Why do you think an adult with PKU can tolerate small amounts of phenylalanine in their diet but a newborn can not?

Adults with PKU have matured enzyme systems that can better manage and process phenylalanine compared to newborns. Newborns are still developing these systems, so even small amounts of phenylalanine can accumulate to harmful levels in their bodies. Monitoring and managing phenylalanine intake is crucial for both adults and newborns with PKU.