The study appears in the May 16 issue of the Journal of the American Medical Association. Researchers at the Health Resources and Services Administration and the Centers for Disease Control and Prevention (CDC) also participated in the study.
"Breastfeeding is extremely valuable for infants boosting their immune systems and their mental abilities, and reducing their risk for infection" said Duane Alexander, M.D., Director of the NICHD. "At this point, exercise, nutrition, and heredity appear to have a much greater influence on young children's weight than does breastfeeding alone."
The researchers undertook the study to reexamine an earlier study conducted by scientists in Bavaria, explained the study's first author, Mary Hediger, Ph.D., of NICHD's Division of Epidemiology, Statistics and Prevention Research. The Bavarian researchers reported that the greater the length of time that infants were breast fed exclusively, the less likely they were to be overweight.
The U.S. examined information from the birth certificates of 2685 children. This information was obtained from The Third National Health and Nutrition Examination Survey (1988-1994), a nationally representative sampling conducted by the CDC. The researchers interviewed the children's mothers when the children were between 3 and 5 years of age, asking for such details as whether the children were breast fed or formula fed, how often they breast fed their children, at what age they stopped breast feeding, and whether the children received infant formula in addition to breast milk. The children were then measured for height and weight as part of a comprehensive physical examination. Extremely low birth weight children, many of whom are likely to be premature, were not included in the study. The researchers determined children's weight status with a formula known as the body mass index (BMI), a calculation used world wide to determine a person's amount of body fat.
The researchers found that children who were breast fed had a 16 percent reduced risk for being overweight. However, Dr. Hediger pointed out, the length of time that children breast fed and the timing of introduction of solid foods had little influence on their risk of being overweight.
Breast fed infants who began eating solid foods later than did other breast fed children had a very slightly reduced risk for being overweight about 0.1 percent for each month they were delayed in eating solid food. The researchers noted, however, that the strongest predictor of child's BMI was the mother's BMI. In fact, children were three times more likely to be overweight if their mothers were either overweight or obese.
Dr. Hediger explained that it is not known whether or not maternal overweight and obesity influences children's risk for overweight because of heredity, environmental factors, or a combination of both. For example, Dr. Hediger said, it is possible that many children of overweight and obese mothers are overweight because of improper diet and lack of exercise and not because their mothers did not breast feed them.
Dr. Hediger added that being overweight between ages 3 and 5 is only slightly predictive of overweight and obesity later in life. With a good diet and exercise, overweight children can avoid becoming overweight or obese in adults.
"It cannot be too strongly emphasized that breastfeeding has numerous attributes that render it the preferred feeding choice for almost all infants," the study authors wrote. "However, duration of full breastfeeding does not appear to be predictive of or necessarily have preventive properties for overweight in early childhood, and encouraging breast feeding for overweight prevention would not be as effective as moderating familial factors in preventing early childhood overweight."