Three main sources of vitamin D (vitamin D3 and vitamin D2) are exposure of the skin to sunlight, various types of food such as fish, fortified foods, and supplements.
The main vitamin D source is exposure of unprotected skin to sunlight while outdoors. When the skin is exposed to sunlight and more specifically the UVB or ultraviolet-B rays present in sunlight, vitamin D3 is manufactured in the skin. As a general rule, about ten to fifteen minutes of direct sun exposure three times a week is required to keep vitamin D levels at the required amounts in the body.
However, despite sunlight exposure being a great and easy way of generating vitamin D, it is not necessarily always the main source of this vitamin for some individuals. Dark skinned individuals, people in northern climates, people that do not get outdoors enough, people in polluted environments, and people who need to take extreme caution with sun exposure due to concerns with skin cancer, are all cases where sunlight may not be one of the main sources of vitamin D generation.
Food is another vitamin D source. However, many foods do not contain vitamin D naturally or a lot of vitamin D naturally and therefore individuals need to be very selective on what they eat to get good quantities of vitamin D into their bodies. Various fish such as salmon, mackerel, and sardines are good vitamin D sources. Cod liver oil is as well. After fish, the best option for obtaining vitamin D via food is by consuming fortified foods. Common fortified foods that are good vitamin D sources are milk and a wide variety of breakfast type cereals. Foods can be fortified with vitamin D2 or D3.
The third vitamin D source is supplements. Vitamin D supplements consist of tablets, liquids, or soft gels, and may contain only vitamin D or the vitamin D may be included as part of a multivitamin package. Supplements that contain vitamin D alone come in a variety of strengths but most commonly in strengths of 400 IUs or 1000 IUs. Very potent strengths of 2000 IUs to 5000 IUs are also available. Multivitamin supplements generally contain 400 IUs of vitamin D.
Other vitamin d resources: