Networking gut bacteria and their role in body weight

A recent study has shown that women with lean body types who eat high-fiber diets have complex, highly interactive bacterial networks in their gut microbiomes, and subsequent experiments in mice showed that these bacteria can impart resistance to obesity for several weeks on a high fat diet. The human gastrointestinal tract is home to a thriving community of bacteria. Studies have shown that some of these bacteria are associated with leaner body types, suggesting that they may protect against obesity. One way that they might do this is by breaking down dietary fiber to produce short-chain fatty acids (SCFAs), which are molecules believed to play an important role in regulating metabolism. The overarching properties of the microbiome that may encourage SCFA production and protect against obesity are unclear, however. For example, little is known about how the many different types of bacteria in the gut interact with each other—one type of bacteria might rely upon another to survive while aggressively competing against other types of bacteria for space and nutrients. Recognizing relationships such as these would allow scientists to conceptualize an ecological network in the gut, wherein a type of bacteria is “connected” to another if it affects its ability to thrive. Scientists could then determine how changes to this microbial network affect health.

In a recent study, scientists sought to gain understanding of human gut microbial networks by analyzing the gut microbiomes of 50 women from rural Ghana and 50 African American women from an urban area of the United States. Roughly half the participants—some from each country—were women who had obesity, while the others had lean body types, allowing the researchers to compare not only the microbiomes between different geographical areas, but also between people with different body types. Sequencing the microbial genetic material from the participants’ microbiomes (obtained from fecal material), the scientists identified the types of bacteria that were inhabiting the women’s guts, along with their relative amounts. They then compared the microbiomes from all the women to gain an understanding of how the types of bacteria relate to each other—whether their quantities tend to increase or decrease in parallel from sample to sample, or if their abundances appear to be unrelated. They found that the Ghanaian women with lean and obese body types—all of whom tended to eat more starches and fiber-rich foods than the U.S. women—had more diverse microbiomes and higher amounts of detectable SCFAs than their respective U.S. counterparts who ate diets higher in protein and lower in fiber. Also, the Ghanaian women with lean body types had microbiomes that formed the most densely interconnected bacterial networks compared to the rest of the study population; the researchers also estimated, with computer modeling, that the Ghanaian microbiomes were more stable and resistant to disruption. To determine how this affects health, the researchers inoculated male mice, which had been treated with antibiotics to deplete their native gut bacteria, with samples of the microbiomes from each group of women (those from the United States or Ghana, and with lean or obese body types). The mice that were given samples of the microbiome from a Ghanaian woman with a lean body type were significantly more resistant to weight gain when fed an obesity-inducing high-fat diet for 6 weeks, compared to the mice harboring the microbiomes from the other groups of women. These obesity-resistant mice also made higher levels of a molecule that interacts with SCFAs, suggesting that bacterial-derived SCFAs may be involved in preventing weight gain in these mice.

Rather than pointing to individual types of bacteria, this was the first study to implicate the characteristics of the entire microbiome network—including how the many types of bacteria in the microbiome relate to each other—in metabolic conditions like obesity. This could be important for future studies to determine the best ways to manipulate the microbiome to improve health.