Good Literature Review On Role Of Vitamin D In Sports Performance: A Literature Review

1. Introduction
Vitamin D is an essential nutrient that is not synthesized by the body and has to be obtained through diet and by the interaction of the UVB rays with the skin (Shuler et al. 2012, p. 496). A study on the levels of vitamin D in an average British adult showed that more than 50% of the adults had vitamin D deficiency and 16% of them exhibit severe insufficiency during winters (Hyppönen and Power 2007, p. 862). This low level of vitamin D synthesis in winter is attributed to the blockage of UVB rays reaching the Earth’s surface during winters due to sun’s angle (Larson-Meyer and Willis 2010, p. 220). This seasonal difference in vitamin D levels is consistent among athletes as well (Wolman et al. 2013, p. 388; Morton et al 2012, p. 798). Variation in vitamin D status among athletes seems to be common with the sportspersons of Finland and Middle East showing the lowest vitamin D status while high altitude US athletes showing the highest vitamin D status (Hamilton et al 2010 p. 1528).
People with pigmented skin such as people of African descent seem to be at a higher risk of hypovitaminosis D because they have the capability to block 99% of the UV rays reaching their skin, thereby inhibiting vitamin D synthesis (Pollock et al 2012, p. 55). Similarly, a sunscreen or sunblock of SPF 15 can block 99% of the UVB reaching the skin (Shuler et al. 2012, p. 496). The past decade has researched extensively on the action of vitamin D on the musculoskeletal health as well as non-musculoskeletal conditions such as cancer, inflammation and immune function (Holick 2008, p. 365). Due to the increasing evidence regarding the role of vitamin D in promoting recovery from tissue injury (Wyon et al 2013, p. 8; Barker et al 2013, p. 1253), muscle fibre differentiation, contractile protein expression (Girgis et al 2014, p. 169) and insufficiency of the same pointing towards increased incidents of upper respiratory tract infections and delayed fracture healing in athletes (URTI) (He et al 2013, p. 86; Inklebarger et al 2014, p. 61), treatment regime for vitamin D insufficiency are becoming prevalent among elite athletes. Currently, the use of exogenous supplemental vitamin D3 and D2 has had inconsistent results with regard to improvement in physical endurance (Close et al 2013, p. 1; Owens et al. 2014, p. 1309; Nieman et al. 2014, p. 63) but positively correlates levels of serum vitamin D to muscular and skeletal health (Koundourakis et al. 2014, p. 1).
2. Physiology of vitamin D
Vitamin D is a secosteroid hormone. Upon exposure of the skin to UVB rays (wavelength = 290-315 nm), the 7-dehydrocholesterol that is present in the cell membrane of the keratinocytes of the dermis and epidermis undergoes biochemical conversion to form precholecalciferol (previtamin D3). Previtamin D3 is converted to vitamin D3 or cholecalciferol via a thermal isomerisation process that occurs over 48 to 72 hours (Tavera-Mendoza and White 2007, p. 65; Larson-Meyer and Willis 2010, p. 220). Vitamin D binding proteins (VDBP) bind to the circulating vitamin D3 and translocate it to the liver where it is converted to 25(OH)D. Vitamin D functions through both endocrine and autocrine mechanisms. Endocrine mechanism occurs when the serum calcium and phosphate levels are lower than the required levels for proper physiological functioning of the bones and muscles (Holick 2007, p. 266). During such times, the parathyroid hormone (PTH) converts the 25(OH)D to 1,25(OH)2D3 (calcitriol) in the kidneys (Holick 2007, p. 266). When the body is insufficient in vitamin D, the PTH induces calcium and phosphorous release from the bones to meet the body’s demands, making the bones porous and susceptible to fracture (Holick 2007, p. 266). During the autocrine mechanism, the body utilizes vitamin D in gene expression, protein synthesis and immune response (Ogan and Pritchett 2013, p. 1857). More than 1000 genes have been identified in the past decade that respond to calcitriol and are associated with bone formation (Tavera-Mendoza and White 2007, p. 65).
