the team showed that obese people (and mice) have different communities of microbes in their guts.10 The most obvious difference lay in the ratio of the two major groups of gut bacteria: obese people had more Firmicutes and fewer Bacteroidetes than their leaner counterparts. This raised an obvious question: does extra body fat tilt the Bacteroidetes/Firmicutes see-saw or, more tantalisingly, does the tilt make individuals fatter? The team couldn’t answer that question by relying on simple comparisons. They needed experiments.
