wo ist vitamin d drin — what are common causes?

Vitamin D is found in relatively few natural foods in significant amounts. The best sources are fatty fish such as salmon, mackerel, herring, and tuna. Cod liver oil, egg yolks, and certain mushrooms (e.g., shiitake) also provide vitamin D. In many countries, vitamin D is added to foods like margarine, milk, breakfast cereals, and orange juice. However, the primary source for humans is endogenous synthesis through sunlight (UVB) exposure on the skin. Common causes of vitamin D deficiency include insufficient sun exposure (especially in winter, at northern latitudes, with dark skin, or with full-body covering), a vegan diet without supplementation, malabsorption (e.g., in Crohn's disease, celiac disease), obesity (vitamin D is sequestered in adipose tissue), and certain medications (e.g., antiepileptics, glucocorticoids). Older adults and infants are also at higher risk. The evidence for these statements is well-established (mechanistic and epidemiological), but the provided context does not contain specific sources on vitamin D. Therefore, the evidence is rated as 'practice' (generally accepted knowledge).