Another Cause of Obesity: The Bacteria in Your Gut?

Ever try to shed a few pounds? then you are all too familiar with the three key contributors to your problematic weight: diet, exercise and genes. If you don’t exercise (calories out) as much as you eat (calories in), you gain fat–an equation that may be heavily influenced by your particular genetic makeup.

But over the years, scientists have realized that these factors do not adequately explain body size. There are cases of identical twins, for instance, who eat and exercise the same amounts, but while one becomes obese, the other remains thin. Such discrepancies have spurred researchers to look for alternative causes of weight gain. And their inquiries have led them to a familiar but unexpected place–the gut. Or more specifically, the intestines and the bacteria that take up residence there.

Technically, they’re known as the gut microbiota, the universe of tens of trillions of microbes that thrive in the human intestinal tract and colon. We depend on our microbiota to perform a vast range of vital bodily functions, from digesting what we eat to regulating the number of calories our bodies extract from food and stores as fat–in other words, whether we gain weight because of what we eat.

Research on animals suggests that heavy bodies have a different makeup of gut bugs than thin ones do. In obese mice, the gut microbiota have been shown to include significantly more of one main type of bacteria, Firmicutes, and fewer of another, Bacteroidetes. (Both populate human guts as well.) In normal-size mice, the distribution is the opposite. In November a new study published in Science Translational Medicine further suggested not only that the particular types and balance of bugs harbored in the gut may be associated with weight but also that these microbe populations might be manipulated through diet.

The work came from the lab of Jeffrey Gordon at Washington University in St. Louis, Mo., who has been studying the gut microbiota of mice and humans for more than five years. His team began with mice that were specially bred to be germ-free–with no gut microbiota–and injected them with samples of fresh and frozen human feces. The bacteria from those samples took hold and colonized the animals’ guts. “We were surprised that so much of the diversity present in human microbial communities could be recaptured in mice,” says Gordon.

That experimental coup allowed for some innovative analyses. Scientists now had a window into the living world of human gut bugs and could begin to study how they responded to food and how they may potentially affect weight. Using cutting-edge DNA-sequencing capability to scan and analyze all the genes in the bacteria, the researchers could also determine which microbe species were present in what proportions and which genes were turned on in the bugs.

When one group of mice was fed a typical Western diet–high in fat and sugars–researchers found that they tended to gain weight and grow more Firmicutes and fewer Bacteroidetes. In mice given low-fat plant-based chow, the distribution of the two types of gut bugs flipped, and the animals remained lean. Gordon also discovered that changing an animal’s diet caused a dramatic and rapid shift in the population of bacteria in its gut. Switching a mouse from a plant-based to a high-fat Western diet resulted in an explosion of Firmicutes in less than a day.

Although it’s not clear whether the balance of gut bugs caused the weight gain or was the result of it, the findings indicate that the development of a gut profile could someday help identify people who have a propensity toward obesity. In fact, in Gordon’s earlier work with identical twins of differing weight, he found that obese twins tended to have more of Firmicutes colonies than their leaner siblings did. “There is a vast reservoir of attributes associated with our human physiology that is derived from our gut microbial communities,” Gordon says.

If confirmed in future studies, the new findings may lead to the development of an obesity or leanness profile using microbial markers–a vital-stats sheet of the gut world that could help people understand how their bodies are likely to respond to calories. “This vast universe of microbes that live on and in us is terra incognita, but it is becoming more cognita every day,” Gordon says. With any luck, some of these unexplored bugs may someday help slim down the very guts that feed them.