- Rapid weight gain during infancy is linked to obesity in childhood and adulthood.
- A new study from 2,500 Canadian mother-infant pairs reports that breastfeeding was inversely associated with body mass index, weight gain velocity, and risk of overweight at 12 months of age.
- Infants that received some breast milk from the bottle gained more weight than infants fed directly at the breast but were still leaner than those fed formula.
- Although direct breastfeeding had optimal growth outcomes, pumping is an integral part of breastfeeding for the majority of women and allows them to continue breastfeeding rather than switch to formula.
If you want to understand how an infant’s diet influences its health, you might ask, “What did the infant eat?” But the results of a new study  on infant diet and weight gain suggest that this simple question is no longer sufficient; in addition to asking what, we need to be asking how, and for how long. Newly published results from over 2,500 mother-infant pairs demonstrate that the longer a mother is able to directly provide breast milk, the more closely the infant’s rate of weight gain over the first 12 months of life matches the World Health Organization’s (WHO) standards . Considering the prevalence of pumping among many breastfeeding mothers, these novel findings shouldn’t be reported without consideration of the many positive outcomes associated with feeding expressed breast milk.
During the first year of an infant’s life they produce a lot of data. Length, weight, and head circumference are measured in the U.S. at least every three months, along with other important developmental milestones such as rolling over, following an object with the eyes, and controlling the head. Rolling over ahead of schedule may be celebrated by parents and their physician as a sign of good coordination and muscle strength in an infant, but getting ahead of the growth charts can be cause for concern. Rapid weight gain during infancy is an established risk factor for obesity later in life; infants that gain weight too quickly and end up with excess body weight at the end of infancy are more likely to carry that weight with them into childhood and adulthood .
What an infant eats obviously plays a role in how quickly it gains weight, and breastfeeding has been associated with a reduced risk for developing obesity in childhood and later in life [1, 2]. However, despite being used by the WHO as the feeding standard for optimal growth outcomes , the mechanism connecting breastfeeding to obesity is still unclear [1, 2].
“The link between breastfeeding and lower obesity risk might be because of differences in bioactives altering metabolic development in bottle-fed babies, leading them to grow faster, or store more fat” explains Dr. Melanie A. Martin, an Assistant Professor of Anthropology at the University of Washington who studies biocultural influences on growth, development, and reproduction but was not involved with this study. “But a second possibility, supported by a good amount of research, is that infants don’t self-regulate as well when they are nursing from a bottle. Caregiver feeding can also play a role in this, in that adults may be conditioned to giving infants a set amount of liquid and thinking they need to finish it rather than looking for infant cues of satiety.”
It is important to notice that Martin distinguishes between breastfeeding and bottle-feeding, rather than breast milk and formula. Bottles are not simply for formula anymore. “Studies consistently show that pumping is part of breastfeeding for American mothers, but few researchers are actually studying women’s experiences with pumping,” says human biologist Dr. EA Quinn, an Associate Professor of Anthropology at Washington University in St. Louis who studies human milk composition and breastfeeding, but was not involved with this study. Considering the prevalence of pumping, it may seem surprising that studies examining infant diet and weight gain rarely distinguish between direct breastfeeding (at the breast) and bottled breast milk feeding.
A new study by Azad et al.  uniquely considered feeding methods in their examination of breastfeeding, infant weight gain, and body composition during the first 12 months of life. Mother-infant dyads (n = 2,553) were recruited as part of the Canadian Healthy Infant Longitudinal Development (CHILD) birth cohort, which collected data on infant body size (weight and length), infant diet (breastfeeding initiation and cessation, feeding of expressed breast milk, use of formula, and introduction of solid foods), and other factors known to influence infant body size (e.g., infant sex, birth weight, maternal age, maternal diet quality, maternal body size, and ethnicity).
Their primary outcome measure was body mass index (BMI) Z score, a known predictor of childhood obesity . BMI Z-scores compare an infant’s weight, adjusted for their age and sex, relative to a reference population; scores can be positive (higher BMI than the reference population), negative (lower BMI than the reference population), and zero (BMI is the same as the mean for the reference population).
Across the entire CHILD birth cohort, breastfeeding was inversely associated with BMI, weight gain velocity, and risk of overweight at 12 months of age . However, the longer an infant was directly breastfed, the closer their BMI Z-score matched WHO infant growth standards (i.e., the closer it was to zero).
Infants that followed the WHO’s recommendation of at least 6 months of exclusive breastfeeding had a mean BMI Z-score of −0.4. Over half (54.6% to be exact) of the infants in the group of exclusively breastfed at 6 months occasionally received some expressed breast milk, and when analyzed separately, their BMI Z-scores were significantly higher (mean BMI Z-score of +0.14) than those that only nursed directly (−0.02) . Importantly, they were still leaner than infants that were only partially breastfed (breast milk and formula, (+0.28) or not breastfed at all (+0.45).
Rather than thinking of breast milk vs. formula, these findings highlight the need to consider the heterogeneity that exists in infant feeding strategies. But when reporting these results, we must also be mindful of why there is variation in infant feeding strategies.
“We need to collectively recognize that every mom and baby are different, have different needs and challenges, and have different pathways to achieving what is optimal for them,” says Martin. “Some mothers are always going to have working conflicts or other obstacles that don’t align with optimal practice. So we need to dually emphasize practical solutions and realities.”
The Message to Mothers
In order to offer practical solutions, we first need to identify the problem. Why would pumped breast milk alter infant BMI and rate of weight gain? “Very, very few studies have looked at the effects of long-term storage under home conditions on human milk,” explains Quinn. “My guess, however, would be that the modification of hormones or other bioactives by freezing is not the concern.”
Instead, both Quinn and Martin believe the issue is the bottle rather than the composition of the breast milk in the bottle. And this is good news, because this can be modified. For example, when infants were fed from bottles that were weighted and opaque, caregivers fed less milk than when using clear plastic bottles . Presumably, they stopped feeding based on infant cues of satiety as opposed to how much milk they could see or feel was left in the bottle. Although it is not identical to infant self-regulation at the breast, these types of changes in bottle-feeding behavior may prevent overfeeding, which in turn may play a role in programming satiety .
When working on messaging to mothers, it is also important to look at the positive outcomes associated with feeding expressed breast milk. The WHO advocates for exclusive breastfeeding for 6 months and continued breastfeeding alongside solid foods for 12−24 months. Pumping may actually help more women meet these optimal infant-feeding standards.
“I think research, public messaging, and conversations can better support the idea that bottle-feeding can help moms achieve prolonged breastfeeding,” says Martin. Quinn agrees. “Pumping allows women to keep breastfeeding and providing their infants with human milk after they return to work when otherwise they would have to use formula. And it should be celebrated for that.”
1. Azad MB, Vehling L, Chan D, Klopp A, Nickel NC, McGavock JM, Becker AB, Mandhane PJ, Turvey SE, Moraes TJ, Taylor MS. 2018. Infant feeding and weight gain: separating breast milk from breastfeeding and formula from food. Pediatrics 142(4): p.e20181092.
2. Li R, Fein SB, Grummer-Strawn LM. 2010. Do infants fed from bottles lack self-regulation of milk intake compared with directly breastfed infants? Pediatrics: pp.peds-2009
4. Ventura AK, Hernandez A. 2018. Effects of opaque, weighted bottles on maternal sensitivity and infant intake. Maternal & Child Nutrition: e12737.
Dr. Lauren Milligan Newmark
Researcher, Science Writer