The Possible Adventures
"The recommended intakes are too low. Virtually everyone working in the field believes that."
— Susan Harris, D.Sc., N87
The research adds some urgency to a discussion that has been going on among researchers, physicians and the government -- exactly how much vitamin D do people need? The current recommended daily Adequate Intake is 200 international units (IU) for children and adults up to age 50, 400 for those ages 51 to 70 and 600 for those over 70. Those numbers were set in 1997 by the Food and Nutrition Board of the Institute of Medicine -- the U.S. advisory agency that recommends nutritional intakes -- and were based on the amount of vitamin D then believed to be needed to maintain bone health. These are also the recommendations used by Canada's national health agency. But based on newer research, many researchers are asking for an update.
"The recommended intakes are too low. Virtually everyone working in the field believes that," says Susan Harris, D.Sc., N87, a scientist in the Bone Metabolism Laboratory at the Jean Mayer U.S.D.A. Human Nutrition Research Center on Aging (HNRCA) at Tufts and an adjunct associate professor at the Friedman School.
And, she says, even by current standards, far too many people in North America aren't getting enough. According to the third National Health and Nutrition Examination Survey (NHANES III), at least 40 percent of men and 50 percent of women in the United States have lower-than-optimal blood levels of vitamin D for at least part of the year.
The ABCs of D
Vitamin D's best-understood job is regulating calcium. It functions as a hormone, triggering the intestines to absorb calcium from food and add it to the blood stream, where it can be brought to organs and muscles that require it to function, or to the bones for bone growth and repair. In addition to helping muscles, the brain, the nervous system, the pancreas and reproductive organs, vitamin D has also been shown to affect immune function and to reduce inflammation.
Back in grade school, you might have learned that vitamin D was the "sunshine vitamin." Your body makes it when ultraviolet B (UVB) radiation from sunlight hits cholesterol compounds in the skin, converting them to vitamin D precursors. These precursors travel to the liver and kidneys, where they are converted to the active form of vitamin D.
The amount of vitamin D you synthesize depends not only on your sun exposure, but on the season (winter is less desirable than summer), your skin color (darker tones absorb less UVB) and where you live (the farther from the equator, the less UVB is available). In fact, Americans who live north of 42 degrees latitude (including residents of Boston, Milwaukee and Seattle) do not get enough D-making UVB from roughly mid-October to mid-March, regardless of how much time they spend outdoors. The sun comes in at such an angle that the necessary rays never hit the earth's surface.
Unlike most other vitamins, adequate vitamin D is tough to acquire solely through a balanced diet, although it is naturally present in some fatty fish and fish liver oils, and in small quantities in beef, liver, cheese and egg yolks. In the U.S., most of the vitamin D we consume is in the form of fortified milk, cereal or juice.
Historically, the result of severe D deficiency was rickets, a childhood disease that softens the bones -- its most obvious symptom is a tell-tale bowlegged stance. Once prevalent, rickets mostly disappeared in the U.S. in the 1930s, with the advent of D-fortified milk.
Occasionally, Dwyer says, health professionals would see rickets surface in populations that didn't consume milk, fish or other animal products, including the children of macrobiotic vegans in the 1970s. Today, vegans can obtain adequate vitamin D through fortified soy products and supplements.
The 'hot vitamin'
In adults, a severe lack of vitamin D can lead to osteomalacia, a painful condition of weakened bone and muscle. But even people who are not D-deficient, but simply have lower-than-ideal levels, may experience health consequences.
"We're no longer talking only about avoiding frank deficiencies," Harris says. "We're talking about maintaining optimum health, and reducing our risks in more subtle ways." (continued)
Feature by Helene Ragovin, senior writer, Office of Publications
Homepage image by istockphoto
This story ran online on Jan. 12, 2008. It originally appeared in the Fall 2008 issue of Tufts Nutrition.