If you’re always slathered up with sunblock, live in the geographic north….or are a vampire, then you need to rely on your food supply and supplements for Vitamin-D. This is important because many studies report that most people, especially athletes, are vitamin D deficient. Beyond common health consequences, even a slight vitamin D deficiency limits strength and performance.D-3 (or cholecalciferol) is the form that is best used by the body, so here is how much you’d need to eat make sure you’re getting significant amounts: