Oxidative stress may be a cause of insulin resistance. This explains the benefits of antioxidants at improving insulin resistance, that has been reported in some studies. Antioxidants are beneficial because they can quench the free radicals that are the cause of the oxidative stress. Antioxidants are able to quench free radical chain reactions because they donate electrons to the free radicals, and this stabilises the radical and prevents it propagating the chain reaction to another molecule. Breaking the chain of reactivity in this way stops damage to cells, which can include the insulin receptor and signal system on the surface of cells. When antioxidants donate electrons, they become radicals themselves because they now have unpaired electrons. However, the antioxidant defences in the body work is synergy, such that when one antioxidant sacrifices itself to prevent a free radical chain reaction, it is quickly recycled by another antioxidant. Therefore as long as antioxidant to free radical remains in favour of the antioxidants, tissue damage is minimised.
Antioxidants are available from three main sources. Endogenous antioxidants are formed in the body and include superoxide dismutase and glutathione peroxidase. We shall not concern ourselves with these antioxidants in this discussion, important as they are. Another source of antioxidants are supplements such as vitamin C and vitamin E. There is evidence that supplementing with antioxidant supplements has distinct health benefits, including improving insulin sensitivity. Both vitamin C and vitamin E, as well as other single antioxidant nutrients such as beta carotene have distinct health benefits and reduce oxidative stress, but care needs to be taken when supplementing with high doses of single antioxidants. This is because antioxidants work synergistically, and large doses of single substances can disrupt the delicate biochemical balance in the body. The third, and likely best way, to boost antioxidant defences in the body is therefore through eating foods high in a variety of different antioxidants.
Many phytochemicals have antioxidative effects in humans. The most well researched groups of these are the polyphenols and the carotenoids. Polyphenols are compounds with multiple ring structures, and when they donate electrons to quench free radicals, the charge from the unpaired electrons are dissipated around their structure, preventing the molecule becoming a damaging free radical. Carotenoids work in a different way, and can absorb the energy from excited singlet oxygen molecules into their own structure and then dissipate this energy as heat. Polyphenols are common in fruits and are responsible for their red, blue and purple colours. Carotenoids are common in vegetables and responsible for their yellow, red and orange colours. Recommendations are to eat a variety of coloured fruits and vegetables to provide a wide a range of carotenoids and polyphenols. Other antioxidants in plants such as the terpenes and stilbenes from olives and red wine, respectively, may also be beneficial at improving insulin sensitivity.
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