Oxidative stress is associated with the aetiology of many of the Western lifestyle diseases such as metabolic syndrome, cardiovascular disease, obesity and type 2 diabetes. Although an association does not prove a cause and effect, studies with animals and humans attest to the ability of dietary antioxidants to favourably benefit plasma levels of biomarkers associated with these diseases. In combination with the epidemiological data, the role for dietary antioxidants in disease prevention is therefore strong. Dietary antioxidants are beneficial because they support other antioxidants (here) forming a network of chemical defences against the generation and propagation of free radical induced oxidative stress. Such oxidative stress can damage cellular components, and this can impair biochemical function. Oxidative stress for example can inhibit the nitric oxide synthase enzyme in endothelial tissue, preventing the formation of nitric oxide, and causing endothelial dysfunction and its associated blood pressure changes.
Increased oxidative stress is also thought to lead to impairment of the insulin receptor and contribute to the development of insulin resistance. Insulin resistance and its associated biochemical changes including non-alcoholic fatty liver disease (NAFLD), abdominal obesity, lipoprotein aberrations, blood glucose irregularities and weight gain are characteristic of the metabolic syndrome. The metabolic syndrome (also called syndrome X) is a cluster of disorders that centre on the development of peripheral and central insulin resistance, which is thought to develop through the consumption of low quality Western foods. In particular fructose, is implicated in the development of insulin resistance because it increases flux through the de novo lipogenesis pathway causing the production of fatty acids that accumulate in tissues and cause insulin resistance. The associated weight gain leads to the infiltration of adipocytes by macrophages, and this produces a systemic inflammatory response and oxidative stress.
Oxidative stress is therefore associated with obesity, and is very likely a cause of many of the secondary health implications of being overweight. However, the ability of antioxidant nutrients from plants to reverse insulin resistance and improve peripheral and central insulin sensitivity is well established, suggesting that oxidative stress is also a cause of weight gain (not just a consequence). The effects of the dietary antioxidants vitamin C, vitamin E and selenium have been investigated in obese children1 to assess their effects on oxidative stress. Overweight children received either a supplement of 400 IU of vitamin E, 500 mg of vitamin C and 50 μg of selenium while participating in a lifestyle programme aimed at addressing their obesity. The antioxidant nutrients caused significant improvements in plasma markers of α-tocopherol (vitamin E), ascorbic acid (vitamin C) and selenium compared to those children not receiving the tablet, suggesting improvements in the antioxidant to pro-oxidant ratios of the children.
In addition, the authors performed liver function tests on the children in order to assess the effect of the antioxidant supplements on markers of non-alcoholic fatty liver disease (NAFLD). Alanine aminotransferase showed significant improvements in those children taking the dietary antioxidant supplements, and aspartate aminotransferase also showed a trend towards a significant improvement. However, the antioxidant nutrients did not affect γ-glutamyltransferase activity. Therefore antioxidant supplements have some beneficial effects on improving the function of the liver. This study used modest doses of common antioxidants and for only four months. Had the treatment continued for longer, incorporated a wider range of dietary antioxidant or involved higher intakes, these effect may have been greater. The antioxidant nutrients also had no effects on markers of inflammation in the children. Had dietary flavonoids been used in the supplemental programme, an anti-inflammatory effect may have been seen. Flavonoids are a group of anti-oxidants that have potent anti-inflammatory effects in humans.
Dr Robert Barrington’s Nutritional Recommendation: Around 600 mg of vitamin C can be obtained from a high quality diet by emphasising plant foods. Therefore it could be expected that the effects of the vitamin C in this study could be replicated through the use of food. Likewise, it is not inconceivable that 50 μg of selenium could be obtained from the diet (a tin of tuna for example could contain this amount of selenium). However, it is unlikely that the 400 IU of vitamin E used in this study would be able to be supplied in a healthy diet as it would necessitate high intakes of fats that would unbalance the diet as a whole. Therefore to obtain 400 IU of vitamin E supplements should be taken, preferably mixed tocopherols with added tocotrienols all in their natural D- form. Of course high quality diets supply far more antioxidants than just vitamin C, vitamin E and selenium, and antioxidants should always be obtained from the diet where possibly by eating a range of different coloured fruits and vegetables.
RdB
