"You might think half a muffin over an entire day wouldn’t matter much, particularly if you exercise regularly. After all, doesn’t exercise turn fat to muscle, and doesn’t muscle process excess calories more efficiently than fat does? Yes, although the muscle-fat relationship is often misunderstood. According to calculations published in the journal Obesity Research by a Columbia University team in 2001, a pound of muscle burns approximately six calories a day in a resting body, compared with the two calories that a pound of fat burns. Which means that after you work out hard enough to convert, say, 10 lb. of fat to muscle — a major achievement — you would be able to eat only an extra 40 calories per day, about the amount in a teaspoon of butter, before beginning to gain weight. Good luck with that."
From the 2009 Time Magazine article: Why Exercise Won’t Make You Thin