In the previous rules, we call for cutting back food intake on rest days. Calories drop by about two to three per pound of bodyweight and carbs are slashed in half. Even protein drops a bit (mostly due to the fact that you’re not drinking a pre- and postworkout shake).
You need to watch calories and carbs on rest days because your body requires less energy when it’s not working. Simple enough. If you ingest the same amount of calories and carbs on rest days as you do on workout days, you run the risk of gaining some fat with that newly developed muscle.
However, increase your fat intake on rest days, so your body doesn’t compensate for a perceived calorie deficit. If you focus on getting extra healthy fats (see rule number 3), you will not only help to keep bodyfat off, but will enhance joint recovery.