Only a few foods naturally contain vitamin D, particularly the flesh of fatty fish (e.g. trout, salon, tuna) and fish liver oils. Beef liver, cheese and egg yolks contain small amounts of vitamin D, but the animal’s diet also affects the amount of vitamin D in its tissues.
Most of the vitamin D from food sources is through the consumption of fortified foods. Most cow’s milk in the US is fortified with ~3 mcg/cup, and in Canada it is mandatory to fortify cow’s milk with 0.88-1.0 mcg/100 mL. Plant-based milk alternatives also often contain similar amounts of added vitamin D. Other commonly fortified foods include cereals, yogurt, orange juices, and margarine. The fortification of infant formula is mandatory in both the US and Canada.
Dietary supplements are also a source of vitamin D, particularly in infants who are not receiving fortified infant formula.
Sun exposure is the source of at least some vitamin D for most people around the world. Type B UV radiation with a wavelength of 290-320 nm penetrates uncovered skin and triggers the synthesis of vitamin D. Various factors influence the amount of vitamin synthesis from sun exposure, including season, time of day, cloud cover, smog, skin melanin content, and sunscreen. This makes it difficult to provide guidelines for sun exposure for sufficient vitamin D synthesis.