Babies fed cow’s milk formula gain weight faster than babies fed protein hydrolysate formula or breast-fed babies, according to a new US study that suggests this finding could be relevant to infants’ risk of developing obesity, diabetes and other diseases later in life.
The study was the work of lead author Dr Julie Mennella, a developmental psychobiologist at Monell Chemical Senses Center, Philadelphia, Pennsylvania, and colleagues, and is published in the 27 December online issue of Pediatrics.
Previous studies had already revealed that formula-fed babies gained weight faster than breast-fed babies, but it was not clear whether this was true of all types of formula.
Mennella said the study found that “all formulas are not alike”:
A key area of interest at Monell is how the composition of food interacts with the senses to influence feeding behavior.
While most infant formulas are based on cow’s milk, other choices include soy-based and protein hydrolysate-based.
Protein hydrolysate-based formulas contains pre-digested proteins and are designed to suit babies who cannot tolerate the intact proteins of other formulas.
It is thought that in adults, pre-digested proteins cause the gut to signal that a meal is coming to an end, resulting in smaller meals and the consumption of fewer calories.
The researchers at Monell wondered, given this information, if infants fed on protein hydrolysate formulas would also eat less and whether this would affect their growth, compared to infants fed formula based on cow’s milk. They thought yes, and set out to investigate, because, as Mennella explained to the press:
“Events early in life have long-term consequences on health and one of the most significant influences is early growth rate.”
For their study, Mennella and colleagues recruited infants whose parents had already decided to bottle feed.
When the infants were two weeks old, the researchers randomly assigned them to be fed one of two types of formula for seven months; one group of 35 babies received cow’s milk-based formula and the other group of 24 babies received a protein hydrolysate formula.
Both formulas had the same amount of calories per bottle, but the hydrolysate formula had more protein, including greater amounts of small peptides and free amino acids.
The babies were weighed and measured every month in the lab, where they were also filmed being fed a bottle of their assigned formula. The feeding continued until the baby signalled he or she was full.
The results showed that over the seven months of the study, the infants fed on protein hydrolysate gained weight more slowly than infants on cow’s milk formula. There was no significant difference in length, suggesting that the difference between the groups resulted from gains in weight not attributable to gain in length.
(To rule out the effects of differences in size and age, the researchers used World Health Organization weight-for-length and length-for-age growth standards also known as “anthropometric z scores”).
From the monthly assessments, Mennella and colleagues also found that the babies fed protein hydrolysate consumed less formula to the point of signalling they were full compared to the babies fed the cow’s milk formula.
There was no difference in the mother’s ratings of how acceptable the babies found the formula.
When they compared the results to the national norms, the researchers found the rate of weight gain for the protein hydrolysate infants was comparable to that of “gold standard” breast-fed babies, whearas the cow’s milk formula babies gained weight at a greater rate than the same breast milk standards.
They concluded that the cow’s milk formula-fed infants’ weight gain was “accelerated”, whereas the protein hydrolysate-fed infants’ weight gain was “normative”.
“These two formulas have the same amount of calories, but differ considerably in terms of how they influence infant growth,” said Mennella.
The study did not determine whether the differences in weight gain were “because of differences in the protein content or amino acid profile of the formulas and, in turn, metabolism”, so this remains unknown.
The researchers recommended that more studies be done on the long-term consequences of these early growth differences.
Mennella pointed out that one of the reasons the babies fed protein hydrolysate formula had similar growth rates to “gold standard” breast-fed babies, was because they consumed less formula during a feed, as determined from the monthly filmed assessments.
The next question for research, said Mennella, is:
“Why do infants on cow’s milk formula overfeed?”
The researcher believe their findings highlight a need to understand the long term effect of formula compostion on feeding behavior, growth and metabolism.
In future studies in this area they plan to use measures of energy metabolism and expenditure to see how individual formulas affect growth, and how they differ from breastfeeding.
“Differential Growth Patterns Among Healthy Infants Fed Protein Hydrolysate or Cow-Milk Formulas.”
Julie A. Mennella, Alison K. Ventura, and Gary K. Beauchamp.
Pediatrics, published online 27 December 2010.
DOI:10.1542/peds.2010-1675
Additional source: Monell Chemical Senses Center.
Written by: Catharine Paddock, PhD