Vitamin D and Exercise
Athletic performance is determined by multiple factors, including both genetic and environmental influences. The former have gotten significant attention recently with many questions still unanswered. Similarly, environmental factors, while well recognized as having the potential to impact athletic ability, have only a limited scientific basis. It is well recognized that vitamin D plays an important role in bone health and calcium regulation. However, recent evidence points to a possible role for vitamin D in regulation of the immune system and skeletal muscle function. Therefore, vitamin D has the potential to impact overall health, training, and performance in athletes. In fact, vitamin D has been proposed as a potential limiting factor on athletic performance when in deficiency, and as a possible performance enhancer when present in abundance (Cannell). This article will briefly review vitamin D; its sources and synthesis, role in overall health, status in athletes and athletic performance, and recommendations for assessment and supplementation.
Sources and synthesis
Vitamin D is a hormone found in the body in 2 forms. Ergocalciferol, or vitamin D2, is present in plants and some fish. Cholecalciferol, or vitamin D3, is synthesized in the skin by sunlight. Humans can meet their vitamin D requirements by either consuming vitamin D or being exposed to the sun for enough time to produce adequate amounts. Vitamin D controls calcium absorption in the small intestine and works with parathyroid hormone to mediate skeletal mineralization and maintain calcium homeostasis in the blood stream. In addition, recent epidemiologic studies have observed relationships between low vitamin D levels and multiple disease states, probably caused by its anti-inflammatory and immune-modulating properties and possible affects on cytokine levels.
Vitamin D3 is synthesized from 7-dehydrocholesterol in the skin. The vitamin D binding protein transports the vitamin D3 to the liver where it undergoes hydroxylation to 25(OH)D (the inactive form of vitamin D) and then to the kidneys where it is hydroxylated by the enzyme 1-α-hydroxylase to 1,25(OH)D, its active form. This enzyme is also present in a variety of extrarenal sites, including osteoclasts, skin, colon, brain, and macrophages, which may be the cause of its broad-ranging effects. The half-life of vitamin D in the liver is approximately 3 weeks, which underscores the need for frequent replenishment of the body's supply.
Vitamin D3 can be manufactured in the skin by way of ultraviolet (UV) B rays. UVB rays are present only during midday at higher latitudes and do not penetrate clouds. The time needed to produce adequate vitamin D from the skin depends on the strength of the UVB rays (ie, place of residence), the length of time spent in the sun, and the amount of pigment in the skin. Tanning beds provide variable levels of UVA and UVB rays and are therefore not a reliable source of vitamin D.
Natural dietary sources of vitamin D include salmon, sardines, mackerel, tuna, cod liver oil, shiitake mushrooms, and egg yolk. Fortified foods include milk, orange juice, infant formulas, yogurts, butter, margarine, cheeses, and breakfast cereals. Given concern about skin cancer, many patients and doctors are cautious regarding sun exposure recommendations. However, exposure of arms and legs for 5 to 30 minutes between the hours of 10 AM and 3 PM twice a week can be adequate to prevent vitamin D deficiency.
Vitamin D and overall health
While it is well recognized that vitamin D is necessary for optimal bone health, emerging evidence is finding that adequate vitamin D intake reduces risk for conditions such as stress fracture, total body inflammation, infectious illness, and impaired muscle function. Vitamin D plays an important role in bone metabolism and seems to have some anti-inflammatory and immune-modulating properties. In addition, recent epidemiologic studies have observed relationships between low vitamin D levels and multiple disease states (Hollick). Low vitamin D levels are associated with increased overall and cardiovascular mortality, cancer incidence and mortality, and autoimmune diseases such as multiple sclerosis (Hollick). Although it is well known that the combination of vitamin D and calcium is necessary to maintain bone density as people age, vitamin D may also be an independent risk factor for falls among the elderly.
Vitamin D assessment
Serum 25(OH)D concentration is the best indicator of vitamin D status in both healthy and ill individuals (Hollis). Along with this blood test, body fat (inversely correlated with vitamin D levels), dietary intake and lifestyle should also be assessed before making recommendations for supplementation.
