Hello everyone. I thought I would bite on this since its what I do. I’m a personal trainer and fitness therapist who also happens to have PKU. In my experience for the guys, there are those who can build mass by looking at a dumbell and those it appears to take an act of Congress to build any muscle mass. I would not suggest extra formula unless your nutritionist has allowed room for a little extra. Mine has done that so it certainly isn’t unheard of. So yes extra protein intake can build mass but it has to be in proper balance with your workout. Most protein supplements in the gym have incredible calorie content.Body builders that take those have to ensure that the workout doesn’t allow for body fat gain. It is different for every body, its very personal. I would be happy to help anyone with fitness goals. HAVE A GREAT DAY!