Depletion of glutathione, an important antioxidant present in red cells, has been reported in type 1 diabetes, but the mechanism of this depletion has not been fully characterized. Glutathione depletion can occur through decreased synthesis, increased utilization, or a combination of both. To address this issue, 5-h infusions of l-[3,3-(2)H(2)]cysteine were performed in 16 diabetic adolescents divided into a well-controlled and a poorly controlled group and in eight healthy nondiabetic teenagers as control subjects (HbA(1c) 6.3 +/- 0.2, 10.5 +/- 0.6, and 4.8 +/- 0.1%, respectively). Glutathione fractional synthesis rate was determined from (2)H(2)-cysteine incorporation into blood glutathione. We observed that 1) erythrocyte cysteine concentration was 41% lower in poorly controlled patients compared with well-controlled patients (P = 0.009); 2) erythrocyte glutathione concentration was approximately 29% and approximately 36% lower in well-controlled and poorly controlled patients compared with healthy volunteers; and 3) the fractional synthesis rate of glutathione, although similar in well-controlled and healthy subjects (83 +/- 14 vs. 82 +/- 11% per day), was substantially higher in the poorly controlled group (141 +/- 23% per day, P = 0.038). These findings suggest that in diabetic adolescents, poor control is associated with a significant depletion of blood glutathione and cysteine, due to increased rates of glutathione utilization. This weakened antioxidant defense may play a role in the pathogenesis of diabetes complications.