Tryptophan Malabsorption in Levodopa‐treated Parkinsonian Patients

Abstract.Subnormal serum tryptophan (Try) response has been measured in parkinsonian patients receiving oral Try loads while under levodopa treatment, indicating malabsorption of Try in the gut. Other amino acids (phenylalanine, tyrosine and glutamic acid) also depressed the absorption of Try. If levodopa was excluded during Try loading, nearly normal serum Try values were measured. Similar tests in a normal subject revealed normal absorption of Try. An explanation of these observations is discussed and an insufficiency (degeneration?) of amino acid carriers in the intestinal mucosa, and possibly also in other membranes, is suggested in parkinsonian patients. Mental disturbances were observed in cases with extremely low serum Try values and were presumed to be correlated with an insufficient synthesis of 5‐hydroxytryptamine (5‐HT) in the brain secondary to a decreased influx of Try. In two cases a considerable improvement of mental symptoms was noted after i.v. and oral Try medication, and in one case a regeneration of the ability to absorb Try during levodopa treatment was noted. A prophylactic Try or protein treatment during levodopa treatment was recommended.