摘要AIM: To investigate whether nicotinamide overload plays a role in type 2 diabetes.METHODS: Nicotinamide metabolic patterns of 14 diabetic and 14 non-diabetic subjects were compared using HPLC. Cumulative effects of nicotinamide and N~1-methylnicotinamide on glucose metabolism, plasma H_2O_2 levels and tissue nicotinamide adenine dinucleotide (NAD) contents of adult Sprague-Dawley rats were observed. The role of human sweat glands and rat skin in nicotinamide metabolism was investigated using sauna and burn injury, respectively.RESULTS: Diabetic subjects had significantly higher plasma N~1-methylnicotinamide levels 5 h after a 100-mg nicotinamide load than the non-diabetic subjects (0.89 ± 0.13 μmol/L vs 0.6 ± 0.13 μmol/L, P< 0.001). Cumulative doses of nicotinamide (2 g/kg) significantly increased rat plasma N~1-methylnicotinamide concentrations associated with severe insulin resistance, which was mimicked by N~1-methylnicotinamide. Moreover, cumulative exposure to N~1-methylnicotinamide (2 g/kg) markedly reduced rat muscle and liver NAD contents and erythrocyte NAD/ NADH ratio, and increased plasma H_2O_2 levels. Decrease in NAD/NADH ratio and increase in H_2O_2 generation were also observed in human erythrocytes after exposure to N~1-methylnicotinamide in vitro. Sweating eliminated excessive nicotinamide (5.3-fold increase in sweat nicotinamide concentration 1 h after a 100-mg nicotinamide load). Skin damage or aldehyde oxidase inhibition with tamoxifen or olanzapine, both being notorious for impairing glucose tolerance, delayed N~1-methylnicotinamide clearance.CONCLUSION: These findings suggest that nicotinamide overload, which induced an increase in plasma N~1-methylnicotinamide, associated with oxidative stress and insulin resistance, plays a role in type 2 diabetes.