Say Hello to the Bugs in Your Gut

What's indigestible to you is a seven-course meal to your gut microbiota. The conditions in the colon—dark, moist and free of oxygen—are just what your gut microbiota needs to ferment indigestible material passed on from the small intestine and produce simple sugars and short, chain-free fatty acids. They do this for their own good, but they also share some of these energy-rich substances with their host—us. Some people get up to 10 percent of their daily calories from substances produced by their gut bugs.

Gordon and his colleagues think that the gut microbes in some people are more efficient at extracting energy than those in other people. This could partly explain why some individuals gain more weight on the same diet that allows others to stay lean. Gordon's hypothesis is supported by a series of elegant experiments.

First, Gordon's team raised generations of mice in sterile conditions. These microbe-free mice downed almost one third more food each day than their ordinary counterparts—yet had 40 percent less body fat. When the researchers took samples of gut microbes from ordinary mice and transplanted them into germ-free mice, the newly inoculated rodents began to gain weight even though they weren't eating any extra food. The team took the work a step further with help from a strain of genetically obese mice. Transplanting gut microbes from these fat mice into lean, germ-free mice led to greater gains in body fat than transplanting gut microbes from normal mice.

To see if fat mice had different gut bugs than lean mice, the Washington University team took genetic snapshots using high-tech DNA sequencers. In ordinary normal-weight mice, bacteria belonging to the group known as Firmicutes accounted for about two thirds of the gut's bacterial community. Members of the Bacteroidetes group made up most of the rest. In contrast, genetically obese mice had even more Firmicutes and many fewer Bacteroidetes. By analyzing the sequence of genes extracted from various microbiota samples, Gordon's team discovered that the bacterial community in obese mice had more genes for breaking down complex starches and fiber. In other words, microbes from obese mice were better at releasing calories from the gut's contents than were the microbes from lean mice.

Think of it this way: the gut community in obese mice is like a fuel-efficient car, extracting more energy from food and passing more along to its host than its gas-guzzler counterpart in lean mice.

Mice are mice. Does any of this apply to humans? To find out, the Washington University team asked a dozen obese men and women to follow a low-fat or low-carbohydrate diet for a year. Before starting these diets, these obese volunteers had more Firmicutes and fewer Bacteroidetes in their guts than did several lean volunteers acting as controls—just as was seen in obese and lean mice. As the volunteers lost weight, their microbial communities underwent a remarkable shift, with an increase in the gas guzzlers (Bacteroidetes) and a decrease in the efficient energy extractors (Firmicutes). The type of diet didn't matter; only significant weight loss sparked the shift.