The Surprising Links Between Breastfeeding, BMI, & Bacteria

Written by Clare Carrick ANutr (BHSc)

Scientifically reviewed by Dr Fathalla Ali, PHD Paediatrics


A Weighty Issue

More kids than ever before are overweight or obese (1). Although trashy magazines and celebrities like Kim Kardashian may have us fooled into thinking being overweight is only an aesthetic concern, we know better than that…Obesity is associated with a whole bunch of potential complications, like type 2 diabetes, cancer and cardiovascular disease, and is the 2nd leading risk factor (behind smoking!) contributing to Australia’s burden of disease (1, 2). For this reason, figuring out how to solve this issue has become extremely important, and no stone is being left unturned in the search to find the potential causes for this massive increase in weight over the last few decades (2).


Why do Overweight Mums Have Overweight Kids?

We know, undeniably, that overweight mums are more likely to have overweight one-year-olds (2). We also know that overweight infants are more likely to become obese later in life (2, 3). But, why is this? Obesity and overweight are complex and multi-factorial (2). The reasons behind childhood obesity will differ between individuals, however, a combination of environmental, dietary, physiological and genetic factors will most likely all be thrown into that mix (2).


Can Gut Bugs Make Overweight Toddlers?

With the rise in research on the importance of the gut microbiome on our health and wellbeing, the possibility that our kids’ gut bugs may play some kind of role in weight gain has been tossed around more than once (2). Science has shown us that there is a strong link between an adult’s gut microbiome and obesity (2, 4). In fact, the microbiome of those in a healthy weight range is quite different to those who are obese (4). In adults, a combination of genetics and diet seems to shape the gut population that promotes obesity, with certain dietary changes, such as going from a low fibre/high fat to a high fibre/low fat diet, triggering changes to the gut population in as little as 24 hours (4)! 


For children, the jury is still out on how much of an impact their gut bugs may, or may not have on their risk of overweight or obesity (2). Some studies conclude that a child’s microbiome could explain more than 1/2 of the variation between the BMIs in their study group, whilst others have placed more emphasis on the many other factors that could be at play, such as genetics, physiology and dietary elements (2, 5). 


Breastfeeding and Gut Bugs

One thing that is clear though, is that whether a child is breastfed or not has a major impact on the composition of their gut microbiome at 6 and 12 months of age (2). Interestingly, breastfed babies have less diverse gut microbiomes than their formula-fed buddies (2). This might seem counter-intuitive because we know that, for adults, a more diverse gut microbiome is associated with good health, but for breastfed babies, this lack of diversity is most likely due to the breastmilk favouring and boosting the growth of beneficial bacteria, like Bifidobacteria, and Bacteroides (2)


A Mother’s BMI and Breastfeeding

Another seemingly random relationship that scientists have uncovered is that women who have a higher pre-pregnancy BMI are more likely to have children with higher bacterial diversity in their gut microbiome (2). There is, however, an easy (yet very interesting!) explanation for this one! In this case, it all comes down to the fact that women with a higher pre-pregnancy BMI, are much less likely to breastfeed their children, which results in this altered gut microbiome (2, 6). Obese women often experience more of a delay in their milk ‘coming in’, a lower milk supply, and overall, a shorter duration of breastfeeding if it happens at all (6). As a result, this then looks like a correlation between a mum’s BMI and a child’s gut microbiome composition, but it is because of the impact of the breastmilk (or lack of) on the gut, rather than the weight of the mother specifically (2).


Yep, It’s Complicated…

So many intricate and complex relationships to examine, so much science…It’s tricky to get your head around all of it sometimes! So, what does this information really all boil down to? In a nutshell, these findings re-emphasise the importance of a mum’s wellbeing in the lead-up to, and during, pregnancy (2). Support and education surrounding healthy diets and behaviours for mums and their bubs will help to build the foundations for thriving gut microbiomes, helping to give these babies the best start in life (2). 


  1. Keramat SA, Alam K, Al-Hanawi MK, Gow J, Biddle SJ, Hashmi R. Trends in the prevalence of adult overweight and obesity in Australia, and its association with geographic remoteness. Scientific reports. 2021 May 31;11(1):1-9.
  2. Haddad EN, Sugino KY, Kerver JM, Paneth N, Comstock SS. The infant gut microbiota at 12 months of age is associated with human milk exposure but not with maternal pre-pregnancy body mass index or infant BMI-for-age z-scores. Current research in physiology. 2021 Jan 1;4:94-102.
  3. Taveras EM, Rifas-Shiman SL, Belfort MB, Kleinman KP, Oken E, Gillman MW. Weight status in the first 6 months of life and obesity at 3 years of age. Pediatrics. 2009 Apr;123(4):1177-83. 
  4. Davis CD. The gut microbiome and its role in obesity. Nutrition today. 2016 Jul;51(4):167. 
  5. Stanislawski MA, Dabelea D, Wagner BD, Iszatt N, Dahl C, Sontag MK, Knight R, Lozupone CA, Eggesbø M. Gut microbiota in the first 2 years of life and the association with body mass index at age 12 in a Norwegian birth cohort. MBio. 2018 Oct 23;9(5):e01751-18.
  6. Turcksin R, Bel S, Galjaard S, Devlieger R. Maternal obesity and breastfeeding intention, initiation, intensity and duration: a systematic review. Maternal & child nutrition. 2014 Apr;10(2):166-83.


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