Evidence check: Nährstoffcheck in the context of fatigue

The idea of a nutrient check for fatigue is intuitive, as deficiencies in iron, vitamin B12, vitamin D, or magnesium can indeed cause exhaustion. However, the evidence for commercial home tests (saliva or blood) to diagnose the cause of fatigue is weak. Most such tests analyze single genetic variants (e.g., MTHFR or VDR) or biomarkers with small effect sizes that are heavily confounded by diet, lifestyle, and seasonal variation. A blood test ordered by a physician (e.g., ferritin, vitamin B12, 25-hydroxyvitamin D) is far more informative and often covered by insurance when clinically indicated. The provided context contains no validated studies on nutrient checks for fatigue; most sources refer to DNA tests with limited clinical utility. Therefore, the evidence grade is 'unclear' to 'marketing'. Caveat: Self-testing without medical guidance can lead to misinterpretation or delayed diagnosis. Persistent fatigue warrants a thorough medical evaluation.