Vitamin D: what's enough? Many people may need much more

Science News, Oct 16, 2004 by Janet Raloff

Second in a two-part series.

A few minutes of sun exposure on a summer day can generate huge quantities of vitamin D in a person's body. A cholesterol-like substance in the skin absorbs ultraviolet (UV) energy and creates vitamin D. Then, a cascade of chemical reactions turns vitamin D into a surprisingly versatile hormone--one that has long been recognized to help the body absorb calcium from the diet to build strong bones. Recent work, however, indicates that vitamin D also bolsters muscle strength, insulin action, immune health, and the body's natural defenses against cancer (SN: 10/9/04, p. 232).

Inhabitants of the tropics typically have plenty of vitamin D. says Robert P. Heaney of Creighton University in Omaha, Neb. However, studies are now showing that people throughout the industrial world lag far behind. Many in temperate and colder climates don't reach the doses currently recommended to protect bone health, much less the far-higher amounts that research has been linking to additional health-promoting functions.

Some scientists are campaigning for additional vitamin-D enrichment of foods. Others advocate that people spend more time outdoors to increase vitamin D-producing sun exposure. Many hold that the boost must come largely from supplements.

What researchers who study vitamin D do agree on is that many people would benefit from more of the vitamin. At issue is only how much.

OUT OF THE TROPICS The Food and Nutrition Board of the National Academies in Washington, D.C., currently recommends that people from infancy through age 50 get 200 international units (IU) of vitamin D per day, that those ages 51 through 70 receive 400 IU daily, and that anyone over 70 get a net of 600 IU from sun, food, and supplements.

That's easy enough to do if you're, say, a white person working outdoors during the summer in New Jersey. In shorts and a T-shirt, such a person can soak up enough ultraviolet rays to produce 12,000 IU of vitamin D within 20 minutes, notes Reinhold Vieth of the University of Toronto.

That production would plummet if the person stayed indoors or slathered on UV-blocking sunscreen or covered up with clothing when out of doors, as recommended for protection against skin cancer.

Global location and skin color also affect the amount of vitamin D a person's skin manufactures. UV intensity falls as one moves from the equator toward Earth's poles, increasing latitude. Evolution compensated by selecting for increasingly, unpigmented skin in northern populations, says Boston University endocrinologist Michael F. Holick.

Melanin pigment protects the skin from the damage of UV rays but also lowers the skin's production of vitamin D. In the March American Journal of Clinical Nutrition, Holick quantifies this effect: Fair-skinned people who sunburn easily and rarely tan need just 2 to 10 percent as much sun exposure to produce a unit of vitamin D as do people with the darkest skin.

Season also matters. Holick has found that from the latitude of San Francisco northward--or from Buenos Aires southward--for 3 to 6 months a year, no amount of exposure will generate substantial vitamin D in even the palest skin.

Holick composed a map of North America that shows the minutes of exposure each skin type needs to generate some 1,000 IU of vitamin D without risking sunburn. For instance, a dark-skinned individual living in Anchorage can get that amount in 20 to 30 minutes midday in July, Holick reports in his new book The UV Advantage (2004, with Mark Jenkins, ibooks). A pale person in Honolulu might do it in 1 minute.

FINDING SUFFICIENCY Severe vitamin D deficiency, softens bones. In children, the result is rickets, characterized by malformed legs. Adults may develop a rare condition called osteomalacia, distinguished by weakened muscles as well as bones. Seventy-five years ago, when the cause of rickets and osteomalacia was first recognized, the remedy was vitamin D-rich cod liver oil. Later, the United States mandated that dairies fortify milk with 100 IU of vitamin D per 8 ounces, and rickets essentially disappeared.

However, rickets has staged a comeback in the U.S. There is no national count, but according to Laurence Grummer-Strawn of the Centers for Disease Control in Atlanta, between 1997 and 1999, "5 per million Georgia children were hospitalized with rickets due to vitamin D deficiency." All were African American, 8 to 21 months old. Numbers could be higher in more-northern locales.

Scientists offer several explanations for rickets' reemergence. Vieth notes that breast-feeding has had a revival and that mother's milk delivers little vitamin D. And Holick points out that doctors have been discouraging parents from letting babies get sun without liberal doses of sunscreen. The Food and Nutrition Board last reviewed its vitamin D recommendations in 1997. As part of that effort, a panel of experts including Vieth, Holick, and Heaney was charged to define how the vitamin should be monitored in people. The active form wasn't deemed suitable because it's manufactured in the body on demand, so it doesn't directly correlate with vitamin D intake and production.