The skin synthesizes vitamin D when it is exposed to ultraviolet (UV) rays of the sun, but many people limit or avoid sun exposure because of the risk of skin cancer, or else they use sunscreen to block UV. Obesity is linked with lower blood levels of D because the vitamin is trapped in fat tissue. Surprisingly few foods supply vitamin D.
But only after the kidneys convert it into its biologically active form, called calcitriol. Vitamin D is actually a group of related compounds with a mind-numbing array of names and a complex metabolism. The vitamin D made in the skin and found in most foods and capsules is called vitamin D3, which is first converted to a compound known as calcidiol—the form usually measured in blood tests. These forms of vitamin D are considered pre-hormones.
Older people need to consume more because the skin’s ability to produce vitamin D from the sun’s ultraviolet rays declines with age. A 70-year-old makes only about one-quarter as much vitamin D, on average, as a 20-year-old from the same sun exposure. What’s more, in older people the liver and kidneys are not as efficient in converting vitamin D to its active form, and vitamin D is not absorbed as well from the intestine.
In 2010 the Institute of Medicine (IOM) raised the RDA for most adults from 400 IU up to 600 IU. Plus, it raised the RDA for people age 70 and older to 800 IU. The IOM said that most Americans don’t need to take supplements to achieve adequate blood levels of vitamin D. However, many experts disagree and recommend supplements of 800 to 2,000 IU a day for people likely to have low blood levels.
Observational studies have linked low vitamin D levels to everything from arthritis, depression, cancer, and heart disease to diabetes, infections, MS, and Parkinson’s disease. But such studies don’t prove causation. Most well-designed clinical trials support vitamin D’s role in keeping bones strong, and some have found muscle benefits. Several large clinical trials on vitamin D are underway, but results are not expected until 2017 to 2020.
Fish such as trout, salmon, and tuna are naturally rich in D. They provide 230 to 645 IU per 3-ounce serving. But milk is by far the main dietary source of vitamin D because it is fortified (100 IU per cup) and we drink so much of it. Some breakfast cereals, orange juice, and other foods are now also fortified with vitamin D. Mushrooms produce vitamin D only when exposed to ultraviolet light from the sun or UV lamps.
Cod liver oil is naturally rich in vitamin D, which is concentrated in the liver. But because the oil is also very high in vitamin A, it is not a good option. Though research is inconsistent, some studies have linked high doses of vitamin A to impaired bone health. Don't take cod liver oil, unless the label says that the vitamin A content has been reduced.
...though some experts would say nearly every day. The standard recommendation has been to get about 10 to 20 minutes of sun exposure between 10 a.m. and 3 p.m. twice a week to the face, arms, and legs without sunscreen to produce adequate vitamin D. However, the skin’s synthesis of vitamin D varies from person to person, depending on factors such as age, skin color, geographic location, and genetics.