ITHACA, N.Y. -- Women with low body iron, yet who are not anemic, have a much harder time sustaining exercise and adapting to training, concludes a new Cornell University study. But after a period of training, iron-deficient women who boost their body iron by taking supplements can improve their exercise endurance twice as much as iron-depleted women.
"Millions of women are working harder than they need in order to exercise or physically work, and they can't reap the benefits of endurance training as easily. As a result, exercise is more difficult so these women are more apt to lose their motivation to exercise," says Jere Haas, the Nancy Schlegel Meinig Professor of Maternal and Child Nutrition and director of the Division of Nutritional Sciences at Cornell.
Iron deficiency that is not as severe as anemia is a common problem, affecting about 16 percent of U.S. women and 40 to 80 percent of women in developing countries, most of whom are unaware of their condition. The new study, published in a recent issue of the Journal of Applied Physiology (Vol. 88, 2000), provides mounting evidence that iron depletion without anemia should be of greater concern. Other researchers have reported recently that moderate iron deficiency also compromises memory and verbal learning in teen-agers.
In a previous study, published in 1998, Haas found that iron-depletion in nonanemic women results in lower capacity for physical work and impaired exercise performance. The new study by Haas and three other researchers shows that iron deficiency impairs the ability to increase aerobic endurance after a period of exercise training. "In other words, we now know that iron-deficient women don't benefit from training as much as women with higher iron status because of impaired metabolic responses to exercise, but that iron supplementation can compensate," Haas says.
In the latest study, 42 iron-depleted (but not anemic) women aged 18 to 33 participated in a randomized, double-blind trial. Half the group received an iron supplement (100 mg of ferrous sulfate per day) and half a placebo while training for four weeks, 30 minutes a day, five days a week at 75 to 85 percent of maximum heart rate.
Although all the women increased their endurance because of the training, those who improved their iron status cut 3.5 minutes off a 15-kilometer time trial on a stationary bike, compared with an average 1.5-minute improvement in the placebo group.
"That represents a 10 percent improvement in endurance performance after four weeks of training for the women who received iron," says Pamela Hinton, a postdoctoral research associate who is the first author of the study paper. "The study shows that women with moderate iron deficiency might not be getting all the fitness benefits of exercise training. They can improve aerobically but not optimally. Exercise for them is more difficult than for women with adequate iron."
Women who are physically active, dieting or vegetarians are particularly at high risk for iron depletion, the researchers point out. "In developing countries, iron depletion can have dramatic consequences on a woman's ability to do physical work and make a living," says Haas, who studies the functional consequences of mild to moderate forms of malnutrition worldwide.
Iron is an essential component of hemoglobin in the blood and plays an important role in oxygen transport and utilization. When people consume iron-deficient diets, they first deplete their iron stores in the liver; at the final stage, they become anemic due to insufficient iron to produce new red blood cells. Haas and his co-authors focused on the first stage, while most medical practitioners are interested in the final stage.
"Because these women aren't anemic, we know they are getting enough oxygen to their muscles, but in iron-depleted, nonanemic women, the lack of iron has an effect at the muscle metabolism level," explains Hinton, whose co-authors at Cornell, besides Haas, were Christina Giordano '98 and now a medical student and Thomas Brownlie, M.S. '99, and now a Cornell doctoral student.
To prevent iron depletion, the researchers recommend red meat; for vegetarians, they recommend citrus fruit and juice (vitamin C) with meals to improve absorption from iron-rich foods such as legumes, whole grains and green vegetables.
The study was supported, in part, by the Mead Johnson Research Fund and the National Institute of Child Health and Human Development.
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-- For an online copy of the paper in the Journal of Applied Physiology, 88:1103-1111;2000.
Or the 1998 paper, 84:1768-1775;1998
-- For more information on Jere Haas
http://www.human.cornell.edu/faculty/facultybio.cfm?netid=jdh12&facs=1
-- For information on the Division of Nutritional Sciences
http://www.human.cornell.edu/units/DNS/index.cfm
Journal
Journal of Applied Psychology