Can Vitamin D 25-OH help with fatigue?

Whether vitamin D (25-OH) can help with fatigue requires nuance. Low 25-hydroxyvitamin D levels are linked to fatigue, muscle weakness, and reduced physical performance. In individuals with confirmed deficiency, supplementation often improves these symptoms. However, evidence for benefit in those with normal levels is weak—randomized controlled trials frequently show no significant effect on fatigue. Importantly, 25-OH is a blood test marker, not a supplement; vitamin D intake should be guided by measured deficiency. High-dose supplementation without deficiency may cause toxicity (hypercalcemia). The provided context lacks direct studies on vitamin D and fatigue, focusing instead on DNA tests and general biomarkers. Therefore, the evidence is graded as 'human-moderate' based on broader literature, not the given sources. Caveat: fatigue has many causes (iron deficiency, thyroid disorders, sleep issues, stress); a vitamin D test is only one piece of a comprehensive workup.