ITHACA, N.Y. -- Among women who are not anemic, only those with tissue-iron deficiencies can benefit from taking iron supplements, concludes a new study by Cornell University nutritionists.
"Supplementation makes no difference in exercise-training improvements in women with low iron storage who are not yet tissue-iron deficient or anemic," says Thomas Brownlie, the first author of the study and a Cornell doctoral candidate in nutritional sciences.
Women with low body iron, but who are not anemic, may not experience any improvements following training if their tissues are low in iron. Whereas women who have low iron storage in their liver only, and who are not anemic, appear to have no functional impairments.
The study, published in the American Journal of Clinical Nutrition (2004; 79:437-43), is the first to show differences between the physical abilities of nonanemic women with low-liver vs. low-tissue iron. The researchers are also the first to show that low iron without anemia does have functional consequences in humans.
About 10 to 12 percent of U.S. women and 40 to 80 percent of women in developing countries are iron deficient but not anemic, yet most are unaware of their condition. The new study provides mounting evidence that mild to moderate iron depletion should be of greater concern.
It has long been known that iron-deficiency anemia compromises physical work capacity because of decreased oxygen delivery to the working muscles and decreased ability to produce energy at the tissue level. In previous work, the Cornell researchers had shown that mild iron deficiency also reduces endurance, the capacity for physical work and exercise performance, and that iron supplementation improves exercise training.
"Millions of women who are mildly iron deficient must work harder than necessary when exercising or working physically, and they can't reap the benefits of endurance training very easily," says Jere Haas, the Meinig Professor of Maternal and Child Nutrition at Cornell and a co-author of the study. "As a result, exercise is more difficult so these women are more apt to lose their motivation to exercise." The new study shows that iron supplementation can help nonanemic women, but only those who have become tissue-iron deficient.
Forty-two iron-depleted (but not anemic) women, ages 18 to 33, participated in the six-week study. They first cycled 15 kilometers (about nine miles) on a stationary bike to determine how quickly they could complete the bout before supplementation. Then, half took iron supplements (100 milligrams of ferrous sulfate per day) while the other half took placebos. After participating in an exercise program during the final four weeks of the study, the women then cycled nine miles again.
Only the tissue-iron-deficient women who did not take iron supplements failed to significantly improve on the endurance test.
A woman's tissue iron status can be assessed with a technique called serum transferrin receptor concentration, which can be given by health professionals. "It would be useful for women who test low for iron but who are not yet anemic to have this test," says Brownlie. "Women found to be tissue-iron deficient will find exercise exceedingly difficult without improving their iron status -- which could be achieved by increasing consumption of iron-rich foods or iron supplementation."
Women who are physically active, dieting or are 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. 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 stores of iron in the liver; at the final stage, they become anemic due to insufficient iron to produce new red blood cells.
To prevent iron depletion, the researchers recommend red meat; for vegetarians, they recommend consuming citrus fruit and juice (vitamin C) with meals to improve absorption from iron-rich foods, such as legumes, whole grains and green vegetables.
The other study authors are Virginia Utermohlen, M.D., professor of nutritional sciences at Cornell, and Pamela Hinton, assistant professor of nutrition at the University of Missouri.
The study was supported, in part, by the Mead Johnson Research Fund and the National Institute of Child Health and Human Development.