All infant formulas may not be equal when it comes to babies' weight gain over their first months of life, a new study finds.

In a study that followed 56 formula-fed infants, researchers found that babies on hypoallergenic formula stayed close to the "normal" weight-gain pattern seen among breastfed infants, while those on standard formula packed on pounds more quickly.

At the age of 7 months, babies on regular formula weighed over 2 pounds more, on average, than those on the hypoallergenic variety.

Past studies have shown that bottle-fed infants tend to weigh more than breastfed ones.

Now these latest findings, reported in the journal Pediatrics, suggest that different types of formula might vary in their effects on babies' growth. "When you compare them to the 'gold standard' of breastfeeding, the babies (on hypoallergenic formula) looked like breastfed babies," said lead researcher Dr. Julie A. Mennella, of the Monell Chemical Senses Center in Philadelphia.

Hypoallergenic formulas, also known as protein hydrolysate formulas, are designed to lower the likelihood of the allergic responses some infants have to standard formula.

Like standard formula, the hypoallergenic products contain cow's milk proteins; the difference is that the proteins are broken down so that they are less likely to spur an allergic reaction compared with the intact proteins in regular formula.

Breast milk is considered the best nutrition for infants. But hypoallergenic formulas are often recommended for infants who aren't being breastfed and either have allergies or are at increased risk of them -- usually because of a strong family history of allergies.

Mennella and her colleagues suspect that infants may gain less weight on hypoallergenic formulas because of the form in which the proteins are provided.

The broken-down proteins may, for example, be absorbed differently or have different effects on infants' metabolism, Mennella said.

The hypoallergenic formula used in the study had the same number of calories as the regular cows-milk formula, but 35 percent more protein (1.9 vs 1.4 grams per 100 milliliters) and slightly less carbohydrate.

Does this mean that all parents who are opting to bottle-feed should go for a hypoallergenic formula?

"No, I don't think we can say that at this point," Mennella said.

Much more research, she said, is needed to understand the effects of different types of formula on babies' overall growth and development, versus breastfeeding.

What's more, the current study did not include infants on soy formula, a popular alternative to cow's milk varieties.

Ongoing research by other investigators, Mennella noted, is looking at the effects of soy formula on growth.

"There is still so much we don't know about the early determinants of infant growth," Mennella said.

Hypoallergenic formulas also have some strikes against them. In addition to the fact that they are normally recommended only for allergy-prone infants, they are much more expensive than standard formula.

Brands like Nutramigen and Alimentum, for example, cost between $20 and $30 for about 20 baby-bottles' worth of formula in powder or concentrated liquid form.

The current findings are based on 56 new mothers who had chosen to bottle-feed and were randomly assigned to use either standard or hypoallergenic formula. All of the infants were healthy.

At the study's start, when the infants were about 2 weeks old, the average weight in the standard-formula and hypoallergenic-formula groups was nearly identical. By the age of 7.5 months, however, babies on standard formula outweighed their counterparts by more than 2 pounds -- even though they were no taller.

When the researchers had moms and infants come to the lab once a month, they did find that babies on hypoallergenic formula tended to consume less at each feeding than the standard-formula babies.

The reasons are not clear, but babies on the anti-allergy formula did not seem to be turning their noses up at it. (The formulas are well-known, among parents, to have a less-than-pleasant scent and taste by adult standards.)

Understanding how early nutrition affects infants' growth is vital, Mennella noted, because experts believe that our first year of life helps determine our risks of obesity, type 2 diabetes and even heart disease years down the road.

"Many factors contribute to infant weight gain," Mennella said. "However, studies like this strongly suggest that we need more research to better understand what regulates infant feeding and weight gain."