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There is a growing amount of evidence, including a study I published in 2012 (Close et al, 2012), to suggest that many athletes are vitamin D-deficient, especially during the winter months. Other reports around the world have shown similar results, suggesting that vitamin D deficiency and insufficiency is widespread among athletic communities.
Although the consequences of such deficiencies are still unclear, impaired muscle function and reduced regenerative capacity, impaired immune function, poor bone health and impaired cardiovascular function have all been associated with vitamin D- deficient athletes. All these factors can have a detrimental impact on performance, so making sure you have a daily source of vitamin D is essential.
Vitamin D synthesis, the process vitamin D goes through for it to be effective within the body, is activated through the reaction of the skin's dermis with ultraviolet B radiation, which forms vitamin D3. Skin pigmentation can have an impact on the effectiveness of vitamin D synthesis, as melanin competes with this process for UVB radiation. This means that those with darker skin require exposure for longer or to a stronger source of UVB radiation to reach the same levels as those with lighter skin.
An alternative route to obtaining vitamin D is through the diet. Foods that provide vitamin D include oily fish, eggs, fortified breakfast cereals, mushrooms and powdered milk. However, large-scale investigations identified that less than 2% of individuals meet the recommended daily allowance from foods. In addition, unlike vitamin D synthesis, that solely produces vitamin D3, dietary intakes provide both vitamin D2 and vitamin D3. There is a huge difference in effectiveness between these two sources; quite simply vitamin D3 is 87% more potent in overcoming a deficiency and is the body's naturally preferred form.
Supplementation is an incredibly effective way of providing the necessary levels of vitamin D. However, in a similar way to foods, supplements can come in the vitamin D2 or vitamin D3 form, so it is important to make sure to choose a supplement that contains vitamin D3.
Regarding dosage: in a previous study I have shown that 5,000 International Units (IU) per day over an 8-week period can overcome a deficiency and maintain sufficient levels of vitamin D (Close and Russell, et al., 2013). However, it is vitally important to remember that vitamin D synthesis via UVB exposure still occurs. For this reason, the US Institute of Medicine has set the tolerable upper intake at 4,000 IU, the same as the guidance set by the European Food Safety Authority, so it is recommended that you choose a supplement with a maximum dose of 4,000 IU per day.
Professor Graeme Close PhD is a former professional rugby league player who also holds a PhD in Sports and Exercise Nutrition from Liverpool John Moore University. He is a nutrition consultant for elite rugby teams and the British Ski and Snowboard association.
Find out more about Professor Graeme Close.
This article is written by nutrition professionals, and is aimed at nutritionists and athletes. It is not intended to replace advice from your own doctor or nutritionist. Please consult a professional before trying supplements.