Excessive nicotinic acid increases methyl consumption and hydrogen peroxide generation in rats.

Recent ecological evidence has showed a lag-correlation between the prevalence of diabetes and consumption of niacin (nicotinamide and nicotinic acid) in the US. Nicotinamide has been demonstrated to induce insulin resistance due to excess reactive oxygen species and methyl depletion, whereas the effect of nicotinic acid is poorly understood. To examine the mechanism of the effect of nicotinic acid on glucose metabolism. Rats were injected with different cumulative doses of nicotinic acid (0.5, 2, 4 g/kg) and nicotinamide (2 g/kg). A glucose tolerance test was given 2 h after the final injection. The role of methyl consumption and reactive oxygen species generation were evaluated by measuring N(1)-methylnicotinamide and hydrogen peroxide. Cumulative doses of nicotinic acid produced a dose-dependent increase in the plasma levels of N(1)-methylnicotinamide and hydrogen peroxide, which was associated with a decrease in liver and skeletal muscle glycogen levels. At the same dosage (2 g/kg), in comparison with nicotinamide, nicotinic acid was weaker in raising plasma N(1)-methylnicotinamide levels (0.7 ± 0.11 µg/mL vs. 4.69 ± 0.24 µg/mL, P < 0.001), but stronger in increasing plasma hydrogen peroxide levels (1.88 ± 0.07 µmol/L vs. 1.55 ± 0.05 µmol/L, P < 0.001). Moreover, nicotinamide, unlike nicotinic acid, did not reduce liver glycogen levels. This study suggested that excessive nicotinic acid, like nicotinamide, might induce methyl consumption, oxidative stress and insulin resistance. Long-term consumption high niacin may increase the risk of type 2 diabetes.