Now, researchers are finding the most commonly used formula, which is based on cow's milk, may be the one most likely to cause weight gain.
Alison Brownrigg, a freelance writer in Seattle, says infant formula was not her first choice. She would have preferred to breast-feed. But, because both of her children were born prematurely, they were in the hospital neonatal intensive care unit where they became accustomed to a combination of pumped breast milk and formula.
Once the children were home, Brownrigg and her husband spent lots of time considering what type of formula to use. They ultimately chose the brand used by the hospital. But it wasn't an easy choice: There were lots of varieties to pick from, everything from vitamin- and supplement-enhanced formulas to organic, soy and predigested protein formulas.
Infant formulas available on the U.S. market are all regulated by the U.S. Food and Drug Administration. Nicolas Stettler, a pediatrician with Children's Hospital in Philadelphia, says they've been proved "safe and effective." If infants like them and eat them, he says, they'll maintain their health and weight.
Main Baby Formula Varieties
For parents who opt to use formula for their baby, here are the main three:
Cow's Milk Formula
Cow's milk formula, which makes up about 80 percent of the market, is by far the most common choice for parents using formula, according to the American Academy of Pediatrics. But it's not pure cow's milk — it's been altered so it's safer for babies to consume and easier for them to digest.
This formula may be an option for parents who want to avoid animal products or for parents whose baby can't digest lactose. However, babies who have an allergy to the protein found in cow's milk formula might have trouble with soy-based formula as well, according to the Mayo Clinic. The AAP says there are few situations where parents should opt for a soy formula over cow's milk formula.
Protein Hydrolysate, or Hypoallergenic, Formula
A third product on the market, protein hydrolysate formula contains broken down, or "predigested," proteins, which can be easier for babies to digest, AAP says. According to the National Institutes of Health, this kind of formula suits babies allergic to cow milk protein. But these formulas also tend to be more expensive.
— Whitney Blair Wyckoff
The problem is formula-fed babies often gain too much weight. And, unfortunately, Stettler says, that trend can continue throughout life.
"There's more and more evidence that infants who gain weight rapidly during the first four months or year of life are at much greater risk of" becoming overweight or obese, he says.
Stettler points to studies both in the U.S. and Europe that show rapid weight gain, including a study at his hospital, which showed infants who experienced rapid weight gain during the first four months of life were five times more likely to be obese by age 20.
And, as children, Stettler says, there are more associated health problems, including liver abnormalities along with elevated blood sugars and high cholesterol.
Two Formulas, Different Results
Scientists at the Monell Chemical Senses Center in Philadelphia wanted to explore average weight gain with different formulas. They compared cow's milk-based formula to protein hydrolysate formulas, which contain predigested proteins and are typically fed to infants who cannot tolerate the intact proteins in other formulas.
Lead author and developmental psychobiologist Julie Mennella investigated weight gain among infants whose parents had already decided to bottle-feed. She compared infants fed cow's milk formula to those fed the broken-down protein formula. After seven months, infants fed cow's milk formula had, on average, gained 2 pounds more than infants fed protein formula.
Mennella says the slower weight gain among infants on the predigested protein-based formula mimicked the average weight gain among breast-fed infants.
"There's something in cow-milk formula, or something lacking in cow-milk formula, that's resulting in babies overfeeding," she says. "I could give the same baby cow-milk formula one day and protein hydrolysate formula on the other, and that baby will satiate sooner and consume less formula on the protein hydrolysate day."
Mennella's study didn't explore the reasons for this, but she has some ideas. In adults, predigested proteins are believed to act in the intestine to initiate the end of a meal, leading to smaller meals and the intake of fewer calories. It could be these infants responded similarly, getting the full message to the brain sooner than infants fed cow's milk formula.
Slow Weight Gain Also Risky
Mennella cautions that the findings are highly preliminary and really only clues that the composition of formula might affect infant weight gain. Other speculation has focused on a parent's overfeeding, intent on emptying the contents of the bottle and not tuning into infant signals that the baby is satiated.
Stettler says the findings suggest certain formulas normalize weight gain, but says more research is needed — particularly over the long term because babies who gain weight too slowly are also at risk.
These infants "have less neurological development and attain a lower IQ during childhood and adulthood," he says, adding that parents, until further research provides conclusive answers, should work closely with their pediatricians to monitor their babys' weight and make sure they don't gain too much or too little. In this case, "average" is definitely best.