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Wednesday, 30 October 2013

Vitamin D, Health and Athletic Performance

I have a guest article on Vitamin D written by Joel Dawson, Sport Scientist at Stcke City Football Club. You can follow him on twitter - @joeldawson9
There is a rising awareness about the value of vitamin D for both health and athletic performance. It is an essential nutrient, which contributes to bone health, muscle functionality and calcium regulation in the blood, to name a few. However, contrary to other vitamins, it is difficult to source and acquire the recommended daily amount with the principal source of vitamin D from ultraviolet B radiation in sunlight. The limited availability of naturally occurring vitamin D in dietary foods may provide a viable argument for the fortification of more foods. Evidence detailing the physiological importance of vitamin D will be reviewed as well as an assessment about the readily source availability of the vitamin and needs of different population groups. Only then can it be argued to whether or not fortifying different food groups is a logical and obligatory option moving forward.
Vitamin D has been found to play an active role in many physiological processes in the body (Ogan and Pritchett 2013). Firstly, Vitamin D is most well known for its active role in osteoclast activity and absorption of calcium in the intestine (Larsen-Meyer 2010). An inadequate amount of vitamin D can lead to bone loss and injury because it has a crucial function in bone growth, density and remodelling (DeLuca 2004). For example, if infants and children are chronically deficient in vitamin D then the risk of rickets becomes a progressive and prominent problem (Hollick 2004). Low status of this vitamin can also impair calcium and phosphorus homeostasis, resulting in a restricted calcium absorption rate in the small intestine (Heaney 2003). Metabolic processes, neuromuscular activity and bone health are compromised when calcium absorption is insufficient, further stressing the importance of maintaining adequate vitamin D status. The identification of vitamin D receptor (VDR) sites in an array of tissues within the body (Holick 2005) is important because VDR regulates hundreds of gene expression that perform vital bodily functions. Epidemiological evidence suggests that the VDR is involved in mediating the noncalcemic effects of vitamin D and could play an imperative role in maintanence of extracellular health and disease prevention (Grant 2005).
                The desirable intake levels of vitamin D are yet to be fully recognised, with recommendations differing amongst medical institutions. New guidelines were released by the Institute of Medicine (IOM) in 2010 recommend children and adults (0-70 years) to intake 400-600 IU/day with older adults (>70 years) advised to have 800 IU/day from the diet. Although, clinically it may be an amount that prevents vitamin D deficiency, it is a recommendation that is insufficient for fully optimised vitamin D levels (Ginde 2009, Heaney 2008). The Endocrine Society, for example, proposes much higher dietary intake recommendations in addition to rational sun exposure. The comparison between the two dietary allowance advocated by the two institutes is exemplified in table 1 below.
Table 1: Recommended vitamin D intake levels of the IOM vs Endocrine society. Taken from Ogan & Pritchett (2013)
In concurrence with the lack of clarity in recommended dietary intake of vitamin D, is the reference values used to characterize serum vitamin D status. Although most researchers display similar reference values, definitive thresholds of deficiency, sufficiency and optimal status are still subject to debate. A table of classification is shown below.
Table 2: Reference values for serum 25(OH)D taken from Larson-Mayer 2013
Reaching levels of >32ng/mL will enhance calcium absorption and it is at this point in which functionality of vitamin D is efficiently demonstrated (Canell 2008). However, the greatest benefit of vitamin D is fully appreciated above the optimal threshold with storage in fat or muscle not occurring if circulating 25(OH)D levels are >40ng/ml (Cannell 2009). Therefore, advanced processes will require frequent daily replenishment of vitamin D, a recurrent source that is currently unattainable from the common diet. This seemingly suggests that the current recommendation of 600IU per day by the IOM is only designed to prevent clinical deficiency of <20ng/mL, with higher levels such as 1000IU per day increasing availability at optimal levels of vitamin D. Holick (2005) has even estimated values of 3000-5000IUdaily to fully meet the physiological needs of the body. In terms of toxicity, daily intakes of 10,000IU can take years to manifest toxic symptoms after extended use (Heaney et al 2008) with no toxicity incidents reported either from sun overexposure UVB exposure (15minutes can provide 10,000-20,000IU) (Ogen & Prtichett 2013). Over supplementation seems to be the feasible cause of intoxication with outdoor workers living near to the equator only showing levels of around 50ng/mL (Cannell 2008). This suggests that dietary sources of vitamin D could potentially be optimised without the risk of toxicity.
