shown to be associated with variability in exercise performance (20,21). This is an exciting area of research, but further clarification of the clinical significance of vitamin D receptor polymorphisms is needed. In fact, we are only just beginning to scratch the surface with respect to our understanding of vitamin D’s role in the maintenance of optimal muscle functioning.

MEASUREMENT OF VITAMIN D

One of the problems of measuring levels of vitamin D is that the active form of the vitamin (1,25(OH)2

D) exists

only in very small amounts in the blood and as such, it is very difficult to measure accurately. Furthermore, even when vitamin D supplies and levels are low, 25(OH)D is still converted to 1,25(OH)2

D, thereby sparing

levels despite an actual whole-body deficiency of the vitamin. By contrast, 25(OH)D is found in higher levels and as a result is the preferred indicator of vitamin D status. The situation is far more complicated than this, however. It is now known that there is significant variability in the measurement of vitamin D levels between laboratories, which casts doubts on the validity of sampling (22). Although 25(OH)D may be measured by several different methods (including chemiluminescent technology and mass spectroscopy) the gold standard assessment is always mass spectroscopy.

VITAMIN D REQUIREMENTS Definitions of what constitutes an adequate 25(OH)D level and what constitutes appropriate supplementation continue to be hotly debated (23–25). It is, however, generally accepted that levels greater than 30 ng/ml are “adequate” for maintaining calcium and bone homeostasis – at this concentration, there is no inverse linear relationship with parathyroid hormone levels, and calcium absorption in the intestine is at an optimum (26). Levels below this are associated with different degrees of depletion and deficiency. The commonly accepted definitions (1) are shown in Fig. 6. Some researchers would argue that levels far greater than 30 ng/ml are needed for optimal functioning (4), but it remains unclear whether levels higher than this will enhance performance, improve muscle

14

function, or aid tissue regeneration in healthy young people.

PREVALENCE OF DEFICIENCY

Deficiency of vitamin D is now seen endemically in many regions of the world, including North America, Europe and the Middle East. In order for people to produce enough vitamin D endogenously from sunlight, they must first have adequate exposure. A number intrinsic and extrinsic factors may also reduce production of vitamin D (Fig. 7). Some people, such as the elderly

and infirm anywhere in the world, simply have inadequate exposure to sunlight. People living at high latitudes also have increased incidence of deficiency. However, a recent study found that young adults in inner city areas of America and skateboarders in sun-drenched Hawaii are also deficient in vitamin D. And despite long hours of sunlight in the Middle East, the deficiency is also observed in women who, in line with cultural expectations, keep their skin covered when outside.

Deficiency among athletes Our facility has recently examined the vitamin D levels of sportsmen in the Middle East and found deficiency in over 80 per cent of them. Clearly, athletes are not immune to vitamin D deficiency, and concern has recently been raised about the vitamin D status of athletes – both elite female Australian gymnasts and Finnish female athletes were recently found to be deficient in two separate studies (27,28). The exact significance of these findings remains unclear, but there may be significant implications for the bone health of athletes, as well as their general long-term health and performance.

Correcting deficiencies The identification and correction of any deficiency is important for the elite or recreational athlete, as well as any

> 30 ng/ml

20–30 ng/ml 10–20 ng/ml > 10 ng/ml

Figure 6: Serum 25(OH)D levels

and definitions of deficiencies (1)

sufficiency insufficiency deficiency

severe deficiency

WITHOUT CALCIUM troponin-tropomyosin blocks binding sites on actin so that actin and myosin filaments cannot interact

actin myosin troponin tropomyosin

WITH CALCIUM troponin-tropomyosin unblocks enabling actin and myosin filaments to interact

actin Ca2+

Ca2+ myosin troponin

Figure 5: Interaction between calcium and muscle proteins that bring about muscle contraction

tropomyosin

patient presenting with atypical aches and pains. Since the predominant source of vitamin D is from sunlight (26), athletes in the UK may be at particular risk during the winter, when a combination of factors means that their actual exposure to sunlight is minimal, whether because of high latitude, cold weather and short days or because the athletes are training in indoor facilities (Box 3). Even during the summer in the UK, dark-skinned athletes may not receive enough ultraviolet light to maintain adequate vitamin D levels. The recommended level of 25(OH) D (Fig. 6) can be achieved either via adequate sunlight exposure or via dietary supplementation. Supplementation, however, depends on access to high-quality vitamin products, and contamination must always be considered when supplementing elite athletes. There is a general feeling that the published recommended daily allowances for vitamin D are inadequate, and maintenance doses of the vitamin of 1,000 IU per day may be necessary for those who are unable to gain adequate vitamin D from other sources. In the absence of adequate ultraviolet B light, 2,000 IU per day of vitamin D3

or equlivalent for a period of

8 weeks has been shown to reverse even a significant deficiency. There are numerous different

BOX 3: FACTORS THAT REDUCE EXPOSURE TO SUNLIGHT

n High latitude n Short daylight hours n Training in indoor facilities

sportEX medicine 2010;43(Jan):11-16 Ca2+ Ca2+