Is Vitamin D 25-OH useful for Sport?

Measuring vitamin D (25-OH) is useful for athletes if deficiency is suspected. Vitamin D supports bone health, immune function, and muscle recovery. Evidence shows that correcting a deficiency can improve performance, especially in strength and power sports. However, supplementing when levels are already sufficient does not provide additional benefits. The evidence is moderate: randomized trials show benefits only in deficient individuals. A blood test is the first step, not a DNA test. Genetic variants (e.g., VDR) have small effects and should not dictate supplementation alone. Caution: vitamin D is fat-soluble and can be toxic in excess. For athletes, a target of 30–50 ng/mL is reasonable; higher levels are not proven beneficial. In summary, 25-OH testing is useful for diagnosing deficiency but is not a universal performance enhancer.