Evidence check: Vitamin D 25-OH in the context of fatigue

The relationship between vitamin D 25-OH levels and fatigue is not clearly established. Observational studies suggest an association between low vitamin D (< 12 ng/ml) and increased fatigue, but randomized controlled trials have not consistently shown that supplementation reduces fatigue in the general population. The evidence is moderate: deficiency may contribute to fatigue, but it is rarely the sole cause. Other factors such as sleep quality, stress, iron deficiency, or thyroid function are often more significant. Testing vitamin D in the context of fatigue can be reasonable, but results should be interpreted with caution and ideally by a physician. Supplementation without confirmed deficiency is not supported by strong evidence. Overall, while low vitamin D is a plausible contributor, it is not a proven independent cause of fatigue.