The study, appearing in the February 17 New England Journal of Medicine, was conducted by Cheryl A. Lovelady, Ph.D. and her coworkers at the University of North Carolina in Greensboro.
"Being overweight may cause serious health problems," said NICHD Director Duane Alexander, M.D. "This study shows that it's safe for overweight women to begin a sensible weight loss program without posing a risk to their infants."
Dr. Lovelady explained that weight gained during pregnancy might contribute to obesity later in life. Losing this extra weight soon after pregnancy may help many women to avoid later obesity and its long-term health effects. An Institute of Medicine report earlier had concluded that overweight breast-feeding women could probably lose about 2 kg (4.4 pounds) per month without affecting their production of milk. However, Dr. Lovelady pointed out, no studies existed to prove whether this assumption was true.
Dr. Lovelady stressed that a woman who is breast feeding should first consult her physician and nutritionist before undertaking any weight loss program. She added that breast-feeding women should not attempt to lose weight if they are only a few pounds overweight.
"Breast-feeding mothers who are only 5 pounds overweight shouldn't try to lose weight," she said. Unless a woman has sufficient fat reserves, dieting may hinder milk production and also cause the woman to feel fatigued.
The researchers recruited 40 overweight, breast-feeding women for the study. The women took part in the study for 10 weeks, beginning at the 4th week after they gave birth. Overweight was defined as having a body mass index of 25 to 30. Body mass index is a mathematical formula used to calculate body fat from a person's height and weight. Dr. Lovelady explained that a 5' 4" woman having a body mass index of from 25 to 30 would weigh between 145 and 175 pounds. The authors wrote that 51 percent of U.S. women have a body mass index of more than 25
Of the 40 women, 21 were assigned to the diet and exercise group, and 19 were assigned to the control group. All the women had given birth to full-term, full-size infants delivered without C-section. Women in the diet and exercise group reduced their food intake by 500 calories, essentially by avoiding fatty and sugary foods. These women also began some form of aerobic exercise-such as brisk walking, jogging, or aerobic dancing-for 15 minutes a day. The exercise time was increased by two minutes a day, until the women were exercising for 45 minutes a day. To help the women stick to their diets, the researchers also provided them with 6 low-fat, low-sugar frozen entrees per week during the course of the study.
The control group exercised no more than once a week, and did not change their dietary habits. All the women received a daily multivitamin containing at least 50 percent of the recommended daily allowances for breast-feeding women.
The women in the diet and exercise group lost an average of about 5 kilograms (about 10 pounds) by the end of 10 weeks. The women in the control group lost an average of .8 kilograms (about 2 pounds). In contrast to the diet-and-exercise group, which lost weight at about the same rate, the control group varied in their weight loss. In fact, a few of the women lost nearly 10 pounds, while a few others gained that amount.
The women in the diet-and-exercise group reported that they seemed to be producing enough milk. Also, they reported that their infants were not crying any more than normal. (Infant fussiness is a possible indication of insufficient milk production.) Similarly, the women did not report feeling tired. In fact, most said that the exercise sessions seemed to give them more energy.
The infants of the women in the diet and exercise group grew at a normal rate, as compared both to the infants of the women in the control group, as well as to those in larger studies of infant growth.
"In conclusion," the authors wrote, "a program of moderate exercise and energy restriction was successful in inducing weight loss in overweight, lactating mothers without harming the growth of their infants in the early postpartum period."
The study builds upon the findings of an earlier study, published in the New England Journal of Medicine, on February 17, 1994. This study found that breast-feeding mothers could not lose weight if they began an exercise program without also cutting the amount of calories they consumed.
"You've got to have the caloric restriction if you're going to see weight loss," Dr. Lovelady said.
The NICHD is one of the Institutes comprising the National Institutes of Health, the Federal government's premier biomedical research agency. NICHD supports and conducts research on the reproductive, neurobiological, developmental, and behavioral processes that determine and maintain the health of children, adults, families, and populations. The NICHD website, http://www.nichd.nih.gov, contains additional information about the Institute and its mission.
An in-depth explanation of body mass index is available on the website of the National Institute of Diabetes and Digestive and Kidney Diseases, at http://www.niddk.nih.gov/health/nutrit/pubs/unders.htm#howis.