A family of proteins called PPARs (peroxisome proliferator-activator receptors) controls how the body uses sugar and fat.  PPAR-gamma drives the formation of fat cells and regulates the storage of fat. It encourages muscles to burn sugar and maintains insulin sensitivity. PPAR-delta regulates how muscles burn fat by stimulating cellular fat-burning pathways and increasing slow-twitch muscle mass, which primarily uses fat as an energy source. Mice that are engineered to produce an overactive version of this receptor in their muscle tissue remain sleek and lean. On a treadmill, these 'marathon mice' run twice as far as normal mice. Drugs that stimulate PPARs might help people slim down and improve health without altering appetite. From the Series:HHMI’s Understanding Biomedical Research Series