Vitamin D is primarily sourced from ultraviolet rays triggering synthesis endogenously when the skin is exposed to sunlight. It is also naturally present is foods such as fatty fish (salmon, tuna, sardines, mackerel), egg yolks, types margarine and fortified in products such as milk, orange juice and cereal (Chen 2007) (see table 1 for examples of food and IU’s per serving). However, much of the nutrient value of dietary vitamin D can be lost within the absorption process, with evidence showing it to be a process that is 50% efficient (Mahan 2004).
Table 3: Selected Food Sources of Vitamin D (U.S. Department of Agriculture, Agricultural Research Service. 2011)

Some population groups have an emphasis on alternative vitamin D stores. For example, individuals with dark skin have greater amounts of melanin in the epidermal layer of the skin which reduces the ability to produce vitamin D from sunlight. A study in 1546 African women showed that > 40% had deficient levels of vitamin D (Nesby’O’Dell 2002) with 82% Asian adults in the UK also deficient in the vitamin (Pal 2003) in the summer months. The need for vitamin D is also more prevalent in younger populations, the elderly and pregnant women. Approximately 40-100% of Elderly men and women from the US and Europe for example are said to be deficient in vitamin D (Holick 2007). Overall, an estimated 1 billion individuals worldwide are deficient or insufficient in vitamin D levels (Holick 2007); therefore any solution for increasing daily intakes of vitamin D should be taken seriously with fortification a viable option given the current low levels attainable commercially in the diet.  However, achieving the recommended daily aim of 1500-2000IU set by the Endocrine Society is difficult to obtain from food with fortified products only containing 40-140IU per serving on average (see table 3).
                Vitamin D and athletic performance is an area of research that is becoming more prominent. Close (2013a) showed that muscle performance in young UK based athletes did not improve following either a dose of 20,000IU or 40,000IU over a 12 week period with supplementation not inducing levels above 40ng/mL. However, a larger study by this study group did show that a daily dose of 5000IU was more superior in raising levels above 40ng/mL with this daily dose improving vertical jump height and 10 meter sprint times (Close 2013b), suggesting that optimal levels of vitamin D can improve the anaerobic concept of performance. It also worth noting that inadequate vitamin D concentration had a negative impact on musculoskeletal performance in athletes. Inflammation markers in endurance athletes have also been associated with low vitamin D status (Willis et al 2012). The requirements for the athlete will be even greater to fully optimise stores during times of physical activity to ensure sufficient stores for increased physiological demands.
                After reviewing the current literature, there is a clear positive influence when daily intakes of the vitamin lead to optimal serum 25(OH)D levels in the body. The inadequacy of vitamin D stores is prevalent amongst a high amount of the population and the need for these optimal stores is also particularly apparent across children, the elderly, dark skinned individuals and athletes. Although the primary source of vitamin D comes from sunlight exposure, the ongoing deficiency exemplifies that suffice levels are not obtained on a daily basis. Therefore, fortification of more food groups seems a reasonable way to proceed in order to increase bodily vitamin D stores, in the form of a readily available daily source. Although larger experimental research exploring the impact of dietary vitamin D is needed, not even regular intakes 30,000IU for an extended period of time will manifest the symptoms of toxicity (Heaney 2008). This indicates that taking in too much vitamin D from the diet will not create a toxic environment, with the digestion process also nullifying vitamin D absorption. There is also a rationale to provide a controlled supplementation to particular groups. Reaching optimal levels of 40ng/mL, will not only meet fundamental metabolic processes but also ensure extra storage for muscular performance gains. Although research in this area is its infancy, the performance gain from optimal vitamin D status is encouraging and should be considered by athletes and coaches alike.