25(OH)D has a longer half-life or 2-3 weeks when compared to 1,25(OH)2D3 (less than 4 hours). 1,25(OH)2D3 is released as required and the levels in the blood are not indicative of the vitamin D stores in the body. Therefore, scientists often use circulating levels of 25(OH)D to gauge the vitamin D status of a person. Accordingly, a person is vitamin D insufficient if his/her serum 25(OH)D concentration is 20–30 ng/ml; vitamin D deficient if the concentration is below 20 ng/ml; and severely deficient if the concentrations are below 10 ng/ml (Hamilton et al. 2010, p. 1528). Since vitamin D synthesis and storage are dependent on age, geographic region and season, athletes living at higher altitudes, training during winter and training for indoor games need to pay attention to their vitamin D levels for maintaining muscle and bone health (Hamilton et al. 2010, p. 1528).
3. Role of vitamin D in musculoskeletal functioning
The role of muscles, ligaments, tendons and bones in athletics have been widely studied and recorded (Angeline et al. 2013, p. 462). A study (Plotnikoff and Quigley 2003, p. 1463) conducted on patients with non-specific musculoskeletal pain revealed that 93% had vitamin D deficiency. Two mechanisms have been proposed that could explain the involvement of vitamin D in muscle health. First mechanism suggests that 1,25(OH)2D3 binds directly to the vitamin D receptor (VDR) that is present throughout the body but most abundantly on three organs, namely, the kidneys, bones and the small intestine, to mediate muscle strength (Minasyan et al 2009, p. 161). This cell process is associated with increased protein synthesis that promotes growth of muscles, which is a critical aspect of sports endurance. A study using VDR knockout mouse revealed that VDR is essential for balance, muscle growth and muscle fibre maturation (Minasyan et al 2009, p. 161). The second mechanism suggests that the presence of vitamin D increases the number of calcium binding sites on the muscle cells to improve muscle contractile function (Ogan and Pritchett 2013, p. 1858).
As mentioned earlier, vitamin D’s role in bone health is mediated by PTH. At a physiological level, bone contains two types of cells, namely, osteoblasts that form bones and osteoclasts that reabsorb bone. PTH triggers mobilization of calcium from the bone by enhancing the formation of osteoclasts when the serum vitamin D levels are less than 30 ng/ml (Angeline et al 2013, 462). On the other hand, the presence of 1,25(OH)D3 stimulates formation of osteoblasts and activates calcium absorption in the intestines, which raises the serum calcium levels for uptake by the osteoblasts (Angeline et al 2013, 462).
4. Role of vitamin D in sports and athletic performance
Athletes perform activities that require aerobic capacity, muscle strength as well as agility, all of which are made possible by the musculoskeletal system. An athlete’s body, during both training and athletic event, requires that the musculoskeletal system be at its peak for optimal performance as well as be healthy enough to endure the stress. Studies show that muscle soreness and stamina could be affected by the levels of vitamin D in the body (Koundouraki et al. 2014, p. 2). One study (Shuler et al. 2012, p. 497) shows that the optimal neuromuscular performance occurs when the concentration of 25(OH)D3 is 50 ng/ml or more. In terms of athletics, this translates to a sun exposure that could only be attained by a person who spends an entire summer basking in the sun, which allows him/her to achieve a 25(OH)D3 level of 64.4 ng/ml, approximately (Shuler et al. 2012, p. 497).
Researchers have found that athletes perform better in the summer when compared to winters, emphasizing the importance of sun exposure and vitamin D (Wolman et al. 2013, p. 388). Similar results were reported many decades back when UV irradiated subjects performed better when compared to non-irradiated control subjects (Cannell et al. 2009, p. 1104). When the non-irradiated group was given oral vitamin D supplements, their performance characteristics were similar to the UV irradiated group, proving that vitamin D improves performance (Angeline et al 2013, 463). Based on current literature, rugby, soccer and jockeys are 62% vitamin D deficient (Close et al. 2013, p. 5), professional UK soccer players are 65% deficient (Morton et al. 2012, p. 798), UK elite dark skinned women athletes are 65% insufficient and deficient (Pollock et al. 2012, p. 55), Middle Eastern athletes are 32% insufficient and 58% deficient (Hamilton et al. 2010, p. 1528), USA field and track athletes are 42% insufficient and 11% deficient (Willis et al. 2012, p. 35) and professional ballet dancers are 100% deficient (Wolman et al. 2013, p. 390).
5. Consequences of vitamin D deficiency in athletes
One of the consequences of vitamin D deficiency is the damage to the type II fibres (Ceglia 2009, p. 630). The type II muscle fibres are one of the first muscles to be recruited when there is a need for power or burst activity and fall prevention, both of which are necessary for a sportsperson. Clinically inadequate levels of 25(OH)D3 in the body result in fatty infiltration and degeneration of the type II muscles (Ceglia 2009, p. 630). Currently, it is not known whether vitamin D supplementation could reverse this degeneration.
