A potential explanation could lie in the trillions of microbes that reside in the gut. In 2009, Rosa Krajmalnik-Brown from Arizona State University in Tempe and her colleagues sequenced the bacterial genes present in faeces from three people who had received a gastric bypass. Compared with obese and normal-weight controls, their guts contained proportionally fewer bacteria from the usually abundant Firmicutes phylum, and excess levels of the Gammaproteobacteria class8. “Even with that small sample size we were able to get statistically significant differences because the microbiota changed so drastically,” Krajmalnik-Brown says.