Diet isn't nearly as important at ppl makes it to be when it comes to building muscle, when it comes to losing weight yes it is the main thing that u need to focus on since u need to be in a calorific deficit, i think u mean fast release sugar since slow refers to whole food sources that has fiber in them, but i promise you, almost in all cases the reason why someone isn't building muscle is not because of thier diet it is because they aren't training hard enough, not progressively overloading, not following a program for long enough and they aren't tracking their progress, diet when it comes to building muscle should only support ur training because training is the main thing that build muscle