wo ist vitamin d drin — what does this mean exactly?

The question 'wo ist vitamin d drin' (where is vitamin D in?) asks about sources of vitamin D. The primary source is sunlight – skin synthesis under UVB rays. Dietary sources are limited: fatty fish (salmon, mackerel, herring), cod liver oil, egg yolks, and in some countries fortified foods (milk, margarine). Mushrooms provide some vitamin D2. Because endogenous synthesis varies by latitude, season, and skin pigmentation, supplementation is often recommended in Central Europe, especially in winter. Evidence for vitamin D's role in bone health is strong; for other claims (immune function, cancer prevention) the evidence is mixed. Direct-to-consumer DNA tests often overstate the impact of genetic variants on vitamin D levels. A blood test (25-hydroxyvitamin D) is the most reliable way to assess status. The question is valid, but the answer should be evidence-based and not driven by marketing.