Myths vs facts: Vitamin D 25-OH and fatigue

Myth vs. fact: Vitamin D (25-OH) and fatigue. A common myth is that low vitamin D levels always cause fatigue and that supplementation will fix it. Observational studies show an association between low 25-OH-D and fatigue, but evidence for a causal effect is mixed. Randomized controlled trials (RCTs) often fail to show significant improvement in fatigue with vitamin D supplementation in non-deficient individuals. True deficiency (25-OH-D < 12 ng/ml) can contribute to muscle weakness and tiredness, but many other factors (sleep, stress, iron deficiency, thyroid) are more common. Levels vary seasonally and by skin type. Direct-to-consumer tests typically measure only 25-OH-D, not the active form. Bottom line: vitamin D deficiency as the sole cause of fatigue is rare; blood testing and medical evaluation are advisable before supplementing. Evidence is moderate but not strong enough for general recommendations for fatigue without deficiency.