The cost of too little magnesium.

Most people work harder than they really need to, a new study suggests. There's a simple solution, though: Eat more magnesium-rich greens and grains.

Magnesium plays an important role in regulating how well the body converts food into energy, notes physiologist Henry Lukaski of the Department of Agriculture's Human Nutrition Research Center in Grand Forks, N.D. Since the majority of adults don't eat the recommended daily allowance (RDA) of this mineral--about 280 milligrams for women and 350 mg for men--he wondered whether the general U.S. population might be using energy inefficiently

To test the idea, Lukaski recruited eight women, age 55 to 70, to live on the metabolic ward of a local hospital for 5 months. There, his team could oversee the preparation and consumption of the women's food, measure their excretion of magnesium, and periodically biopsy muscle to verify diet-related changes in the amount of magnesium their bodies incorporated.

For the first month, the women received a magnesium-enriched diet (about 390 mg per day). For the next 3 months, the magnesium content of their otherwise balanced diet was only about two-thirds of the RDA. Magnesium intake was boosted again--to 420 mg per day--for the remainder of the study.

At the end of each dietary phase, the women rode stationary bicycles to keep their hearts pumping at 70 to 80 percent of an age-adjusted maximum rate. "Regardless of the diet, they were able to do the same amount of work," Lukaski says. "However, their oxygen consumption was about 15 percent higher when the diet was low in magnesium than when it was adequate." The women's average heart rate was nine beats per minute faster throughout the low-magnesium phase of the study--an increase "that was biologically significant," he says.

"These two indicators tell us that the women were under more physiological stress when they had low magnesium in the diet." He compared the low-magnesium effect to the extra effort required of a person carrying a backpack equivalent to 15 percent of his or her weight while climbing stairs: "You're going to use more energy to do that work."