Overweight and obese women have significantly less success breast-feeding their babies than their normal-weight counterparts, according to a new Cornell University/Bassett Hospital study, and biological factors largely may be why.

And the heavier the mother, the researchers found, the less successful she was at initiating and maintaining breast-feeding.

"Although other research has looked at obese rats and lactation, this is the first study to look at obesity and lactation in women," said Kathleen Rasmussen, professor of nutritional sciences at Cornell. She conducted the study, which was published in the December issue of the American Journal of Clinical Nutrition (Vol. 66, pp. 1371-78), with first author Julie Hilson, a Cornell graduate student, and Chris L. Kjolhede of Bassett Hospital in Cooperstown, N.Y. The study is also the first to separate the effects of being overweight (up to 20 percent above ideal body weight or a body mass index [BMI] between 26 and 29) from those of being obese (20 percent or more above ideal body weight or a BMI greater than 29), she said.

"In addition, since our sample was a very homogenous group of white women who tried breast-feeding, confounding racial, ethnic and socioeconomic (SES) factors that could have influenced attempts to begin and maintain breast-feeding were eliminated," she added.

Animal models provide strong evidence that the link between obesity and poor breast-feeding results could be biologically based, Rasmussen pointed out. "We have found that obese rats have both hormonal and physiological irregularities that could contribute to poor establishment and maintenance of lactation."

Mechanical difficulties also may play a role. "Obese women have large, flat breasts with large aerolas that make latching on more challenging for the infant," wrote the researchers who looked at a sample of 810 rural, white women who were disadvantaged, primarily economically; all the women attempted breast-feeding after giving birth at Bassett Hospital, which encourages breast-feeding.

Whether and how long mothers breast-feed is important because breast milk can protect children from a variety of childhood illnesses, said Rasmussen, an expert in pregnancy and lactation.

"To increase the rates of breast-feeding, which the Surgeon General has identified as a goal for the coming years, our research suggests that we'll need to focus on the obese and better understand this potential biological risk factor that we had not been aware of before," said Rasmussen who is now seeking funding to further explore the biological influences of obesity on lactation.

In a related preliminary study, published in the November issue of Obesity Research (Vol. 5, No. 6, pp. 538-541), Rasmussen, Kjolhede and Cornell undergraduate student Kristen Elliott and research technician Effie Gournis looked at the link between breast- or bottle-fed babies and the weight of the children during adolescence. After examining the medical charts of 136 white women who gave birth at Bassett between 1977 and 1980 and their children's height and weight charts through adolescence, they found that the babies who were breast-fed for less than two months were almost two and one-half times more likely to become obese adolescents.

When the researchers controlled for gender, birth weight and SES, however, the link between bottle-feeding and adolescent obesity remained significant only for the lower SES group.

"This means that even among this racially homogenous sample, other factors are important in explaining the relationship between infant feeding and the later development of obesity," Rasmussen said. "However, our findings suggest that it is important to direct health education messages about promotion of breast-feeding and the prevention of obesity toward the lower SES groups."

Examining factors that contribute to obesity in adolescence is important because obese adolescents often grow into obese adults, other research has shown, and obesity is related to many adult diseases, including diabetes, cancer and cardiovascular disease. Other studies also have shown that breast-fed infants are leaner than formula-fed infants at one year of age and that obese adults were often obese as children, and, conversely, obese children are more likely to become obese adults. "Obesity in adolescence also increases the risk of morbidity from several diseases in both adult men and women, regardless of obesity in adulthood," said Rasmussen.

About 30 percent of the U.S. adult population is obese, and more than 20 percent of U.S. youth is obese.