I was reading some scientific papers on nutrition and bone health which led my attention off in an interesting direction. The topic I had originally been researching was the role of calcium in maintaining bone health (spoiler: calcium is important!), but what really sparked my interest was the emerging science of vitamin D. Vitamin D is something I've long thought of as just an adjunct to calcium intake; much like vitamin C boosts iron absorption, so too does vitamin D boost calcium absorption. But as it turns out, there is much more to vitamin D. For one, it's really more of a hormone than a nutrient, and strictly speaking it's not a "nutrient" at all—even an extremely healthy and nutritious diet hardly contains any vitamin D at all. Most of the vitamin D in your body is synthesized by exposure to sunlight.
With some interesting chemistry, your skin is able to capture some of the energy from the sun's ultraviolet light, using it to kick-start the reaction that synthesizes vitamin D from a cholesterol-like precursor. Since this is the primary source of vitamin D for most people, blood levels of vitamin D tend to vary by season, peaking in the late summer and dropping sharply by mid-fall. Additionally, people who spend a lot of time indoors (including indoor athletes like gymnasts), live in northern climates (like Minnesota, my home state), and who have dark skin are less likely to be able to synthesize enough vitamin D through their day-to-day sun exposure.