, probably due to the fact that vitamin D may increase the amount of one type of fast-twitch muscle fibers and decrease the amount of another type. , the recommended amount of vitamin D is 600 international units per day. But levels vary person-to-person, because ultimately, a recommendation is just that—a recommendation.
, Lewin says. She recommends that all runners take the 25-hydroxy vitamin D blood test to gauge their own dietary needs, but says that you can also look at your diet to see if you’re getting enough of the vitamin.Some common symptoms of a vitamin D deficiency include fatigue, bone or muscle soreness, recurring stress reactions and fractures, frequent illnesses or infections, and even depression.
Since we absorb the vitamin in the sun, some athletes assume that if they’re frequently outside, they’ll be fine—but even the sunniest climates can’t always provide enough D, especially if you exercise with sunscreen on, which blocks your skin from absorbing it.