Another consequence is the occurrences of URTI during winters (He et al. 2013, p. 86; He et al. 2014, p. 8). Vitamin D is thought to regulate the secretion of salivary immunoglobulin A (SIgA) and mediate the production of monocytes and lymphocytes (Holick 2007, p. 271) that are necessary for fighting infection. A significantly higher occurrence of URTI was noticed in a group of vitamin D deficient athletes during a 6-week winter training period but not in those individuals who were vitamin D insufficient (He et al. 2013, p. 86). Therefore, based on the percentage of athletes who are deficient and insufficient in vitamin D, the possible clinical consequence of such a prolonged deficiency and the benefits of vitamin D on strength and endurance, it is necessary to educate the athletes regarding the positive influence of exposure to the sun on the musculoskeletal health, endurance capacity and performance peak.
6. Evaluation of vitamin D status and treatment of vitamin D deficiency in athletes
Literature suggests that athletes who follow restricted, vegetarian or vegan diet (Calvo et al. 2005, p. 314) and train indoors are prone to fractures and need to be assessed a minimum of twice a year during late summer and late winter for 25(OH)D3 concentration in their blood (Holick 2007, p. 267). Currently, athletes have three options to revert vitamin D insufficiency and deficiency (Cannell et al. 2008, p. 5). The first method is basking in the noon sun for about 15 minutes every day at least twice a week, which would make a light skinned individual vitamin D sufficient. The second method is artificial UV irradiation using tanning beds (Cannell et al. 2009, p. 1104). However, such an artificial exposure makes the individual susceptible to non-melanoma skin cancer and ages the skin. The third option is using vitamin D3 supplements (Heaney 2008, p. 1537). A person who is severely deficient in vitamin D, would require 1000 IU/day of colecalciferol (vitamin D3) for many months to achieve sufficiency (Cannell et al. 2008, p. 5). However, if the supplements are given as an adjunct to natural sun exposure therapy, the possibility of attaining sufficiency would be much quicker. In England and the US, the most commonly used supplement is ergocalciferol (vitamin D2), even though its bioavailability is low and is not as effective as D3 supplements (Houghton and Vieth 2006, p. 695).
A study by Close et al. (2013, p. 692) revealed that vitamin D3 supplements did help in elevating the serum 25(OH)D3 concentrations to 50 ng/ml and higher within 6 weeks but did not help the athletes in improving their physical performance. Wyon et al. (2014, p. 8) discovered that vitamin D3 supplements improved the strength of the muscles (which has been corroborated by Koundouraki et al. 2014, p. 1) but had no effect on the power of the muscles. The vitamin D3 supplement also significantly reduced the frequency of injury among the subjects when compared to the control group (as also found by Barker et al 2013, p. 1253). A study on NASCAR pit crew athletes using vitamin D2 supplement for 6 weeks increased 25(OH)D2 concentrations but also significantly reduced the concentration of the essential 25(OH)D3 leading to exercise-induced muscle damage (EIMD) (Nieman et al. 2013, p. 72). Vitamin D3 supplements were found to accelerate stress fracture healing in soldiers who presented hypovitaminosis-D (Inklebarger et al. 2014, p. 62). Owens et al. (2014, p. 1317) discovered that vitamin D3 supplements neither improve nor reduce skeletal muscle functioning.
7. Conclusion
Vitamin D is an essential nutrient required for maintaining muscle and skeletal health. Literature reveals that, many athletes are vitamin D insufficient and deficient, which could put pressure on their muscle and skeletal systems. Since athletes depend extensively on their musculoskeletal system, the role of vitamin D in their lives has become a topic of interest in the past few decades. Athletes train many hours a day and often experience muscle soreness and fatigue as part of their training sessions, which, as existing data suggest, could be reverted by introducing vitamin D in the body. Even though there are contradicting evidence regarding the efficacy of vitamin D as a performance enhancer, there are positive evidences that point towards the healing capacity of vitamin D during stress fractures and muscle damage. Therefore, it can be concluded that vitamin D is essential for maintaining current musculoskeletal health, for promoting healing and preventing falls, fractures and muscle damage owing to extensive physical stress.

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