Sunlight

Sunlight exposure provides most people with their entire vitamin D requirement. Children and young adults who spend a short time outside two or three times a week will generally synthesize all the vitamin D they need. The elderly have diminished capacity to synthesize vitamin D from sunlight exposure and frequently use sunscreen or protective clothing in order to prevent skin cancer and sun damage. The application of sunscreen with an SPF factor of 8 reduces production of vitamin D by 95%. In latitudes around 40 degrees north or 40 degrees south (Boston is 42 degrees north), there is insufficient UVB radiation available for vitamin D synthesis from November to early March. Ten degrees farther north or south (Edmonton, Canada) this “vitamin D winter” extends from mid October to mid March. According to Dr. Michael Holick, as little as 5-10 minutes of sun exposure on arms and legs or face and arms three times weekly between 11:00 am and 2:00 pm during the spring, summer, and fall at 42 degrees latitude should provide a light-skinned individual with adequate vitamin D and allow for storage of any excess for use during the winter with minimal risk of skin damage.

Food sources

Vitamin D is found naturally in very few foods. Foods containing vitamin D include some fatty fish (mackerel, salmon, sardines), fish liver oils, and eggs from hens that have been fed vitamin D. In the U.S., milk and infant formula are fortified with vitamin D so that they contain 400 IU (10 mcg) per quart. However, other dairy products such as cheese and yogurt are not always fortified with vitamin D. Some cereals and breads are also fortified with vitamin D. Recently, orange juice fortified with vitamin D has been made available in the U.S. Accurate estimates of average dietary intakes of vitamin D are difficult because of the high variability of the vitamin D content of fortified foods. Vitamin D contents of some vitamin D-rich foods are listed in the table below in both international units (IU) and micrograms (mcg).

Supplements

Most vitamin D supplements available without a prescription contain cholecalciferol (vitamin D3). Multivitamin supplements for children generally provide 200 IU (5 mcg) and multivitamin supplements for adults generally provide 400 IU (10 mcg) of vitamin D. Single ingredient vitamin D supplements may provide 400-1,000 IU of vitamin D, but 400 IU is the most commonly available dose. A number of calcium supplements may also provide vitamin D.

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