Babies given extra amino acids in their formula ate less and felt sated sooner than with regular cow's-milk formula, in a new study whose results challenge the idea that bottle-feeding diminishes a baby's ability to regulate its own food intake.
When the amino acid glutamate was added to a typical cow's milk formula, researchers found that babies drank significantly less of it and showed no signs of being hungry. Human breast milk has naturally high levels of glutamate, a building-block of proteins, but cow's milk, which is used to make most infant formulas, has less of it.
"What food is fed may be at least as important as how it is fed," said Gary Beauchamp, director of the Monell Chemical Senses Center in Philadelphia, who worked on the study.
Formula-feeding infants has been linked to faster weight gain in the babies' first year, which puts them at greater risk of obesity later in life. Adding glutamate to regular infant formula might shorten feeding time, Beauchamp told Reuters Health.
"The gold standard you want to mimic is consumption at the same rate as human milk," he said.
Thirty parents and their infants participated in the research, which is published in the American Journal of Clinical Nutrition.
The babies, who were less than four months old, were bottle-fed two consecutive formula meals a day in a laboratory setting over three days.
Infants drank one of three formulas during each first meal: regular cow's milk formula; cow's milk formula with added glutamate; or so-called hydrolyzed formula, which contains broken-down or predigested milk protein. Hydrolyzed formulas, which also have high glutamate levels, are typically given to babies who have bad reactions to whole protein.
Researchers allowed the infants to drink formula at the first meal until they indicated they were full. When the infants showed they were hungry again, they were given a second feeding of cow's milk formula.
Parents fed their baby at their customary pace, starting and ending only when the baby signaled hunger or satisfaction with cues such as sucking on hands or turning away from the bottle. They were unaware of what type of formula their baby was getting.
During the first meal, the infants drank significantly less hydrolyzed formula and cow's milk formula with added glutamate compared to when they got regular cow's milk formula, leading researchers to conclude that even on formula, babies may know to stop eating when they're full.
"Since the parents feeding the infants did not know what formula they were feeding, the infants must have been able to control the amounts that they consumed," Beauchamp explained in an email.
The time between the first and second feedings did not differ markedly depending on which formula a baby was given at the first meal. The infants who first got hydrolyzed or added-glutamate formulas also did not make up for drinking less at the earlier meal by having extra formula at the next one.
That showed infants were just as satisfied after the two high-amino-acid formulas despite drinking less, said Beauchamp.
The study was funded by the National Institutes of Health and Ajinomoto Inc, a maker of amino acids, but neither was involved in the research or the written report, the authors note.
Beauchamp's team concludes from the infants' behavior that glutamate may trigger a signal in the body that tells babies they have eaten enough.
"Extra glutamate seems to control the baby's appetite," said Dr. Ian Holzman, a neonatologist at Mount Sinai School of Medicine in New York, who was not involved in the study.
Although human milk remains best for babies, Holzman explained, some babies have to be fed formula for various medical or social reasons.
In those cases, the study suggests adding glutamate to infant formula could make it "closer to breast milk," he told Reuters Health.
"We should try to make formula as good as it can be," he added.