The type of formula fed to infants influences how quickly they gain weight, according to a new study. This is important, the researchers say, because rapid weight gain during the first year increases the baby's risk later in life of becoming obese or developing diabetes and other diseases.

Over the seven months they were studied, infants who drank formulas made from cow's milk gained 2 pounds more (0.9 kilograms) than those who drank formulas made from protein hydrolysates, which are designed to be easier to digest, said study researcher Julie Mennella of the Monell Chemical Senses Center in Philadelphia.

Because the formulas contained the same amount of calories, "we were surprised by the difference," said Mennella , a developmental psychobiologist at the independent nonprofit research institute.

The study showed that, while many factors contribute to an infant's weight gain, there are important differences in the types of infant formula that parents choose, Mennella said.

It has long been known that formula-fed babies gain more weight than breast-fed babies, Mennella said, and some had suggested that parents giving formula are less attentive than breast-feeding mothers to the babies' signals that they are full. But this study shows that there may be more to it.

A formula for weight gain?

The researchers observed that infants being fed cow's milk formula drank more than those fed protein hydrolysate formula before they gave signals of being full. It may be that the reactions in the intestines that come when the baby is full are triggered in part by broken-down proteins called free amino acids, and those free amino acids are far more abundant in the hydrolysate formula than in the cow's milk formula. Hydrolysate-based formulas are similar to breast milk in their level of free amino acids.

While the difference in the infants' weight gain may be due, in part, to the difference in how much they drink, the free amino acids are likely to play yet-undiscovered roles, Mennella told MyHealthNewsDaily. Cow's milk formulas don’t have many of these molecules. Hydrolysate-based formulas also have more protein than cow's milk formulas.

Formula based on protein hydrolysates are typically given only to babies who are allergic to cow's milk or who have a hard time digesting cow's-milk-based formula, Mennella said. Protein hydrolysate-based formulas can cost twice as much as cow's milk formulas. (Soy-based formulas are also available, but the study did not examine these.)

The researchers found that the babies fed hydrolysate-based formulas gained no more weight than breast-fed infants; it was only those given the cow's-milk-based formulas who saw the accelerated weight gain during their first seven months.

"We think it may the free amino acids that help babies to regulate their hunger and satiety," Mennella said.

The study was based on 64 infants, and the researchers accounted for the babies' weight and length at the start of the study, as well as their growth throughout the study.

The weight gain of breast-fed babies was used as the "gold standard" against which formula-babies were compared. The infants were weighed and measured once a month.

Early weight gain, later health risks

"We need to understand why cow's-milk-based formula-fed babies are gaining weight at a faster rate than breast-fed babies," Mennella said.

The finding that formulas are not alike in the ways they affect weight gain is important, she added.

"Numerous studies have shown that rapid rates of growth during first year are linked to obesity, metabolic syndrome, mortality," Mennella said. "Lifelong health begins during infancy."

The researchers' next step will be to look more closely at the effects of free amino acids and other components of formula, to disentangle their effects, Mennella said.

The study was published online Dec. 27 in the journal Pediatrics, and was funded by the National Institute of Child Health and Human Development.

Pass it on: Feeding a baby protein-hydrolysate based formula may help him to gain weight at the same rate as breast-fed baby, instead of the accelerated rate often seen in babies fed cow’s milk-based formula.

Follow MyHealthNewsDaily managing editor Karen Rowan on Twitter @karenjrowan.