NEW YORK (Reuters Health) - A large study from Scotland is adding new evidence that breast-fed infants are less likely than their formula-fed peers to become obese as children.

Among the more than 32,000 young children researchers followed, those who had been exclusively breast-fed in early infancy were about 30% less likely to be obese than those who had been strictly formula-fed.

The findings suggest that breast-feeding has a "modest benefit" against obesity--an idea that should be taken along with the fact that it has "lots of other benefits and no major disadvantages," said the study's lead author, Dr. John J. Reilly of the University of Glasgow.

He and his colleagues report the findings in the June 8th issue of The Lancet.

Previous research has uncovered a link between breast-feeding and lower odds of childhood obesity, but the meaning of the findings has been unclear due to shortcomings in the studies, according to Reilly's team. Many studies, they note, have been too small or have failed to account for other factors that could explain the relationship between breast-feeding and later obesity--such as family income and education.

In their study, Reilly and his colleagues did factor in families' socioeconomic status--which, he told Reuters Health, "we felt was the main confounder."

The investigators found that among children who were being exclusively breast-fed at 6 to 8 weeks of age, roughly 7% had become obese by the time they were about 3.5 years old. That compared with about 9% of children who were getting only formula when they were 6 to 8 weeks old.

When the children's socioeconomic status, sex and birth weight were factored in, breast-fed children were 28% less likely to be obese and 30% less likely to be severely obese, according to the report.

Reilly acknowledged that not all the variables that affect a child's weight could be accounted for in this study--the major one being the mother's body mass index (BMI), a measure of weight in relation to height.

However, there are also "biologically plausible" reasons that breast-feeding would lower childhood obesity odds, he said.

One theory, Reilly explained, is that formula-feeding takes control away from babies and disrupts their "innate ability" to regulate their food intake.

In addition, there is evidence of metabolic differences between babies reared on breast milk or formula. "Formula-feeders," Reilly said, show higher blood levels of the sugar-regulating hormone insulin, which in turn may encourage fat deposition. On the other hand, breast milk contains growth factors that appear to inhibit fat cells.

This study, according to Reilly, adds "another piece of evidence that 'breast is best'."

SOURCE: The Lancet 2002;359:2003-2004.

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