Vitamin D is best known for its role in promoting calcium absorption which is essential for normal bone mineralization and therefore crucial for bone growth and remodeling. Vitamin D is also known for its important role in modulation of cell growth by activating various gene-encoding proteins that regulate cell proliferation, differentiation, and apoptosis. It also plays a part in the inflammatory process, neuromuscular and immune function. These are just a handful of reasons why sports clinicians should care about Vitamin D.
Although most of us will generally meet adequate Vitamin D requirements through exposure to sunlight; season, time of day, length of day, cloud cover, smog, skin melanin content, and sunscreen can potentially affect ultraviolet (UV) radiation exposure and Vitamin D synthesis. Additionally, UV B radiation does not penetrate glass. This means that athletes participating in indoor activities who attempt to increase their exposure to sunshine through windows may not receive enough UV exposure to produce Vitamin D. Research has shown that the incidence of Vitamin D deficiency in elite indoor athletes is up to 94% among basketball players and 83% in gymnasts (Shuler et al. 2012). Sunscreen has shown to block UV radiation, particularly those with a sun protection factor (SPF) of greater than eight. As a result of these environmental factors it is important that sports clinicians routinely monitor Vitamin D levels in athletes, especially those in high risk sports such as gymnastics where athletes spend prolonged periods indoors.
Vitamin D is a fat soluble secosteriod. In the body, UV radiation triggers Vitamin D production through the skin and therefore sun exposure is vital. Vitamin D is also found in some foods (e.g. the flesh of fatty fish such as salmon and tuna), is added to others for preventative medicine (such as milk and breakfast cereals to reduce incidence of rickets), and is available as a dietary supplement. The active form of Vitamin D goes through two hydroxylations in the liver and kidneys i.e. a chemical process which converts fatty compounds into water-soluble products that are more readily excreted and absorbed by the body. In the liver Vitamin D is converted to calcidiol (25(OH)D), which is measured in serum to determine Vitamin D levels. In the kidney it is converted to calcitriol (1,25(OH)2D), which is the active form of Vitamin D.
Vitamin D and Research
There has been a recent influx in research into Vitamin D levels around the world, particularly in athletes. For example there now is emerging evidence that Vitamin D levels can be linked to an increased risk of stress fractures. Endurance athletes may already be aware that Vitamin D plays a role in controlling inflammation and aids the body’s fight against infections.
A recent literature review published by Associate Professor Shuler and colleagues from the Department of Orthopaedic Surgery, Marshall University, West Virginia provides an overview of Vitamin D research and its benefits in sports (titled “Sports Health Benefits of Vitamin D”). This review reports that Vitamin D deficiency is common amongst athletes and should always be investigated when athletes present with stress fractures, musculoskeletal pain, and frequent illness. Research has also shown that Vitamin D deficiency is associated with muscle fibre atrophy, slow peak muscle contraction, prolonged time to muscle relaxation, and increased risk of chronic musculoskeletal pain. In particular type II (fast-twitch) muscle fibres are known to be particularly sensitive to changes in Vitamin D levels. Increases in muscle protein synthesis, ATP concentration, strength, jump height, jump velocity, jump power, exercise capacity, and physical performance are achieved with increased Vitamin D levels. In summary, Vitamin D levels will directly affect sporting performance.
As clinicians it is also important to acknowledge that some medications can significantly impair Vitamin D levels. Corticosteriods for example can reduce calcium absorption and impair the metabolism of Vitamin D (Searing et al. 2010). Some weight loss drugs and cholesterol lowering drugs are also known to reduce the body’s absorption of Vitamin D.
Searing, DA, Zhang Y, Murphy JR, Hauk PJ, Goleva E, & Leung DYM: ‘Decreased serum vitamin D levels in children with asthma are associated with increased corticosteroid use,’ Journal of allergy & clinical immunology 2010 125:995-1000
Shuler FD, Wingate MK, Moore GH & Giangarra C: ‘Sports Health Benefits of Vitamin D,’ Sports Health: A Multidisciplinary Approach 2012 4: 496