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The Microbiome and Obesity

One of the more important functions of the microbiota is to aid in digestion. For example, ruminants such as cows have multiple stomachs to serve as bioreactors where specialized populations of bacteria break down the otherwise-indigestible starches found in grass. Within the human intestine, the gut microbiota is responsible for vitamin biosynthesis and harvesting inaccessible nutrients to produce bioavailable nutrition for the human host. This is a truly mutualistic relationship, in that the bacteria and the host benefit by providing for each other's nutrition.

However, recent studies have revealed that some bacteria may actually be too good at scavenging nutrients, with possible links to obesity. The microbiota plays a direct role in regulating fat storage, with microbe-free mice depositing much less fat than their normal counterparts. Studies in both mice and humans have revealed that lean and obese subjects have a different balance of bacteria within their microbiota, and that low-calorie diets can cause the obese microbiota to more closely resemble that of lean mice. Furthermore, the microbiota of an obese mouse is far more efficient at extracting energy from the diet than that of a lean mouse, suggesting that gut bacteria might actually cause obese subjects to take in more calories from the exact same food.

These studies raise the hope that a whole new approach to fighting obesity could work by targeting the gut microbiota. One crucial caveat, however, is that they report correlation but not causation. Does a high calorie diet or the host genetic makeup cause the microbiota to extract more energy from the diet, promoting fat accumulation? Or does a genetic propensity or random chance result in the acquisition of a fattening microbiota, which then causes the host to become obese?