by Graeme L Close PhD, ASCC, CSCS and James Morton PhD
In 2008, Willis et al published a review paper entitled “Should we be concerned about the Vitamin D status of athletes?” As practicing sports nutrition consultants as well as a University Lecturers and researchers, this excellent review made us sit up and take notice of the emerging literature on vitamin D and athletes and the more the authors read the more we actually did get “concerned”. Since this review, there has been much discussion on the effects of vitamin D status on the health and performance of athletes however, definitive studies on athletic populations are unfortunately still lacking. This article will look at the theory behind why vitamin D status may be important to athletes and attempt to draw some conclusions and recommendations for the strength and conditioning/sports nutrition professional.
What is Vitamin D?
The term Vitamin D refers to a group of fat-soluble pro-hormones. The discovery of vitamin D can be largely credited to the observation that supplementation with cod liver oil prevented rickets (osteomalacia). One of the components of cod liver oil credited with preventing rickets was identified and termed vitamin D as it followed the earlier discovery of vitamins A, B and C (Zhang and Naughton, 2010). There are 2 major precursors to vitamin D known as Vitamin D3 (cholecalciferol) and Vitamin D2 (ergocalciferol). Due to these biological precursors Vitamin D could be described as a unique vitamin since under optimum conditions physiological sufficiency could be met through endogenous synthesis (Willis et al., 2008). Vitamin D3 is synthesized when the skin is exposed to UVB radiation, i.e. sunlight. UVB radiation converts 7-dehydrocholesterol (found in the skin) to pre-vitamin D3, which is then converted to Vitamin D3. There are many factors that can impair the conversion of 7-dehydrocholeterol to pre-vitamin D3 including aging, skin pigmentation, clothing, cloud cover, sunscreen use, and perhaps most importantly the time of the year (Willis et al., 2008). During the winter months when the sun is low in the sky it has been reported that it is very difficult for UVB rays to pass through the atmosphere and consequently there may not be sufficient UVB radiation to synthesise vitamin D. It has recently been reported that in the UK between October and March synthesis of vitamin D may be almost impossible and therefore the majority of the UK population may be vitamin D deficient during the winter months (Webb et al., 2010).
Vitamin D2 in contrast is mainly derived from the diet (Wolpowitz and Gilchrest, 2006) although it must be stated that very few foods naturally contain vitamin D and therefore many foods are fortified with it including some milk and orange juices (see Table 1). Vitamin D2 and D3 are converted to 25-hydroxyvitamin D (25-OHD) in the liver and therefore measurement of circulating 25-OHD is perhaps the best (and most widely cited) measure of total vitamin D status. The RDA for vitamin D in the UK is 400IU although many authors suggest that this value is too low and there is certainly no consensus on the optimum daily Vitamin D intake for athletic performance. The USA has recently increased their RDA of vitamin D to 600IU per day up the age of 70 and 800IU for those aged 70+.
Table 1. Example of foods containing vitamin D including the absolute amount in International Units and the % RDA. Data adapted from the US department of agriculture.
|Food||IU per serving (% RDA)|
|Cod Liver Oil||1,360 (340)|
|Wild cooked Salmon||800 (200)|
|Mackerel (cooked)||400 (100)|
|Fortified orange juice||100 (25)|
|Beef Liver (cooked)||46 (12)|
|Egg (whole)||20 (5)|
|Fortified Milk||120 (30)|
Are athletes deficient in Vitamin D?
For many years the vitamin D status of athletes has been largely ignored. At no point in our education as undergraduate or post graduate students was vitamin D ever discussed and during the first authors career as a rugby league player not once did anyone measure or even try to estimate vitamin D levels. The reason for this apathy in relation to vitamin D may be a consequence of the general assumption that vitamin D levels were adequate in the general population combined with a lack of knowledge as to the importance of vitamin D in many aspects of health. There are also equivocal suggestions from vitamin D experts as to what constitutes a vitamin D deficiency. Zittermann (2003) defined sufficiency as between 100-250nmol/L and described a range of sub-optimal levels based on the effects on physiological function (Figure 1). It is important to note that this figure shows that sufficiency is between 100-250 nmol/L and as yet there is no evidence that being at the top end of this scale is optimum compared with being at the lower end of the sufficient scale.
