Supplemental Iron Improves Submaximal Exercise Performance in Non-anemic Iron Depleted Women

Recent estimates of the prevalence of anemia among non-pregnant Mexican women of reproductive age range from 15 percent for a nationally representative sample from the 1988 National Nutrition Survey to 29 percent for low socioeconomic women from the PROGRESA baseline survey of 1998 (Mexican Ministry of Health, unpublished report). The cause of most of this anemia is iron deficiency. It can be assumed that time between 15 and 30 percent of Mexican women also experience iron deficiency without also experiencing anemia. In some studies of anemic women, iron deficiency has been shown to result in reduced physical performance as measured by VO2max (the oxygen consumption at maximum physical exertion), presumably due to reduced oxygen (02) transport related to the reduced hemoglobin (Hb) concentration. However, no effects of iron deficiency have been shown on VO2max in women who were not anemic. Given that Hb concentration is not reduced in iron deficient non anemic women, it is expected that VO2max remains unaffected. However, this less severe form of iron deficiency may still affect physical performance. The important role of iron in the synthesis of many of the enzymes involved in oxidative metabolism suggests that moderate levels of iron deficiency may impair muscular energy transformation at levels of exertion that are not as severe as those reflected in VO2max. The choice of the appropriate sub maximal physical performance test is therefore critical to successful demonstration of the effects of moderate iron deficiency on response to exercise stress.

In this study the investigators proposed to test the effects of iron deficiency on physical performance using a randomized double blind design that assigns marginally iron depleted, physically active women between 18 and 45 years to two treatments. One half of the women will be randomly assigned to a group that receives a daily oral iron supplement (20 mg of elemental iron as slow release ferrous sulfate) while the remaining women received a placebo. Investigators anticipated that iron supplementation would replete iron body stores in all women and increase Hb only in those who are marginally anemic. Investigators tested the physical performance response to iron treatment with two exercise tests. It was hypothesized that VO2max would not be affected by iron supplementation. A graded submaximal exertion test (30 60% of VO2max) to estimate energy expenditure (EE), gross efficiency (GE), and net efficiency (NE) was administered to test for an increase in work efficiency due to iron supplementation.

This study has important implications for the definition of dietary iron requirements in this segment of the Mexican population. The physical performance test used in this study (EE, GE, and NE), while well recognized, have never been applied to this type of research question nor has it been examined in relation to endurance performance in Mexican women. Investigators believe that previous reliance on the VO2max test to measure performance has biased conclusions toward concern only for the anemic subject, while the less severe forms of iron deficiency have been considered benign.

This study, if successful in supporting the stated hypotheses, will serve as further confirm the deleterious effects of iron deficiency without anemia and justify future research on more practical measures of performance related to female economic productivity and time and effort allocated to child care and household responsibilities.

Inclusion criteria

• non-smoking

Exclusion criteria

• acute or chronic injury or illness at time of screening, hemolytic anemia
• physician-diagnosed asthma, musculoskeletal problems, or eating disorders
• pregnant or lactating, pregnant with the previous year
• excessive alcohol consumption or recent use of recreational drugs
• recent history of eating disorders, or use of prescription medications and/or vitamin or mineral supplements that would interfere with dietary iron absorption