In 2016, Eran Elinav and coworkers at the Weizmann Institute of Science in Rehovat, Israel, advanced Gordon’s studies by determining exactly what was happening in the intestines of thin mice that allowed them to remain thin.

First, they repeated Gordon’s studies, finding that germ-free mice that received the feces from obese mice became obese and those that received the feces from thin mice remained thin. Then they studied the yo-yo effect. They found that when obese mice lost weight, they were much more likely to regain the weight and regain it quickly if they retained the microbiomes of obese mice. On the other hand, if obese mice received fecal transplants from thin mice, they would not only lose weight, they wouldn’t regain it. The researchers had eliminated the yo-yo effect.

Then the Israeli researchers compared the metabolites produced by the bacteria in obese mice with those produced in the intestines of the thin mice. One thing jumped out: Mice that were obese no longer produced certain flavinoids, specifically apigenin and narigenin. (Flavinoids are a byproduct of certain plants and fungi.) Then they found out why. The bacteria from obese mice made enzymes that destroyed apigenin and narigenin, so less of these flavinoids were available. They also found that high-fat diets promoted the growth of bacteria that destroyed the flavinoids.