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

The link between vitamin D (25-OH) and fatigue is clinically plausible but evidence is nuanced. Epidemiological studies consistently associate low serum 25-hydroxyvitamin D (<20 ng/mL) with increased fatigue, muscle weakness, and reduced physical performance. However, randomized controlled trials (RCTs) show a clear benefit of supplementation on fatigue only in individuals with baseline deficiency. In replete individuals (>30 ng/mL), additional vitamin D does not reliably improve fatigue. Fatigue is a multifactorial symptom; causes include iron deficiency, thyroid dysfunction, sleep disorders, and depression. A single vitamin D test without comprehensive workup is insufficient. MyBody-X offers blood-based vitamin D measurement, but results should be interpreted by a clinician. Caveat: Self-diagnosis and supplementation based solely on a lab value can be misleading and potentially harmful (e.g., toxicity with excessive intake). Evidence: human-moderate (predominantly observational, some RCTs).