Cystine deficiency during dietotherapy of homocystinemia

Cystine deficiency was inadvertently produced in a boy receiving specific dietary therapy for homocystinuria. This was manifested as a loss in weight, the reappearance of significant amounts of homocystine in the plasma and urine, and the elevation of the plasma methionine level. In addition, there was a significant reduction in the level of cystine in the plasma. This reduction in plasma cystine level differentiates cystine deficiency from loss of biochemical control due to failure to keep the prescribed diet. The addition of cystine to the regime of this child, without any other dietary modification, resulted in a complete remission.