Effect of L-tryptophan and phenylalanine on burning pain threshold

The purpose of this study was to determine the effects of ingestion of L-tryptophan or phenylalanine on burning pain threshold using a double blind, pretest-posttest control group design. Sixty healthy, female student volunteers were assigned randomly to L-tryptophan, phenylalanine, or placebo groups, (Groups 1-3, respectively). The pretest radiant heat tolerance was determined for all subjects. Burning pain tolerance was defined as the amount of time in seconds from the initial exposure of the left fifth distal phalanx to a 250-W infrared lamp until the subject's detection of an "intense burning sensation." The subjects in Groups 1 and 2 ingested four 500-mg tablets of phenylalanine or L-tryptophan a day for 14 days. The placebo group ingested four placebo tablets a day for 14 days. Immediately after the 14th day, the radiant heat pain tolerance of all subjects was remeasured. The results of a one-way analysis of covariance showed no significant difference in the posttest pain tolerance values of the three groups.