wie merkt man — what are common causes?

Common causes of nutrient deficiencies include inadequate intake (e.g., restrictive diets, picky eating in children), increased requirements (e.g., endurance athletes needing more iron), malabsorption (e.g., gut disorders), and genetic variants (e.g., TPMT or COMT, though their clinical impact is often small). Noticing a deficiency is tricky because early symptoms are vague – fatigue, pale skin, brittle nails, frequent infections – and overlap with many conditions. The most reliable way is a blood test: 25(OH)D for vitamin D, ferritin for iron stores, serum selenium, etc. Home test kits can give a first hint but should be confirmed by a doctor before supplementing. Toxicity from supplements is rare but real (e.g., selenium above 300 µg/day per EFSA). Evidence is mostly practice-based, with some human studies for selenium. Genetic tests from consumer companies often overstate single-gene effects. In short: if you suspect a deficiency, consult a healthcare professional rather than relying solely on self-testing.