It should also be highlighted that excessively high vitamin D levels (greater than 250nmol/L) can be toxic and may result in hypercalcemia and hyperphosphatemia. Vitamin D intoxication can present as nausea, fatigue, constipation, back pain and memory loss where as prolonged hypercalcemia from excessively high vitamin D intake can cause soft tissue calcification and elevated blood pressure. To our knowledge, no study has ever reported vitamin D levels in the toxic range from a normal diet high in vitamin D or through sunlight exposure, only through the intake of mega dose supplementation. Moreover, even with supplementation vitamin D intoxication is reported to be extremely rare (Zhang and Naughton, 2010). Data on intoxication have reported doses as high as 50,000IU per day where as doses traditionally classed as very high (10,000IU per day for 5 months) have shown no signs of toxicity (Vieth, 2004). However, the possibility of toxification should not be ignored, especially in athletes due to the high prevalence of uncontrolled supplement intakes in athletic populations.
In a comprehensive review of the field, Zitterman (2003) suggested that during the winter months both children and older adults in the UK are reported to have levels <50nmol/L (insufficiency) and that these levels are only marginally better in the summer with values approximately 60-80-nmol/L (hypovitaminosis). It is also important to consider that dark skinned children living in the UK have very low vitamin D status (Zitterman, 2003). This is because dark skin has high levels of melanin which competes with 7-dehydrocholesterol for the absorption of UVB. Clemens et al., (1982) have suggested that dark skinned individuals may need up to 50 times the sunlight exposure of an individual with pale skin complexion.
There is a lack of published studies on the vitamin D status of athletes and even less on UK based athletes, however the extant data does suggest that like the general public athletes do appear to be deficient. In a collaboration between Liverpool John Moores University and Professor Bill Fraser at The University of Liverpool, we have recently tested the vitamin D status (using the highly sensitive technique of tandem mass spectrometry) of several groups of elite athletes including professional soccer players, rugby players and jockeys. These data were collected from December-February and have suggested that athletes may be deficient with mean values for these athletes being between 40-60nmol/L with some athletes less than 10nmol/L and classed as being deficient (unpublished observation). Moreover, in keeping with the suggestions of Clemens et al (1982), we also observed particularly low levels in dark skinned athletes. These data are currently being prepared for publication.
What is the physiological role of vitamin D in relation to athletes?
Bone Health – For many years it has been known that vitamin D is essential for optimal bone health through its regulation of calcium homeostasis with deficiencies being clearly linked with osteomalcia. Although the chances of osteomalcia are highly unlikely for athletes (although not impossible especially in athletes competing in a severely weight restricted state) the risk of stress fractures are high in athletes with some authors suggesting rates as high as 21% in track and field athletes (Snyder et al., 2006). Studies have suggested that low vitamin D status is related to a higher incidence of stress fractures suggesting that hypovitaminosis could be a major problem for the bone health of athletes. Interestingly, Dawson-Hughes et al (1991) reported that the low vitamin D status observed in the wintertime resulted in a transient fall in bone mineral density suggesting that athletes may be particularly susceptible to bone problems during hard training in the winter. It should however be noted that this study was in older non-athletes and this study is yet to be repeated on athletic populations.
Muscle function – The sudden and marked increase in the interest in vitamin D status of athletes probably corresponds with the discovery of a vitamin D receptor on human muscle tissue (Peng et al., 2004) which may regulate protein synthesis in a dose-response manner. It has therefore been suggested that restoring adequate vitamin D status may stimulate protein synthesis and improve musculoskeletal performance (Bischoff-Ferrari et al., 2004b, Bischoff-Ferrari et al., 2004a). There is also evidence that vitamin D could interact with Insulin like growth factor 1 (IGF-1) and through its splice variant mechano-growth factor (MGF) stimulate muscle specific stem cell differentiation and proliferation respectively (Ates et al., 2007, Kuang and Rudnicki, 2008). Muscle specific stem cells are essential for cell growth and repair and therefore any impairment in function could have severe consequences for athletes. At the moment, this suggestion that low vitamin D concentrations may impair muscle structure and function in athletes remains hypothetical and this is something that we are currently researching in our laboratories. Preliminary data indeed suggests that correcting low vitamin D status does improve athletic performance (1-RM muscle strength) but these studies are early and require further analysis.
Immune Function – A third important role of vitamin D with respect to athletes is in relation to immune function. In winter 2008 in this journal, Mike Gleeson explained the effects that exercise has on the immune system suggesting that athletes may be especially susceptible to infection. In our experience working with elite athletes, it is important that nutritional plans are not only devised to improve performance but also to maintain (or even enhance) immune function during times of intense training. There is now significant evidence that vitamin D plays a key role in the immune system and this may be related to their association with monocytes (Hewison, 2010).
Do you UK based athletes require vitamin D supplementation?
Much more research is needed in athletic populations before a definitive answer could be given. However, it is interesting to note that in the Harvard Universities version of the food pyramid (http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/pyramid/), daily vitamin D supplementation is advised for most people. Given the key role that vitamin D plays in health and potentially athletic performance it would appear wise to ensure that athletes are not deficient and data would suggest that this is highly likely especially during the winter months. It therefore may be wise to get your athletes tested for vitamin D to allow sports nutritionists / dietitians / medics to make informed decisions on the need to supplement or not.