Cannell, J.J.; Hollis, B.W.; Sorenson, M.B.; Taft, T.N.; Anderson, J.J.B. Athletic performance and vitamin D. Med. Sci. Sport Exerc. 2009, 41, 1102–1110.
Chen, T.C.; Chimeh, F.; Zhiren, L.; Mathieu, J.; Person, K.S.; Zhang, A.; Holick, M.F. Factors that influence the cutaneous synthesis and dietary sources of vitamin D. Arch. Biochem. Biophys. 2007, 460, 213–217.
Close, G.L.; Leckey, J.; Patterson, M.; Bradley, W.; Owens, D.J.; Fraser, W.D.; Morton, J.P. The effects of vitamin D3 supplementation on serum total 25(OH)D concentration and physical performance: A randomised dose-response study. Br. J. Sports Med. 2013, in press.
Close, G.L.; Russel, J.; Cobley, J.N.; Owens, D.J.; Wilson, G.; Gregson, W.; Fraser, W.D.; Morton, J.P. Assessment of vitamin D concentration in non-supplemented professional athlettes and healthy adults during the winter months in the UK: Implications for skeletal muscle function. J. Sports Sci. 2013, 31, 344–353.
Cannell, J.J.; Hollis, B.W. Use of vitamin D in clinical practice. Altern. Med. Rev. 2008, 13, 6–20.
DeLuca, H.F. Overview of general physiologic features and functions of vitamin D. Am. J. Clin. Nutr. 2004, 80, 1689–1696.
Ginde, A.A.; Liu, M.C.; Camargo, C.A. Demographic differences and trends of vitamin D insufficiency in the U.S. population, 1988–2004. Arch. Intern. Med. 2009, 169, 626–632.
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Heaney, R.P. Vitamin D in health and disease. Clin. J. Am. Soc. Nephrol. 2008, 3, 1535–1541.
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Institute of Medicine of the National Academies. Dietary Reference Intakes for Calcium and Vitamin D; Catharine Ross, A., Taylor, C.L., Yaktine, A.L., Eds.; The National Academy of Sciences: Washington, DC, USA, 2011.
Larsen-Meyer, D.E.; Willis, K.S. Vitamin D and athletes. Curr. Sports Med. Rep. 2010, 9, 220–226.
Larson-Mayer, D.E.; Burke, L.M.; Stear, S.J.; Castell L.M. A–Z of nutritional supplements: dietary supplements, sports nutrition foods and ergogenic aids for health and performance: Part 40. Br J Sports Med. 2013, 47, 118-120.
Mahan, L.K.; Escott-Stump, S. In Krause’s Food, Nutrition and Diet Therapy, 11st ed.; Gallagher, M.G., Ed.; Elsevier: Philadelphia, PA, USA, 2004,  83–88.
Nesby-O’Dell, S.; Scanlon, K.S.; Cogswell, M.E.; Hypovitaminosis D prevalence and determinants among African American and white women of reproductive age: third National Health and Nutrition Examination Survey, 1988-1994. Am J Clin Nutr. 2002, 76:187-192.
Ogen, D.; Pritchett.K. Vitamin D and the athlete: risks, recommendations, and benefits. Nutrients. 2013, 5, 1856-1868
Pal, B.R; Marshall T.; James C.; Shaw N.J. Distribution analysis of vitamin D highlights differences in population subgroups: preliminary observations from a pilot study in UK adults. J Endocrinol. 2003, 179, 119-129
Willis, K.S.; Peterson, N.J.; Larson-Meyer, D.E. Should we be concerned about the vitamin D status of athletes? Int. J. Sport Nutr. Exerc. Metab. 2008, 18, 204–224.