'After just nine days, they were only able to run 50 percent as far on a treadmill as those that remained on the low-fat diet,' said the lead author of the study, Dr. Andrew Murray, now at the University of Cambridge. Endurance depends on how much oxygen can be supplied to muscles and how efficiently muscles release energy by burning the fuel supplied by food. Fat as a fuel is less efficient, but studies on how various diets affect physical performance have been mixed, the authors said. The rats in the study showed a 30 percent decline in running stamina after only five days. They also began making more mistakes in a maze task. The rats also had significantly bigger hearts after only nine days. The study 'shows that high-fat feeding even over short periods of time can markedly affect gene expression,' Kieran Clarke, chief of the research team at the University of Oxford, said in a news release. 'By optimizing diets appropriately we should be able to increase athletes' endurance and help patients with metabolic abnormalities improve their ability to exercise and do more.' The study is published in the current edition of The Journal of the Federation of American Societies for Experimental Biology, at www.fasebj.org. 