This article is from the WSSF 2017 AFRMA Rat & Mouse Tales news-magazine.

Scientists have found that by altering one gene that produced a form of the delta protein that was permanently on, they made mice that were super fit by increasing the slow-twitch muscles which made them resistant to obesity and have more endurance. These slow-twitch muscle fibers burn fat more rapidly and the mice were able to stay in peak condition even without exercise or when on a high-fat diet. These marathon mice were able to run more than a mile more than their normal counterparts.

Story Modified Mice Stay Super-Fit – Without Exercise National Geographic News

Study Regulation of Muscle Fiber Type and Running Endurance by PPARδ Public Library of Science Biology (PLoS Biology) August 24, 2004.