As many of you current moms may know, losing the extra weight you put on during pregnancy is no easy task. For the most part, the assumption has always been that this is due to hormonal changes that follow after pregnancy. While that is likely a big piece of the puzzle, there is another variable lurking that could play as big of a role, if not a bigger, in your fat loss troubles: Your gut bacteria.
In a study published in Cell back in 2012, researchers took a good luck at how the microbiome changed for expectant human mothers throughout pregnancy. The authors found that a woman's microbiome changes significantly from the first trimester to the third and that, in the third, gut bacteria changes similar to those seen in obesity change the metabolism of the pregnant mother(1). These changes lead to increased inflammation, insulin resistance, and hyperglycemia. When the babies had their microbiome tested, it more closely resembled
the mother's during the first trimester and became more similar to the
mothers as the children aged. Finally the microbiome of women in their first or third trimester was given to germ free mice. The mice given the third trimester microbiome gained significantly more weight than germ free mice given the microbiome from the women in their first trimester, and the the mice given the third trimester microbiome had the same metabolic changes associated with obesity.
One of the more interesting aspects of this study is that the changes in the microbiome didn't really correlate with the mother's diet. This supports the notion that there is an evolutionary advantage for this change and that the mother's physiology is likely impacting this change in one way or another, not her diet. Another interesting aspect of this study is that it sort of implicates that this microbial change, which is highly advantageous in pregnancy to conserve energy for the baby, may be a significant contributor to the obesity epidemic in all humans. Since a child's microbiome most closely resembles their mother's at the first trimester and becomes more similar as they age, it is unlikely that a child gets any significant microbial transfer from their father outside of the environment they share, which is also the same as the mother's.
Despite their being many similarities between the third trimester and obese microbiomes, including decreased diversity, some of the microbial changes seen in the third trimester of pregnancy diverge from the obese microbiome. The obese microbiome sees a decrease in Bacteroidetes and an increase is Firmicutes which leads to increased sugar absorption while the third trimester microbiome sees no such change. This change is primarily driven by prolonged excess calorie intake. Could excess calorie intake from the third trimester through the first year or so of a child's life help promote or reinforce this this change in the mother? It's likely, but that wouldn't necessarily mean that this change is required. Insulin resistance blocks fat burning so any metabolic change that induces insulin resistance will reduce a person's ability to burn fat.
So now that we have an understanding about what the study shows, how does this help us? Hormonal changes should not be your only concern and there is the potential that this weight doesn't need to be so difficult to lose. Since there is likely significant crosstalk between the hormonal and microbial systems, eating a diet that promotes a healthy and diverse microbiome should put you in the best position to lose the weight when these two systems will allow it. This includes ample amounts of fiber from fruits and vegetables to help control the inflammation associated with the third trimester microbiome as well as a diverse diet to promote microbial diversity. This is likely the diet that new human mothers ate for hundreds of thousands of years, and it is likely the diet that will provide the quickest, smoothest transmission back to a more healthy microbiome.
Depending on your level of insulin resistance, it may be a good idea to reduce carbohydrate intake for a little while until you restore proper blood glucose control. Finally, if there is ever a good time to make sure you're not stocking up on excess calories and scarfing down Ho Hos, the period right after you give birth is that time. Even if you're breastfeeding, there really isn't a need to increase your calorie intake. Once delivered, the nutritional demands of the baby are met through the milk, but since the baby is no longer inside of you there is no need to increase caloric intake beyond what you did for your third trimester.