After you train, it's difficult to gain body fat. Why? Depleted, broken-down muscles soak up both protein and carbohydrates for growth and recovery. If you eat too little at this time, you may actually set yourself back by impeding recovery; supporting recovery and growth actually increases metabolism while impeding it slows metabolism. In terms of spurring recovery and growth, just about the most counterproductive thing you can do after a hard workout is starve yourself.
This may explain why the fat-burning effects of eating more protein were confirmed in a study published in the American Journal of Physiology. One group was fed a high-protein diet (just over 1 gram per pound of body weight per day) while the second group consumed an amount closer to the lower recommendation of the RDA (recommended dietary allowance). The group eating the higher-protein diet burned the most fat.