Vitamin D status in athletes and performance
Studies in athletes have found that vitamin D status is variable and is dependent on outdoor training time (during peak sunlight), skin color, and geographic location. It was shown quite some time ago that seasonal performance within the northern hemisphere varies with peak performance occurring during the late summer months and then declining to a low point during the winter (Hettinger). Although research has found that athletes generally do not meet the U.S. dietary reference intake for vitamin D, inadequate endogenous synthesis is the most probable reason for insufficient/deficient status.
Muscle pain and weakness are well-documented but frequently forgotten symptoms of vitamin D deficiency that improve with vitamin D supplementation (Hollick). Recent evidence from both animal and in vitro studies suggest that vitamin D is important for calcium handling across the muscle?s sarcolemma as well as expression of proteins necessary for muscle contraction (Barker et al). Although no conclusive studies in athletes have been published on vitamin D status and muscle performance, there are some intriguing data from the Russian and German literature suggesting that UVB exposure leads to a positive impact on athletic performance (Larson-Meyer et al). Two recent studies involving non-athletic adolescent females found a positive association between serum 25(OH)D concentration and both aerobic fitness (Foo) and jump height, velocity, and power (Ward). Most cross-sectional studies show that 25(OH)D levels are directly associated with musculoskeletal performance in older individuals. The vast majority of randomized controlled trials, again mostly in older individuals, show that vitamin D improves physical performance. While arguments for and against vitamin D supplementation exist, additional research is required to determine whether vitamin D supplementation is beneficial to athletes and in particular, under which conditions and for which athletes is it most beneficial. In a recent randomized, controlled trial supplementation with 20 000 and 40 000 IU vitamin D(3) over a 6-week period elevated serum 25[OH]D concentrations above 50 nmol/l, but neither dose given for 12 weeks improved our chosen measures of physical performance (1-repetition maximum bench press and leg press and vertical jump height) (Close).
Treatment recommendations vary depending on the cause of the deficiency. For example, patients with chronic kidney disease are recommended to have 1000 IU of vitamin D3 daily. The expected blood level response to a given vitamin D dose varies, probably because of differences in the cause of the deficit as well as the starting point for correction. To prevent vitamin D deficiency in healthy patients, the 2010 Institute of Medicine recommendations suggested a daily vitamin D intake of 600 IU for males and females from ages 1-70 y; and 800 IU for adults older than 70 y. The upper limit recommended was 4000 IU daily. However, some experts consider this to be too low and recommend that children and adults without adequate sun exposure consume 800 to 1000 IU daily to achieve adequate serum vitamin D levels. Over-the-counter multivitamin supplements frequently contain 400 IU of vitamins D1, D2, or D3. Alternatively, over-the-counter vitamin D3 supplements can be found in 400, 800, 1000, and 2000 IU strengths. Prescription-strength supplementation choices include vitamin D2 (ergocalciferol), which provides 50,000 IU per capsule, and vitamin D2 liquid (drisdol) at 8000 IU/mL.
The idea that dietary supplements can improve athletic performance is popular among athletes. The use of antioxidant supplements is widespread among endurance athletes because of evidence that free radicals contribute to muscle fatigue during prolonged exercise. Furthermore, interest in vitamin D supplementation is increasing in response to studies indicating that vitamin D deficiency exists in athletic populations. The issue of whether athletes should supplement with vitamin D is somewhat controversial. Nevertheless, based upon the growing evidence that many athletic populations are vitamin D deficient or insufficient, it is recommended that athletes monitor their serum vitamin D concentration and consult with their health care professional and/or nutritionist to determine if they would derive health benefits from vitamin D supplementation. Given the recent findings, it may beneficial to help athletes achieve a serum 25(OH)D concentration of >or=32 and preferably >or=40 ng/mL. Further research is needed to determine the effect of vitamin D status on injury, training, and performance in athletes.
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Foo LH, Zhang Q, Zhu K, et al. Relationship between vitamin D status, body composition and physical exercise of adolescent girls in Bejing. Osteoporos Int 20: 417-425, 2009.
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