At this point, it's obvious to me that vitamin D helps athletic performance.
Vitamin D deficiency is known to occur among athletes, even in sunny countries (where people tend to have more ultraviolet-deflecting melanin, and where they make serious attempts to stay out of the sun). Vitamin D deficiency causes muscle weakness, slower reaction times, and is associated with a lower VO2 max.
Ultraviolet radiation (which makes vitamin D in the skin) was known in the early 20th century to help athletes. However, this has been forgotten by the medical field, or perhaps ignored because of the skin cancer risks. In a 1938 Russian study, athletes subjected to UV irradiation improved 100-meter sprint times by 7.4 percent, while controls undergoing the same training improved by 1.7 percent. In Germany in 1944, medical students getting UV irradiation improved their work on a bicycle ergometer by 13%. In 1952, German schoolchildren irridadiated in their classrooms for months increased their physical fitness scores by 56%. The researchers were able to duplicate this effect by giving other children a single dose of 250,000 units of vitamin D, basically confirming that the UV light was having its effect through vitamin D. (Please don't take 250,000 units of vitamin D.)
Vitamin D may protect against overtraining. The evidence is only circumstantial, but it's suggestive. Researchers at the University of Wyoming have found that in long-distance runners, levels of TNF-alpha, a pro-inflammatory hormone, are higher when they don't get enough vitamin D. TNF-alpha is one of the hormones elevated in overtraining syndrome. And vitamin D is known to reduce inflammation in general.