Mushrooms are a source of vitamin D when raised under ultraviolet light. Most other dietary sources of vitamin D come from animals. The best sources are wild-caught salmon and mackerel. Other sources include water-packed tuna, vitamin D-fortified milk and yogurt, oil-packed sardines, beef and calf liver and egg yolks.
Human skin synthesizes vitamin D from exposure to sunlight. It is possible to meet the body's need for vitamin D solely through photosynthesis, but cloudy weather, shade, use of sunscreen and naturally dark skin all reduce the body's ability to make its own vitamin D, and the skin cannot use sunlight coming in through a window. The Recommended Daily Allowance of 600 international units daily for most adults is based on an assumption of minimal sun exposure.
Manufacturers add vitamin D to many breakfast cereals and to some brands of orange juice, soy beverages and margarine, enabling vegetarians to get vitamin D in their diets without consuming animal products. Meeting the RDA for vitamin D is important for maintaining healthy teeth and bones in both children and adults. Vitamin D also plays a role in muscle contraction, in helping neurotransmitters function properly and in maintaining healthy immune system function. In addition, research suggests that low vitamin D levels may increase the risk of developing some kinds of cancer.