(American Journal of Clinical Nutrition) -- Iron deficiency without anemia occurs in about 12% of premenopausal women in the United States, and may negatively affect adaptation to aerobic training in women by reducing their endurance capacity. Brownlie et al., publishing in the American Journal of Clinical Nutrition, investigated the effect of iron supplementation on aerobic fitness in women who were previously untrained and iron-deficient.

The 41 healthy women in the study, ages 18-33, were all iron-deficient but none was anemic. They were given either a 50 mg iron supplement (22 subjects) or a placebo (19 subjects) twice daily for 6 weeks in a randomized, double-blinded trial. During the last 4 weeks of the study, they trained 5 days a week on an ergometric cycle equipped with a heart rate monitor and digital output of cadence and work. They gradually increased their workloads until they were cycling for 10 minutes at 75% and 15 minutes at 85% of their maximum heart rates. Exercise tests were conducted on a cycle ergometer before and after the aerobic training.

The iron-supplemented group experienced twice the improvements in maximal oxygen uptake and respiratory exchange ratio than the placebo group. The women with the most depleted initial iron status experienced the most improvement in their adaptation to aerobic exercise. The study suggests that otherwise healthy women may realize added benefits from their exercise programs by improving marginal iron deficiency.