I know in order to gain muscle you have to be in a calorie surplus, and I know losing fat and gaining muscle don't mix together.
But I'm just wondering why can't it be possible for someone who's in a calorie deficit to gain muscle just by replacing their normal calories with protein and muscle building calories?
Let's say I have to consume less than 1800 calories a day in order to lose fat. Can I hit the gym before one of my main meals, lunch for example, and then instead of having a normal lunch I would take proteins (chicken breast, eggs, potatoes, etc...) so that hopefully I can gain muscles ?