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SPORTS MEDICINE VITAMIN D DEFICIENCY
VITAMIN D AND PERFORMANCE In the early part of the twentieth century, athletes were allegedly exposed to ultraviolet radiation to enhance their performance. There have been numerous (mainly anecdotal) reports from both the USA and Germany about the success of this strategy (3). In the UK, athletes typically head south for the winter, for “warm- weather training”, seeking benefits that include increased flexibility and reduced risk of training injuries in warmer climates (as opposed to indoor stadiums in the UK during the winter). It is possible that increased exposure to ultraviolet light may improve athletes’ vitamin D levels, resulting in the benefits described. A recent review of historical
scientific literature on the impact of ultraviolet light on performance suggests it has a significant effect (4). Of course, the technological and methodological limitations of the early 20th century prevent us from over-interpreting the data nowadays, but that early research did suggest that ultraviolet exposure increases both the generation of power and physical endurance. Despite incomplete understanding of the physiology or adequate technology to assess vitamin D levels at the time, the early 20th- century results still represent the direct impact of vitamin D on athletic performance (4). More recent research,
BOX 2: SIGNS AND SYMPTOMS OF VITAMIN D DEFICIENCY
IN CHILDREN Rickets
IN CHILDREN - Impeded growth
- Deformity of long bones - (tibial and femoral bowing) - Costochondral hypertrophy - Pigeon chest - Impaired tooth development
IN ADULTS Osteomalacia
- Increased bone fragility - Proximal muscle weakness - Bone fragility - Musculoskeletal pain
Osteoporosis
- Reduced bone mineral density - Increased bone fragility
POTENTIALLY ASSOCIATED CHRONIC DISEASES n High blood pressure n Peripheral artery disease n Bowel cancer n Diabetes
Y ASSOCIA n Multiple sclerosis n Seasonal affective disorder
n Cognitive impairment n Parkinson’s disease n Periodontal disease
ON MUSCLE AND EVEN ON ATHLETIC PERFORMANCE
however, suggests that the vitamin has a marked impact on muscle and this may provide valuable insights into its potential to affect performance.
VITAMIN D AND HUMAN SKELETAL MUSCLE Muscle loss and ageing Vitamin D-deficient osteomalacia is known to be associated with a proximal myopathy which is known to involve the loss of type II muscle fibres (5). This loss has been shown to be reversible and preventable if vitamin D supplements are taken. Yet more evidence about the impact of vitamin D on muscle has been found with the age-related decline in muscle mass – the so-called sarcopenia of ageing. However, there are a large number of potential confounding variables to research in this area (6), so opinion in the literature remains divided over the precise role of vitamin D in this form of progressive muscle loss. Cross-sectional studies do appear to show a relationship between age-related sarcopenia and vitamin D, but prospective controlled trials have had mixed results (7–10). Of particular note is the observation that the number of vitamin D receptors found in muscle reduce with increasing age (11); this is one potential mechanism by which vitamin D impacts on age- related sarcopenia.
Studies of functional ability Two studies assessing functional activity in young women with vitamin D deficiency also showed mixed results. In one study, Lebanese girls who were given vitamin D supplements had improved lean mass and bone mass, but no increase in grip strength (12). In post-menarchal British girls (13) with vitamin D deficiency, vitamin levels had a positive relationship with jump height, jump velocity and power.
Protein transcription and muscle function
As explained above, the actions of vitamin D depend on vitamin D
RECENT RESEARCH SUGGESTS VITAMIN D DEFICIENCY MAY HAVE A MARKED IMPACT
receptors; this was first recognised in the middle of the 1980s, and the presence of the receptors in human skeletal muscle was confirmed in 2001 (14). The importance of these receptors in muscle structure has recently been illustrated in mice depleted of vitamin D receptors (known as VDR null mice) whose muscles are 20% smaller than those in mice with the receptors (15). As is typical with all steroid hormones, the binding of vitamin D to its receptor results in enhanced transcription of proteins, particularly those involved in calcium metabolism (16) as described above.
Calcium is obviously a critical modulator of skeletal muscle function and therefore any disturbance in calcium handling may have an impact on the contractile properties of the muscle (Fig. 5). It is very likely, therefore, that vitamin D may affect muscle function through both calcium- related protein transcription and total body calcium levels.
Recent research has shown that vitamin D has a transcription-enhancing role on proteins other than those involved directly in calcium metabolism. One such protein is insulin-like growth factor binding protein 3, or IGFBP-3. This protein regulates the activity of insulin-like growth factor (IGF-1) by influencing its clearance, and it may also have an independent, direct transcriptional role within tissues (17). In turn, IGF-1 has many roles within muscle, including the induction of muscle hypertrophy and tissue repair, which is why it appears on the World Anti-Doping Agency Prohibited List. Obviously if vitamin D enhances the transcription of IGFBP-3, it also will affect IGF-1 activity (18). The significance of this became clear recently when a direct relationship was revealed between changes in growth in children with rickets (following vitamin D supplementation) and IGF-1 levels (19). In addition to this, numerous polymorphisms of the vitamin D receptor have now been described and
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