Hard training dramatically increases your body's demand for vitamin D, which acts like a hormone regulating muscle function, immune response, and recovery. At the same time, athletes often train indoors, wear sunscreen, and are out before sunrise — all of which cut off natural vitamin D production. Dr. Kevin Sprouse notes that insufficient vitamin D is so common in his elite athlete patient population that he now expects it.

D-5,000 (5,000 IU) is the standard daily maintenance dose for most athletes to sustain healthy vitamin D levels year-round. D-10,000 (10,000 IU) is a higher-dose option designed for athletes who have tested low or are actively working to restore deficient levels, ideally under practitioner guidance. Bloodwork is the best way to determine which dose is right for you.

Vitamin D3 is the form naturally produced by your skin when exposed to sunlight, and research consistently shows it raises blood vitamin D levels more effectively and stays active in the body longer than vitamin D2, the form used in many cheaper supplements. Thorne uses exclusively D3 in all of its vitamin D products.

Dr. Sprouse recommends pairing vitamin D with an omega-3 supplement as part of a daily athlete protocol — the two work synergistically for immune function and anti-inflammatory support. Many athletes also pair D3 with vitamin K2 to ensure calcium is directed to bones and not arteries when vitamin D levels are elevated.