Can formula make your baby fat? According to a study from researchers at Monell Chemical Senses Center in Philadelphia, babies fed cow's milk formula put on more weight than other babies and continued to gain weight faster than their counterparts during the first 7.5 months of life.
In the study, published online December 27, 2010, in the journal Pediatrics, a group of 2-week-old infants whose parents had already decided to bottle feed were randomly assigned either a cow's milk-based formula or a protein hydrolysate formula (PHF), a type of formula that contains pre-digested proteins (PHFs are typically fed to infants who cannot tolerate the intact proteins in other formulas). Both formulas contained the same calorie content.
Infants were weighed once a month for the next seven months. When researchers compared growth data to national norms for breastfed babies, the rate of weight gain for PHF-fed infants was comparable to the breast-milk standards—in contrast, infants fed cow's milk formula gained weight at a greater rate than the same breast-milk standards. Researchers found that after only two and a half months, babies who received cow's-milk formula had significantly higher weight-per-length than the babies on PHF formula. By three and a half months, the cow's-milk formula babies also had significantly higher weight-per-age than the PHF babies, whose weight (per length and per age) matched those of breastfed babies. Higher weight gains among cow's milk formula feeders persisted even after all babies started solid foods.
Researchers aren't sure why the cow's milk formula led to more weight gain, but they have a leading theory. Certain compounds in PHF called free amino acids appear to stimulate receptors in the mouth and gut that signal to the brain that the stomach is full and it's time to stop eating. As proof of this, researchers note that though babies in both groups spent about the same amount of time eating at each feeding (between 11 and 12 minutes), babies on the PHF formula drank less during that time before pushing the bottle away.
"One of the reasons the protein hydrolysate infants had similar growth patterns to breastfed infants, who are the gold standard, is that they consumed less formula during a feed as compared to infants fed cow's milk formula," says study lead author Dr. Julie Mennella. "The next question to ask is: Why do infants on cow's milk formula overfeed?"