Vitamin D3 (cholecalciferol) is synthesized by the human body itself. Human skin contains a kind of cholesterol, which becomes vitamin D3 after being irradiated by sunlight. Therefore, if the child can fully accept the direct sunlight to the skin for more than 4-6 hours, the vitamin D3 synthesized by himself can basically be satisfied.

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Vitamin D3 can also come from animal foods, such as liver, especially cod liver oil extracted from marine fish liver. Vitamin D3 exists in some animal foods.

7- dehydrocholesterol in skin is mainly formed by ultraviolet radiation, while 7- dehydrocholesterol is formed by cholesterol conversion, so some people call it sun vitamin.

Baidu encyclopedia-vitamin D3