Vitamin D can be obtained from sun exposure, foods, and supplements. The body produces vitamin D in response to sun exposure, and most people in the world meet at least some of their vitamin D needs through exposure to sunlight. However, factors such as season, time of day, length of day, cloud cover, smog, skin melanin content, and sunscreen can affect UV radiation exposure and vitamin D synthesis.

Natural food sources of vitamin D are limited, especially for vegetarians or those who do not like fish. The best sources of vitamin D are the flesh of fatty fish and fish liver oils. Smaller amounts are found in egg yolks, cheese, and beef liver. Certain mushrooms contain some vitamin D2, and some commercially sold mushrooms contain higher amounts of D2 due to intentionally being exposed to high amounts of ultraviolet light. Many foods and supplements are fortified with vitamin D, such as cow’s milk, soy milk, orange juice, cereals, and oatmeal.

It is important to note that the vitamin D content of fortified foods can vary widely, and it is necessary to check the labels to find out the vitamin D content. Vitamin D2 is manufactured using UV irradiation of ergosterol in yeast, and the Food and Drug Administration (FDA) has approved UV-treated mushroom powder as a food additive for use as a source of vitamin D2 in food products.

While vitamin D is critical for optimal health, it is also important to note that excessive sun exposure can increase the risk of skin cancer. Therefore, it is recommended to obtain vitamin D from a combination of sun exposure, food sources, and supplements.