Is Vitamin D 25-OH useful for Müdigkeit?

Measuring 25-hydroxyvitamin D (25-OH-D) is often marketed for fatigue, but the evidence for routine use is mixed. Severe vitamin D deficiency (<30 nmol/L) is linked to fatigue and muscle weakness, and supplementation can improve symptoms in deficient individuals. However, in non-deficient populations, randomized controlled trials show no consistent benefit for fatigue. The test is useful when deficiency is suspected (e.g., limited sun exposure, malabsorption, dark skin). For general fatigue without risk factors, the predictive value is low. The U.S. Preventive Services Task Force does not recommend routine screening. The test is a biomarker, not a treatment. Over-reliance on a single value may lead to unnecessary supplementation. Evidence: mixed (observational studies support deficiency link, but RCTs show limited benefit in unselected populations).