Moderate aerobic exercise has no adverse effects on the quantity or quality of breast milk produced by nursing mothers, and can significantly improve the mothers' cardiovascular fitness, according to researchers at the University of California, Davis. Findings from the new study on the effect of aerobic exercise on human breast milk, headed by Kathryn G. Dewey, a professor of nutrition and an authority on maternal and child nutrition, will appear in the Feb. 17 issue of the New England Journal of Medicine. "After the birth of a child, many women are eager to lose weight, and although exercise has often been considered safer than dieting, there has been little information available about its impact on lactation," said Dewey. "This study indicates that breast-feeding women can safely participate in frequent, moderate aerobic exercise without adversely affecting the volume or composition of their breast milk." For 12 weeks the study monitored 33 women, beginning six to eight weeks after the births of their children. All were exclusively breast-feeding their infants, with no formula supplements, and had not previously been exercising. Eighteen women were randomly assigned to an exercise group and 15 to a nonexercising control group. The exercise group participated in individual aerobic exercise programs, including rapid walking, jogging or bicycling, for 45 minutes each day, five days per week. Measurements of energy expenditure, resting metabolic weight, body composition, dietary intake and breast-milk volume and composition were made for all participants at the beginning, midpoint and end of the study. Heart-rate monitors were used to estimate energy expenditure, and the mothers recorded their dietary intake. Maximal oxygen uptake, a measure of cardiovascular fitness, as well as blood levels of the hormone prolactin, an important regulator of lactation, were measured at the beginning and end of the study. In addition, milk volume was measured and samples were collected during the three monitoring periods to determine milk protein, fat and lactose levels. At the end of the 12-week study, Dewey and colleagues found: • Women in both groups experienced weight loss. The rate of weight loss and the decline in the percentage of body fat after childbirth did not differ between the exercise and control groups, because women in the exercise group compensated for their increased energy expenditure by eating more. • There was an important improvement in the aerobic fitness of the exercising women, as measured by maximal oxygen consumption. • Despite increased activity, by the end of the study, the exercising group's energy expenditure was not significantly different than that of the control group because the exercising women cut back on other activities. This suggests that, among breast-feeding women, high levels of energy expenditure may be difficult to sustain because of fatigue or time constraints, according to Dewey. • There was no significant difference between the two groups in terms of infant breast-milk intake, energy output in the milk or infant weight gain. • Prolactin levels in the breast milk did not differ between the two groups, suggesting that previously observed short-term increases in the level of that hormone among nonlactating women following exercise do not influence the basal level of prolactin. • There was no indication of nursing difficulties following exercise sessions. Collaborating with Dewey on this study were Bo Lönnerdal, a UC Davis professor of nutrition; post-doctoral researcher Cheryl A. Lovelady; doctoral candidate Megan McCrory and staff research associate Laurie A. Nommsen-Rivers. The research was funded by the National Institutes of Health.