Frequently Jon has Garfield go on various diets, usually after some exaggerated example of the cat's weight (i.e. bouncing on a hardwood floor, or causing the middle panel in one strip to bend). During the week Garfield usually has to eat carrots or a leaf of lettuce and often has hallucinations of anamorphic food or Jon and Odie as food. So far only one diet has succeeded in causing Garfield to lose weight due to the cat's often cheating on them. On one occasion all he lost on a fourteen day diet was two weeks. At one point Jon comments that for Garfield diets aren't about losing weight, but slowing the gain. Twice Garfield has mentioned that the best he has done on a diet is gaining one pound, although this is contradicted when on one diet, Jon said that Garfield had stuck to his diet and actually lost a few pounds. Jon also occasionally joins Garfield on diets. In his first diet strip, Garfield described it as "die with a t".

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