Milk does not naturally contain vitamin D; fish, mushrooms, and egg yolks are the only natural sources, but the mushrooms have to be exposed to sunlight (which

Milk does not naturally contain vitamin D; fish, mushrooms, and egg yolks are the only natural sources, but the mushrooms have to be exposed to sunlight (which is unlikely; they grow in the shade) and nobody eats that many eggs, so only pescetarian cultures retain darker skin at northern latitudes.