I can understand your confusion about the lack of symptoms. I was off-diet for about 30 years, but haven’t had any noticeable symptoms. I’m back on-diet now, as well as being on Kuvan – and responding well. I have also been quite intelligent, graduated from college with honors and a double major – while off-diet, and got my master’s degree with around a 3.8 GPA on a 4.0 scale – while working a full-time and a part-time job, and being on-diet only part of the time. In my case, I have variant PKU, with one mild and one severe mutation. As a result, my levels off-diet were never really high…12-15, maybe. From something I read written by Dr. Richard Koch, it isn’t unusual for those with vPKU to go 20-30 years off-diet with no negative side effects. Seems a little unusual with your low phe tolerance, but…every PKUer is different in how they handle phe – even those with the same mutations vary in their phe tolerance. Good that you’ve had the experience you’ve had…despite the low phe tolerance.