a vital role in bone health. Adequate amounts of this vitamin and supplementation may help preserve bone health. This is why athletes can benefit from it.

Conclusion

The supplementation of vitamin D in athletes whose levels are inadequate has shown positive effects on muscular and skeletal functions, as well as preventing injuries. Vitamin D’s effect on inflammation, on the other hand, needs further study to determine its beneficial effects on athletes. The immune system benefits from optimal levels of vitamin D by preventing upper respiratory tract infections. In order to have a precise recommendation of serum levels and adequate supplementation in athletes, more scientific evidence needs to be collected with larger samples. In conclusion, it is important for every athlete to evaluate their serum 25-hydroxy vitamin D levels, in order to prevent complications and to determine if supplementation is necessary. A global evaluation of the athlete should be conducted before supplementing, and a high vitamin D food diet should always be an option.

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