Vitamin D. Vitamin D is one of the more debated nutrients in regard to its impact on many physiological processes, including health and performance parameters relevant to athletes. Vitamin D is often known as the “sun vitamin” because we get most of our vitamin D from sun exposure. Small amounts are found in food sources (see Table 8.4), though the amount in these foods is variable and most individuals do not consume adequate quantities of these foods to meet their daily needs. Even the amount that we get from the sun varies, depending upon geographical location, latitude, seasonality, skin color, age, use of sunscreen, and other variables. Given the dearth of dietary sources, and the variability in optimal sun exposure, there is concern that many individuals, including athletes, are in poor vitamin D status. In fact, it is estimated that up to 77 percent of Americans are considered to be “insufficient” in vitamin D (serum 25(OH)D levels 32 ng/dL). Because toxicity of vitamin D is rare and would only result from excessive supplementation, consuming food sources in vitamin D would be prudent, such as salmon and fatty fish, especially given the benefit of these foods for a variety of reasons previously described.