The study, which followed more than 840 young children, found that formula-fed infants introduced to solid food before four months of age had a 6.3-fold increased odds of obesity at age 3, Dr. Susanna Y. Huh of Children's Hospital Boston and colleagues reported online in Pediatrics.
However, the same didn't hold for breastfed infants -- who showed no impact of timing of solid food introduction on obesity at age 3, Huh and co-authors wrote.
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The findings from the longitudinal study support guidelines recommending holding off on solids until at least 4 months of age and preferably to six months of age, the researchers noted.
"Additionally, it further confirms the tremendous long-term nutritional value of breast feeding during the first six months of life," commented Dr. Cliff Nerwen of Cohen Children's Medical Center of New York City in a statement sent to reporters.
Increasing adherence to the guidelines might be one preventive strategy to reduce the rising incidence of childhood obesity, seen even among infants and pre-school age children, Huh's group suggested.
"One possible reason why we saw an association among formula-fed but not breastfed infants is that formula-fed infants may increase their energy intake when solids are introduced," they wrote in the paper.
"Breastfeeding may promote self-regulation of an infant's energy intake, and the mother may learn to recognize her infant's hunger and satiety cues," they explained.
The researchers followed 847 children in Project Viva, a prospective, longitudinal prebirth cohort study of mother-offspring pairs. The mothers were recruited between 1999 and 2002 at the obstetrical offices of a multispecialty group practice in eastern Massachusetts.
Six months after delivery, the mothers filled out a questionnaire detailing what type of food their baby ate and when solid foods -- including infant cereal, teething biscuits, and fruit -- were first introduced.
At age 4 months, 67 percent of the children were breastfed and 33 percent were formula-fed.
Breast feeding was linked a more normal pattern of growth and to a slower introduction of solid foods.
For the primary outcome of obesity at 3 years of age, breastfed children were less likely to have a body mass index at or above the 95th percentile for age and gender at that point compared with those who had been fed formula (7 percent versus 13 percent).
Although early introduction of solid food didn't appear to impact obesity risk in breastfed children, bottle-fed infants who tried solid food before 4 months were more likely to be obese at 3 years (25 percent versus 5 percent introduced at 4 to 5 months).
The finding that this effect of early solid foods was independent of early growth was important, according to Huh's group, since "rapid weight gain during the first few months of life has been associated with an increased risk of obesity, and maternal perceptions of infant hunger or large infant size seem to influence the decision to introduce solid foods early."
Late introduction to solid food, after 6 months of age, had no effect on breastfed babies, but showed a trend for increased obesity risk at 3 years, which the researchers said they couldn't rule out given the low statistical power due to few children in this group.
They noted that they couldn't exclude the possibility of residual confounding either and noted that maternal education and income levels were relatively high in their study cohort -- which may limit generalizability to more disadvantaged populations.