Objective. Previous research reported that breast milk lactic acid (LA) levels increase after lactating women complete a bout of exhaustive exercise, resulting in poor infant acceptance of the postexercise breast milk. This highly publicized finding may not apply to more practical, everyday exercise conditions of lactating women. The purpose of the present study was to reexamine the composition and infant acceptance of postexercise breast milk while controlling maternal diet, exercise intensity, and the method, timing, and assessment of infant feeding.

Research Design and Methods. Twenty-four women, 2 to 4 months’ postpartum, completed 3 test sessions: a maximal oxygen uptake test, a 30-minute bout of moderate exercise, and a resting control session. One hour before and 1 hour after each session, participants fully expressed their milk, placed it in a bottle familiar to the infant, fed their infant, and rated their infant’s acceptance of the milk. Each feeding was videotaped and viewed individually by 3 lactation consultants who rated infant acceptance; consultants were blinded to the test sessions. Milk was analyzed for LA and infant milk consumption was measured.

Results. There were no differences in presession versus postsession values for maternal skin temperature, breast milk temperature, and infant milk acceptance as judged by either the mothers or lactation consultants. These results prevailed despite a small but significant increase in breast milk LA premaximal versus postmaximal exercise (0.09 vs 0.21 mM, respectively); there was no difference in milk LA premoderate versus postmoderate exercise, or prerest versus postrest.

Conclusion. These data support the hypothesis that moderate or even high-intensity exercise during lactation does not impede infant acceptance of breast milk consumed 1 hour postexercise.

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