Low serum tryptophan to large neutral amino acids ratio in idiopathic infantile autism.

The serum tryptophan to large neutral amino acids ratio (Try/LNAA) is considered a reliable marker of tryptophan availability for brain serotonin synthesis. A dysfunction of brain serotonergic activity has been postulated to exist in autistic disorder and supported by recent studies. On this basis, we determined the serum amino acids levels in 40 children with idiopathic infantile autism as well as in 46 control children. A significantly lower serum Try/LNAA ratio was observed in the autistic subjects compared to the normal controls. In 14 autistic children (35%) this ratio was 2 SD below the mean value obtained in the control group. These results suggest that a low brain tryptophan availability due to a low serum Try/LNAA ratio could be one of the possible mechanisms involved in the alteration of serotonergic function in autism.