For me, I have always continued to follow my low-protein diet. When I was a teenager, I was not very good with taking my formula which provided me with the essential proteins a person needs for proper nutrition. I was deficient in protein. This caused me to get very sick. I was extremely thin and very weak. I had a hard time concentrating. I was irritable and moody. I also was extremely fatigue often. Once I got myself back into a routine of taking formula consistently, I felt a HUGE difference in my self. I was much happier of a person.
I am sure you will hear from some more people on here what it is like to go completely off diet. During your project, make sure you emphasize the importance that PKU is a lifetime diet! Without remaining on diet, a person's health (and life) can be greatly impacted.