If it fits your macros right? IFFYM = If it fits your macros. You gotta understand that concept first. If your macros are 200g P, 150g C and 50g of fat and you can somehow eat McDonald's while fitting their nutritional values into YOUR macros, then by all means go for it. I think you misunderstand the fact that a caloric deficit isn't simply the best way to go while cutting. Yes, it's calories in vs calories out but at the end of the day you want to make sure you're getting an adequate amount of protein and fats to minimize loss of LBM. And it's a lot harder to hit those macros with a diet based on junk food entirely (or mostly).