Recent research has been focused on vitamin D supplementation for athletes. Although the body can make this stuff when the skin is exposed to sunlight (by converting cholesterol to vitamin D), this mechanism of vitamin D “intake” depends greatly on the season and your sun bathing habits. Beyond sun exposure, vitamin D is found in fortified dairy products, fatty fish (salmon, mackerel), and cod liver oil. Classically, vitamin D is intimately involved in calcium handling, bone metabolism, and neuromuscular function—however the latest research shows it does much more.
The recent interest in vitamin D supplementation for athletes has spawned from data showing that skeletal muscle has vitamin D receptors that are involved in muscle strength production and muscle growth through skeletal muscle regeneration. To add to this, it has been suggested that most athletes are vitamin D deficient, especially in the winter months largely as a consequence of inadequate sun exposure, combined with low variety diets.
Signs of deficiency? Many who are vitamin D deficient complain of tiredness, overall weakness, and muscle aches. Since these signs are universal for many problems, your best bet is to take a blood test for 25-hydroxyvitamin D (25[OH]D), which can be done through your family physician or by purchasing an online test kit.
If you are an athlete and tend to avoid sun exposure, then you are likely vitamin D deficient. Supplementation varies, but the daily upper limit of vitamin D intake in adults is generally reported as 2000 iu, however studies in athletes have shown that up to 10,000 iu per day is safe, well tolerated, and beneficial.