Tryptophan loading test is done to detect deficiency of:

Tryptophan loading test detects xanthurenic acid in urine which indicates deficiency of Vitamin B6 i.e. PLP (Pyridoxal phosphate). In the absence of Vitamin B6 the excess of 3-OH-Kynurenine is conveed to Xanthurenic acid after given a load of tryptophan (see Fig below) Figure: Tryptophan catabolism pathway Note:- Histidine load test detects FIGLU (Form-Imino Glutamate) in urine which indicates deficiency of Vitamin B9 Deficiency Common metabolite Specific metabolite Vitamin B6 Homocysteine Xanthurenic acid Vitamin B9 Homocysteine FIGLU Vitamin B12 Homocysteine L-methyl malonic acid