Research suggests that obesity and the composition of the human gut microbiomeare closely related and influence how energy is both extracted and stored in our bodies. In our review of the microbiome(PAGE XX), we discussed how the gut bacteria is responsible for breaking down plant polysaccharides into a digestible form of energy for the body. So, it makes sense that differentiations in the gut microbiota will affect how the gut bacteria breaks down and extracts energy from what we eat. The connection between gut microbiota composition and obesity risk was initially suggested by researchers who observed lower body fat percentages in mice bred without gut microorganisms (i.e., germ-free mice) compared to mice with normal gut microbiota, even though the normal mice consumed less food than the germ-free mice.1 In other words, the mice without gut bacteria were able to access, burn, and store less calories from their food than the normal mice. Additional research in mice suggests that the gut microbiome can also affect the genes that modulate fat storage and increase the amount of dietary fat that is stored in fat cells.2
While these effects on energy extraction and storage have only been observed in mice thus far, other research has found differing compositions of gut bacteria between people with obesity and those of normal weight—mainly, variations in overall diversity and differences in the ratio of Firmicutes to Bacteroidetes (the two most abundant phyla in the gut). In a study of fecal samples of people with overweight and obesity, less bacterial diversity in these samples were associated with higher weight, higher blood triglycerides (i.e., dyslipidemia), impaired insulin sensitivity, and higher levels of inflammation.3 In a separate study, researchers compared the gut microbiota of people with obesity to individuals of normal weight, finding that people with obesity had a microbiota abundant in Firmicutes relative to Bacteroidetes, with 90-percent less Bacteroidetes than the participants of normal weight.4 In addition, weight loss in these participants resulted in diminishing levels of Firmicutes and a flourishing of Bacteroidetes, but levels never seemed to become qual to those of originally lean individuals. Similar bacterial differences have been observed in children and infants who later, as adults, were either lean or obese, leading some researchers to suggest that these variations in gut microbiota composition precede the onset of obesity.2 So, what does the research suggest we should be eating to shift our microbiota in favor of metabolic health and a
lower risk of obesity? In mouse studies, the typical Western diet, characterized by foods containing high amounts of fat and refined carbohydrates, increases Firmicutes and decreases Bacteroidetes in the gut.5 This difference has also been observed in humans, notably in a study that evaluated the microbial composition of European children and African children.6 The European children ate a modern Western diet, while the African children ate a diet high in fiber-rich plants and low in animal protein and fat, noted to be more closely aligned with traditional diets of early agricultural civilizations.1
Looking to improve your Firmicutes to Bacteroidetes ratio? Here’s some good news: research has shown that dietary shifts can yield changes in the composition of the gut microbiota in as little as 24 hours and persist for as long as the diet is maintained.7
