A study published in Science magazine found that mice without a protein known as toll-like receptor 5 (TLR5) in their gut gain excessive weight and develop full-blown diabetes and fatty liver disease when fed a high-fat diet.

Think of the gut flora as a community, TLR5 is like a neighborhood police force that can keep the bad guys in check. Without TLR5, bad bacteria can get out of control.

The study authors found that these bad bacteria caused a low-grade inflammation in the mice, which caused them to eat more and develop insulin resistance.

But the most interesting part of this study is what happened when the researchers transferred the gut flora from the TLR5-deficient overweight mice into the guts of skinny mice: the skinny mice immediately started eating more and eventually developed the same metabolic abnormalities the overweight mice had.