The body still needs a certain amount of protein in childhood growth to grow properly. Decreasing protein to low amounts I believe causes the body to have to find it somewhere else, their own body the body if it doesn't get enough protein or calories starts to draw protein from other areas, muscle, tissue. If that happens your levels go way up where they shouldn't be. I know this is true if they don't get enough proteins and I was led to belie
When my son was an infant if the test came back to low they would have me put 1 oz of cows milk to bumb his level up. I still do this today when his blood test come back below a 2.
Usually I would think if your working with a nutritionest they would contact you if it was a cause for alarm. Or call them and talk with them, there's lot's to learn about PKU.