In a perfect world, eating more food would directly correlate to more lean muscle mass. Unfortunately, it doesn't work that way. Except for those on steroids who are able to elevate protein synthesis around the clock, natural lifters need a moderate surplus to avoid gaining too much fat. What you need to do is calculate your calories and shoot for 300-500 calories above maintenance. There are dozens of equations out there to help you do this, but here's a simple one:
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• Body weight (pounds) x 15 = maintenance
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So, if someone weighs 160 pounds it would look like this:
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• 160 pounds x 15 = 2,400 calories per day
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So, this hypothetical person would need to consume 300 to 500 calories on top of that, so 2,700 to 2,900 calories per day.
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If you gain fat easily, shoot for the lower end of this range. If you have a crazy metabolism, shoot for the top range or a bit over. You'll have to experiment and see what's best for you, but this will get you in the neighborhood. You can adjust from there after a few weeks. This sensible increase equates to one or two protein shakes and a banana or two per day above maintenance calories. Alternatively, this is an extra sweet potato and a chicken breast each day, not a 1200-calorie burger from a fast food joint. If you greatly exceed this 300 to 500 calorie guideline, you'll end up gaining far too much fat. Activity levels play a role here too, but this guideline will get you on the right track.