A study published this month in Archives of Pediatric and Adolescent Medicine looks at the relationship between infant feeding practices and weight gain (1). Breast milk vs. formula? Nope, it isn’t that simple.
Led by Dr. Ruowei Li of the CDC, this prospective longitudinal study tracked feeding and weight gain in 1900 infants during their first year of life. Each month, mothers were asked how they fed their babies in the last 7 days, and from their replies, infants were grouped into the following categories across ages:
- Breastfed only
- Breastfed and human milk by bottle
- Breastfed and formula by bottle
- Human milk by bottle only (i.e. exclusive pumping)
- Human milk and formula by bottle
- Formula by bottle only
The mothers in this study were mainly white, married, and had at least a high school education. A third were on WIC. About 50% were overweight or obese. Statistical methods were used to adjust the findings for a range of maternal factors, including BMI, as well as infant sex, gestational age, birth weight, and age of solid food introduction.
The most important finding from this study was that infants fed by bottle only – whether fed formula or breast milk – gained more weight than those fed breast milk at the breast. Formula-fed infants gained an additional 71 grams (P<0.001), and babies exclusively fed breast milk by bottle gained an additional 89 grams (P=0.02) per month. Babies that were breastfed and given breast milk by bottle did not have increased weight gain.
Many earlier studies have found that formula-fed infants grow faster than breastfed infants. (I’ve written before about the importance of using the WHO growth charts instead of the CDC charts for breastfed babies, for this very reason). Studies have also found an association between formula-feeding and risk for obesity later in childhood, although it is not clear if this is a causal relationship (2). There are a range of hypotheses to explain these observations. Is it biological – something in the milk, like the hormones found in breast milk but not formula? Or is it behavioral – the act of breastfeeding itself?
The data from this study support the idea that the differences in weight gain are at least in part due to feeding behavior. The babies that were fed by bottle gained the most weight, and this was true whether they were fed formula or breast milk. I can guess how this happens. If you are feeding a baby a bottle, then you have prepared a particular amount, and it is hard to not hope that the baby will finish it. Particularly if you have gone through the labor of love of pumping that milk, then you are probably especially invested in seeing your baby polish off the entire bottle. Even if baby is showing some signs of being full or drifting off to sleep, you might gently encourage her to drink just a little more. By doing so, you’re over-riding baby’s natural sense of satiety.
What is needed is a randomized trial with an intervention teaching moms and dads to read and respond to baby’s hunger and satiety cues. If bottle-fed babies in this group showed slower weight gain, similar to breastfed babies, then we’d know that feeding behavior mattered to weight gain. This kind of research is important. The more we understand about the mechanisms behind differences in bottle and breastfed babies, the more information we have to help parents give their babies the best start, regardless of how they are fed.
There is an alternative explanation to these data. During each breastfeeding session, the composition of breast milk changes, with fat content being higher in the hindmilk (at the end of the session) than the foremilk. This could give a signal to the breastfeeding baby that it is time to wrap things up. These compositional changes don’t occur when baby is fed a bottle of pumped breast milk.
Some further caveats to these findings: First, we don’t really know if faster weight gain during the first year actually causes increased risk of obesity or has any other negative effects. Second, the number of babies fed breast milk exclusively by bottle was relatively small, but this group is what makes this study so interesting. And finally, a small group of babies fed both breast milk and formula by bottle did not gain more weight, making interpretation of the findings just a little sticky.
I don’t consider this study to be conclusive by any means, but it sure is interesting. It is a reminder that no matter how you feed your baby, it is important to pay attention to her cues and let her take the lead on how much and how often to eat.
P.S. – Since I was on a Facebook fast last week during our trip to Sedona, I have no idea if this study was a big topic of conversation. So I apologize if it is old news, but it was one of the few papers I printed out to bring on our trip, and it was the only one I actually read what with all the sleep deprivation and recreation.
1. Li R, Magadia J, Fein SB, Grummer-Strawn LM. Risk of bottle-feeding for rapid weight gain during the first year of life. Arch Pediatr Adolesc Med. 2012;166(5):431-436. doi:10.1001/archpediatrics.2011.1665.
2. American Academy of Pediatrics. Breastfeeding and the Use of Human Milk, Policy Statement. Published online February 27, 2012. www.pediatrics.org/cgi/doi/10.1542/peds.2011-3552