This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. For human females lactation is the most energy-demanding phase of reproduction; it amounts to an increase in energy needs of approximately 30% above non-pregnant non-lactating levels. Although the assumption has been that women will meet this increase in energy need by increasing food intake and/or mobilizing body fat accrued in pregnancy, empirical studies have failed to demonstrate the increases in food intake and utilization of body fat stores expected. This has prompted researchers to examine a third option, i.e. reductions in energy expenditure. The purpose of this study is to improve our understanding of the energy expenditure in lactation by examining the effect of lactation on one measure of energy expenditure, energetic efficiency in exercise. Specifically, we hypothesize that energetic efficiency in exercise is higher in the lactating than the post- lactating state. To test this hypothesis we will measure energetic efficiency in exercise in a sample of 39 lactating women practicing exclusive breastfeeding at peak lactation (2-3 months postpartum) and two months after breastfeeding has ceased completely. At both peak lactation and post-lactation, energetic efficiency will be measured on two non-consecutive days within a 5-7 day period using an identical graded exercise test on a cycle ergometer. We expect to find that lactating women use energy more efficiently in exercise, and that his finding will help explain how they meet the energy demands of lactation.
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