HYDROXYPROLINE and proline are nonessential amino acids; both are synthesized in the body. Thus, the three known diseases which are associated with defective degradation of these amino acids may prove to be untreatable by dietary means.
When an essential amino acid is accumulated because of an inherited defect in a degradative enzyme, as in phenylketonuria (PKU) or maple syrup urine disease, the blood level of the accumulated amino acid can be lowered by limiting the amino acid (s) in the diet. When, however, the amino acid is freely synthesized in the body, dietary treatment may not be successful in lowering the concentration of the affected amino acid in the blood.
Certain disorders with blocks in the degradation of nonessential amino acids which are intermediates in metabolic pathways may also prove amenable to dietary therapy. An example is homocystinuria which apparently responds to removal of most of the essential amino acid
EFRON ML. Treatment of Hydroxyprolinemia and Hyperprolinemia. Am J Dis Child. 1967;113(1):166–169. doi:10.1001/archpedi.1967.02090160216037