Ask people what they do for nutrition during their off days and most will tell you they lower calorie or carb intake. It's a common recommendation, but it's wrong.
People think that since they won't be training and burning as much fuel, they shouldn't consume the same amount of carbs or else they'll risk gaining fat. If your goal is STRICTLY fat loss there may be some value to this. But if you're looking to build as much muscle and strength as possible, you should see your off days as days where you're trying to do everything you can to maximize growth and performance, not days where you don't do anything. Think of your time off as an investment.
With this in mind, what should you do nutrition-wise? Cut carbs and calories, leaving you with muscles that aren't fully replenished with glycogen, leaving you unable to take advantage of the anabolic properties of insulin? No! Do the opposite.
Accelerate Rest Day Gains
Off days are growth days. And insulin is the most anabolic hormone in the body. It shuts down catabolism (protein breakdown or muscle wasting) and ramps up anabolism – protein synthesis or building muscle. It also activates mTor which triggers muscle growth and increases glucose storage inside the muscles. Muscles being full of glycogen is itself anabolic. As a bonus, a fuller muscle is a stronger muscle.
If your goal is maximum muscle growth and strength gains, it's important to consume plenty of quality carbs and protein during those off days. See each rest day as the only day in the week where the body is allowed to build muscle. And if that's the case, what would you do on that day to make sure that you get every possible ounce of muscle growth? Certainly not lower your calorie or carb intake!
At the very least, keep calorie and carb intakes the same or increase them a bit. I'd also increase protein intake slightly to take advantage of the more anabolic state. Mag-10® pulsing would be perfect for this. I simply add three servings or "pulses" to my regular protein intake on off days.