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Your Own RDA

Study suggests future with individualized nutrient quotas

WEDNESDAY, Nov. 21, 2001 (HealthDayNews) -- One day you may have your own tailor-made Recommended Daily Allowance for various nutrients based on your body's chemical "thermostats," instead of relying on the government's generalized list, suggests new research.

Nutritional requirements now are little more than educated guesses at what your body is getting from its diet and what it really needs, says Roger Sunde, professor of biochemistry and nutritional science at the University of Missouri-Columbia. He says he hopes his study on selenium will be the start of a new approach to meet individual nutritional needs based on readings of chemical markers.

Selenium is an essential trace element found in soil. It's used by the body in a number of ways, most notably in the thyroid. It also plays a part in fighting cancer, reducing the risk of heart disease and arthritis and improving fertility, according to previous research.

Sunde studied rats fed a diet lacking selenium. He found that the enzyme glutathione peroxidase (GPX) senses the amount of selenium in the body and regulates how much of the nutrient each cell should store. He says GPX is a good indicator of selenium status because it falls dramatically when you don't have enough selenium, and other nutrients don't seem to affect its levels.

Sunde, who has studied selenium for 30 years, discussed his findings at last month's annual meeting of the American College of Nutrition.

"Many of our requirements, like iron and calcium and zinc, are based on factorial analysis where we are trying to make guesstimates about what is required," he says. "Humans can adapt to a wide variation in calcium or iron or zinc and maintain relatively normal body concentrations. The body does that by increasing or decreasing absorption or excretion. Simply setting a requirement based on how much is in the diet may or may not have the desired effect on the status of the individual because of these mechanisms. This is driven by thermostats, which sense an individual's nutrient status and regulate absorption and excretion, so we need to know what those thermostats are set at."

Body thermostats are a recognized concept in other areas of science, Sunde says.

"They've been the Holy Grail people have looked for in biology," he says. "The thermostat that regulates body weight or satiety has something that's been long sought and certainly important in terms of treating human obesity. The relevance here is that if we can use GPX as an indicator of what the selenium thermostat is saying, then we're actually letting the human [body] tell us what their selenium status is, rather than making a guess."

Though the concept is new in the study of nutrients, Sunde says he hopes his study will be a model in the search for thermostats for other nutritional needs, which can vary dramatically from person to person.

"We've been setting nutrient requirements for populations when we know there are differences for individuals," he says. "We can read the thermostat [in the future], not only for young adults or rapidly growing children, but for someone who is old, or has diabetes, or is pregnant. These are all individuals for whom assessing nutrient requirements is very difficult. They probably all have slightly different disease patterns. They're eating differently, and their nutritional needs may not match well with a young adult, the typical college student used for medical studies."

Dr. Raymond F. Burk, professor of medicine and pathology at Vanderbilt University School of Medicine who has studied selenium since 1964, says Sunde's research is important.

"The Institute of Medicine has determined that what it wants to do is not determine a requirement, but what's best for optimal health," Burk says. "What Dr. Sunde and I are trying to do is find out how much selenium does it take to satisfy the body's requirement to make selenoproteins, with the assumption that this gives you the optimal health." Selenoproteins, like GPX, are enzymes that help metabolize certain compounds.

Despite all the research and new information about selenium, Burk says a great deal still is not known about the trace element and how it interacts with other essential nutrients.

"You can make an animal deficient in selenium, and if you made him deficient in vitamin E at the same time, the liver would necrose [die] and the animal would die," he says. "There was something significant going on there. There's still a lot to be learned about how it [selenium] functions and what it does for us."

What To Do

For detailed information on selenium, read this fact sheet from the National Institutes of Health Clinical Center.

Wondering just how much of any nutrient you're getting? Try plugging your daily diet into this Interactive Healthy Eating Index from the U.S. Department of Agriculture.

SOURCES: Interviews with Roger Sunde, Ph.D., professor of biochemistry and nutritional sciences, University of Missouri-Columbia; Raymond F. Burk, M.D., professor of medicine and pathology, Vanderbilt University School of Medicine, Memphis, Tenn.; Oct. 4-7, 2001, annual meeting, American College of Nutrition, Orlando, Fla.
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