Scientists put rodents on a high-fat diet of bad foods, and the outcome is what we expect. After nine days of gluttony on fatty foods, the rats became overly sluggish and lost cognitive abilities.
The rats’ running stamina decreased by 50% when relegated to the fatty diet after the experiment. They made more mistakes in running maze patterns than rats on a low-fat diet.
http://latimesblogs.latimes.com/booster_shots/2009/08/dieting-obesity.html