The nutritionists from Cornell University are the first to show differences between the physical abilities of non-anemic women with low-liver iron compared to low-tissue iron.

The researchers are also among the first to show that low iron without anemia does have functional consequences in humans.

The study, published in the March issue of the American Journal of Clinical Nutrition (2004; 79:437-43) investigated 42 iron-depleted (but not anemic) women, over a period of six weeks. They first cycled 15 kilometers to determine how quickly they could complete the bout before supplementation. 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 the same distance again. The women's tissue iron status was assessed with a technique called serum transferrin receptor concentration.

Only the tissue-iron-deficient women who did not take iron supplements failed to significantly improve on the endurance test.

Thomas Brownlie, author of the study, explained: "Supplementation makes no difference in exercise-training improvements in women with low iron storage who are not yet tissue-iron deficient or anemic."

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, which play an important role in oxygen transport. Anemia decreases oxygen delivery to the working muscles as well as a decreased ability to produce energy at the tissue level.

Brownlie added: "Women found to be tissue-iron deficient will find exercise exceedingly difficult without improving their iron status. This could be achieved by increasing consumption of iron-rich foods or iron supplementation."

It has been estimated that 10 to 12 per cent of US women and 40 to 80 per cent of women in developing countries are iron deficient but not anemic, yet most are unaware of their condition. High risk groups include women who are physically active, dieting or vegetarians.

The new study provides mounting evidence that mild to moderate iron depletion should be of greater concern.

"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," said 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."

A recent study from Pennsylvania State University also demonstrated another effect of mild to moderate iron depletion. It found that even modest levels of iron deficiency have a negative impact on cognitive functioning in young women. It is also thought to be the first evidence to show that taking iron supplements can reverse the impact on cognition in this age group.