Whether you want to lose weight, build muscle, or improve athletic performance what and how much you eat is of critical importance.
If you’re interested in building muscle you have to over eat tremendously. You also have to lift tremendously heavy weights (relative to your personal ability). If you eat a lot, but don’t lift a lot, you will not build muscle. You will get fat instead. It’s the exercise, or lack of it that determines how your body uses the excess calories.
If you’re interested in athletic performance you have to eat a lot more than normal to be able to fuel that performance (marathon runners carbo load before race days).
If you’re trying to lose weight you must eat less than you burn off .
Here’s a great article by a great coach who’s workshops I attend every chance I get: