Vitamin E is a generic name for a group of compounds that all have the same activity as alpha tocopherol. Vitamin E is only produced in plants and is usually associated with unsaturated fatty acids, the plant using the vitamin to prevent oxidation of the delicate oils. Eating a plant rich diet therefore supplies a good intake of vitamin E, particularly from nuts and seeds which are high in unsaturated oils. However, vitamin E is also available as a supplement, and it is possible to obtain far higher intakes of vitamin E from a tablet or capsule than it is from original plant material. Studies investigating the effects of vitamin E on insulin resistance have not surprisingly therefore used supplements of vitamin E to provide much higher doses than could be obtained from the diet. Such studies have generally shown benefits to insulin sensitivity and also benefits to markers of oxidative stress, suggesting that the vitamin E is able to reduce oxidative stress and thereby improve insulin sensitivity.
However, vitamin E has other non-antioxidant roles in human physiology, and so it is not clear if the effects of vitamin E on insulin resistance are due to its antioxidant capacity or its non-antioxidant effects. In particular, vitamin E is able to inhibit a cellular signal molecule called diacylglycerol through activation of its degradation enzyme diacylglycerol kinase. By degrading diacylglycerol more quickly, vitamin E may inhibit the activation of another enzyme called protein kinase C, that in turn is able to decrease the sensitivity of the insulin receptor by causing conformational (shape) changes to particular amino acids in the receptor. In this way vitamin E inhibits one of the primary mechanisms by which the insulin receptor may be desensitised and thus maintains cellular insulin sensitivity. However, while vitamin E can improve insulin sensitivity and decrease oxidative stress, studies have not demonstrated significant weight loss effects for the vitamin as has been shown for other antioxidant foods such as cinnamon.