One of the major problems with supplementation is prescribing a dose as there are no definitive recommendations to correct marginal deficiencies. If there are clinical deficiencies present, supplements can be prescribed from a medic and this often involves 20,000IU (Dekristol) being given once per week for 6 months followed by once per month for 6 months (personal communication, Professor Bill Fraser), however this must be prescribed and monitored by a doctor.
Another option is for athletes to obtain sensible skin exposure to sunlight. Recommendations include 10-30 minutes twice per week of sun exposure between the hours of 10am-3pm without sunscreen (Holick et al., 2007), however, this again should be advised after consultation with a doctor to ensure your skin type is suitable for this level of exposure and that you do not get sunburn and thus increase the risk of skin cancers.
Perhaps the safest option is to attempt to consume a vitamin D rich diet utilising some of the foods outlined in Table 1 and during the winter months consider a daily 1000-5000 IU supplement for 8-12 weeks depending on the magnitude of deficiency and in consultation with a qualified individual (Close et al., Unpublished Observations). It has been argued that the common dose of 400IU found in multivitamins is not sufficient to increase vitamin D levels and therefore a separate supplement is required.
We hope this brief commentary has provided a useful insight into a contemporary area in sports nutrition but furthermore we hope that it has stimulated interest amongst readers to conduct further research in this area. Moreover, we would encourage practioners to monitor the vitamin D status of their athletes throughout the year looking for seasonal variations. Through our collective efforts we may be able to improve the health, wellbeing and performance of our athletes.
- Ates, K., Yang, S. Y., Orrell, R. W., Sinanan, A. C., Simons, P., Solomon, A., Beech, S., Goldspink, G. & Lewis, M. P. 2007. The IGF-I splice variant MGF increases progenitor cells in ALS, dystrophic, and normal muscle. FEBS Lett, 581, 2727-32.
- Bischoff-Ferrari, H. A., Borchers, M., Gudat, F., Durmuller, U., Stahelin, H. B. & Dick, W. 2004a. Vitamin D receptor expression in human muscle tissue decreases with age. J Bone Miner Res, 19, 265-9.
- Bischoff-Ferrari, H. A., Dietrich, T., Orav, E. J., Hu, F. B., Zhang, Y., Karlson, E. W. & Dawson-Hughes, B. 2004b. Higher 25-hydroxyvitamin D concentrations are associated with better lower-extremity function in both active and inactive persons aged > or =60 y. Am J Clin Nutr, 80, 752-8.
- Clemens, T. L., Adams, J. S., Henderson, S. L. & Holick, M. F. 1982. Increased skin pigment reduces the capacity of skin to synthesise vitamin D3. Lancet, 1, 74-6.
- Dawson-Hughes, B., Dallal, G. E., Krall, E. A., Harris, S., Sokoll, L. J. & Falconer, G. 1991. Effect of vitamin D supplementation on wintertime and overall bone loss in healthy postmenopausal women. Ann Intern Med, 115, 505-12.
- Hewison, M. 2010. Vitamin D and the immune system: new perspectives on an old theme. Endocrinol Metab Clin North Am, 39, 365-79, table of contents.
- Holick, M. F., Chen, T. C., Lu, Z. & Sauter, E. 2007. Vitamin D and skin physiology: a D-lightful story. J Bone Miner Res, 22 Suppl 2, V28-33.
- Kuang, S. & Rudnicki, M. A. 2008. The emerging biology of satellite cells and their therapeutic potential. Trends Mol Med, 14, 82-91.
- Peng, L., Malloy, P. J. & Feldman, D. 2004. Identification of a functional vitamin D response element in the human insulin-like growth factor binding protein-3 promoter. Mol Endocrinol, 18, 1109-19.
- Snyder, R. A., Koester, M. C. & Dunn, W. R. 2006. Epidemiology of stress fractures. Clin Sports Med, 25, 37-52, viii.
- Webb, A. R., Kift, R., Durkin, M. T., O’Brien, S. J., Vail, A., Berry, J. L. & Rhodes, L. E. 2010. The role of sunlight exposure in determining the vitamin D status of the U.K. white adult population. Br J Dermatol, 163, 1050-5.
- Willis, K. S., Peterson, N. J. & Larson-Meyer, D. E. 2008. Should we be concerned about the vitamin D status of athletes? Int J Sport Nutr Exerc Metab, 18, 204-24.
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- Zhang, R. & Naughton, D. P. 2010. Vitamin D in health and disease: current perspectives. Nutr J, 9, 65.
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