Vitamin D is a fat-soluble nutrient that your body can produce but is not found naturally in many foods. Also known as calciferol, vitamin D promotes the absorption of two minerals—calcium and phosphorus—and then helps deposit them in bones and teeth to make them strong and healthy.
Your body produces vitamin D when the sun’s ultraviolet rays stimulate your skin to synthesize it. This is the primary way by which humans obtain vitamin D. If you have limited sun exposure, it is essential that you obtain vitamin D from dietary sources.
Research also suggests that vitamin D has been linked with lower incidences of cancers and lower rates of immune-related conditions, such as type 1 diabetes and multiple sclerosis.
Very few foods are naturally rich in vitamin D. Therefore, most people obtain their vitamin D intake from fortified foods, such as milk, soymilk and fortified cereals. Naturally occurring vitamin D is found in the flesh of fatty fish (mackerel, salmon, sardines) and in egg yolks.
Vitamin D insufficiency results in rickets (defective bone growth) in children and osteomalacia (soft bones) and osteoporosis (porous bones) in adults. These skeletal diseases are the result of inadequate bone mineralization or demineralization of the skeleton. Vitamin D-deficient diets are often found in those who have milk allergies, lactose intolerance or in those following a vegan diet (devoid of animal products, including milk and eggs).
Some groups may require supplemental vitamin D, particularly if they are not receiving exposure to sunlight. These include:
Infants exclusively breast-fed: Human milk contains very little vitamin D. Many infants—especially those who live in far northern latitudes—may not be exposed to sufficient sunlight.
Older Adults: Older adults are more prone to vitamin D deficiency, especially those living in northern industrialized cities of the world. This is due to lack of sunlight combined with a decreased ability to synthesize vitamin D in the skin.
Dark skin and sunscreen: People with dark skin or those using topical sunscreen are at increased risk of deficiency due to a reduction in the body’s ability to produce vitamin D in the skin.
Because fat-soluble vitamins like vitamin D are stored in the body, too much can be toxic and cause hypervitaminosis D. Signs of hypervitaminosis D include kidney stones, bone loss, weakness, weak bones, anorexia, nausea, vomiting and reduced renal function. But in case you were wondering: there is no evidence that vitamin D generated by the body via sun exposure can contribute to vitamin D toxicity, because your body can regulate its production.
The following table lists the recommended intake for healthy people based on current scientific information.