Rats where split into 2 groups, one group was injected with a single does of streptozotocin at a dose of 60 mg/kg body weight, which is a chemical that is used to induce diabetes. After 12 weeks it was found that the diabetes group was only able to convert half as much tryptophan to niacins as the control group. The rats in the diabetic group had twice the excretion of xanthurenic acid in their urine. Xanthurenic acid is a waste product that is produced when tryptophan is not converted into niacin.