Vitamin C and vitamin E are important water and fat soluble vitamins in humans, respectively. Because of their antioxidant effects, both vitamins decrease systemic oxidation and are therefore beneficial against diseases characterised by oxidative stress. Both vitamin C and vitamin E have been shown to improve insulin sensitivity in individuals with diabetes and impaired fasting glucose (IFG), because oxidative stress is believed to cause cellular signalling dysfunction that results in insulin resistance. Antioxidant nutrients such as vitamin C and vitamin E may improve the physiochemical integrity of the plasma membrane in skeletal muscle through increases in microviscocity, and this can have a beneficial effect on the uptake of glucose through the action of insulin. The efficacy of combined vitamin C and vitamin E at improving insulin sensitivity has been demonstrated at higher intakes, and shows promise as nutritional therapy for IFG.
For example, researchers1 have investigated the combined effects of vitamin C and vitamin E on glucose metabolism by supplementing volunteers (mean age 67 year) with 1000mg/d α-tocopherol and 1000 mg/d vitamin C for 4 weeks. The effects on glucose metabolism were assessed using the hyperinsulinaemic euglycaemic clamp method both before and after supplementation. Following supplementation there was an increase in plasma levels of vitamin E (from 8.3 to 64.9µmol/L) and vitamin C (from 35.9 to 79.4µmol/L) as would be expected. Significant reduction were seen in plasma insulin, glucose, lipids, tumour necrosis factor alpha (TNF-α) and isoprostane levels, suggesting that oxidative stress and inflammation was reduced, and that glucose metabolism was improved. Increases in vitamin E correlated with the decrease in TNF-α and isoprostane, but no such associations were found for vitamin C. Combined vitamin C and vitamin E plasma levels correlated with improvements in whole body glucose disposal.
These results suggest that vitamin C and vitamin E are beneficial at reducing insulin resistance at high intakes. The inverse association between plasma vitamin E and both TNF-α and isoprostane suggests that vitamin E may play a more prominent role at reducing oxidative stress and inflammation, when compared to vitamin C. However, this may also reflect the fact that vitamin E plasma status was poor at baseline and was improved to a greater degree following supplementation compared to vitamin C, the latter bring within normal range. Low intakes of vitamin E or vitamin C, as found in poor quality diets, may therefore result in oxidative stress and inflammation that interferes with cellular insulin signalling, ultimately causing insulin resistance and abnormalities to glucose metabolism. The greatest improvement from antioxidant supplements might be expected to be seen in older individuals, as demonstrated in this study.
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