Nutritive value of L-, DL- and D-tryptophan in the chick

The present study was undertaken to determine the nutritive value of L-, DL- and D-tryptophan in chicks. Day-old chicks were fed commercial chick starter ration for one week and then they were given an experimental diet containing zein as a protein source for three weeks. The experimental results were analyzed by a slope ratio technique (weight gain vs. tryptophan intake). The relative biological utilization of DL- and D-tryptophan compared to L-tryptophan was approximately 55 and 15%, respectively. At the end of the experiment, the chicks were sacrificed and the concentration of free tryptophan in plasma was measured. The free tryptophan concentration in plasma corresponded closely with the increment level of tryptophan in diet when there was a normal level of dietary protein. But there was less correspondence when chicks were fed a diet low in protein.