Babies who are bottle-fed early may consume more calories later in infancy than babies who are exclusively breast-fed, a new study suggests.
Researchers found that among 1,250 infants they followed for the first year of life, those who were bottle-fed during their first six months - whether formula or pumped breast milk - showed less appetite "self-regulation" later in infancy.
The investigators say this so-called "bottle effect" could be one reason that studies have found a correlation between breast-feeding and a lower risk of childhood obesity.
In most research on the question of whether and how breast-feeding might protect against excessive weight gain, the focus has been on the components of breast milk. For instance, breast milk contains certain hormones, including leptin and adiponectin, which could help regulate infants' appetite and metabolism.
But the new findings suggest that the way infants are fed also matters, says lead researcher Dr Ruowei Li, of the US Centers for Disease Control and Prevention. That is, breast-feeding may encourage greater appetite self-regulation in the long term.
In this study, self-regulation was measured when the babies were 7, 9, 10 and 12 months old; mothers were asked how often their babies drank an entire bottle or cup of milk (formula or pumped breast milk).
Li's team found that among infants who had been exclusively breast-fed during their first six months, 27 percent always or usually finished their cup or bottle. That compared with 54 percent of babies who had been both breast- and bottle-fed, and 68 percent of those who had been only bottle-fed.
When the researchers accounted for a number of variables - like mothers' weight and education, family income and race - bottle-feeding itself was still linked to less self-regulation later on in infancy.
Babies who had had more than two-thirds of their feedings via bottle in early infancy were twice as likely to routinely empty their milk cups than babies who'd had less than one-third of their feedings via bottle.
What's more, the pattern was seen whether those early bottle-feedings contained formula or pumped breast milk.
Li points out that obesity is a complex matter, with factors ranging from genetic susceptibility to social and economic factors, to exercise habits all coming into the equation.
But bottle-feeding may be one controllable early-life factor, she says.
It's not certain why breast-feeding might encourage better eating self-regulation. But Li explains that when infants breast-feed, they are in control of how much milk they consume; when parents bottle-feed, they may try to get the baby to empty the bottle each time. It's possible that this interferes with infants' innate ability to regulate their calorie intake in response to internal "appetite cues".
Many women who breast-feed use a breast pump at some point, particularly after they return to work. Li suggested that parents who use bottles pay attention to cues that their baby is full - such as trying to push the bottle away with the tongue or shaking the head to move away from the bottle.
"You don't have to push until the bottle is empty," she says.