I'm a 17 year old Turk with PKU. I had to be a strict vegan from birth or else I would get intellectual disability. AMA!
I was diagnosed with PKU one week after birth, meaning that if I consumed high levels of protein during my lifetime (especially childhood), it would cause irreversible brain damage.
My body can't digest the phenylalanine in proteins, and undigested blood phenylalanine above 6 mg/dL is a neurotoxin. So, I've never eaten any sort of meat or dairy product during my lifetime, which are the main sources of protein.
To be able to grow like everyone else who consumes protein, I have to take in "pre-digested" protein so that my body doesn't have to do the work and fail miserably (also if you don't feed it digested protein twice or thrice every day it tries to digest itself and gets your blood phenylalanine mg/dL skyrocketing, so that's cute and convenient).
The possibility of getting intellectual disability (and related stuff) still haunts me to this day.
Ask me anything!
You can find the box of the "pre-digested" protein and my username here: https://ibb.co/zNgcCn0 (Imgur is banned in Turkey.)
Edit: If you're curious, PKU stands for phenylketonuria (derived from phenyl ketone in urine), and it's just the abbreviated name of the condition I've described. Nobody says "phenylketonuria" because it doesn't describe the condition better than just saying "PKU."