In 2005, researchers led by Jeffrey Gordon at Washington University in St Louis, Missouri, reported that the gut microbiota of leptin-deficient mice contains about 50% fewer Bacteroidetes and 50% more Firmicutes, each a major group of bacteria, compared with normal mice3. “This was the first direct evidence that there were differences in the microbial communities between lean and obese mammals,” says Robin Knight, a computational biologist at the University of Colorado, Boulder, who collaborated on the study.

The following year, researchers in Gordon’s lab identified the same pattern in humans: obese individuals have fewer Bacteroidetes and more Firmicutes than lean people. Moreover, the proportion of Bacteroidetes increases when individuals lose weight4. In fact, shifts in the balance of these two types of bacteria crop up again and again in research into the gut microbiota, diet and obesity.

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