The oxidative stress theory of insulin resistance suggests that excessive free radical production is a causative agent in the development of insulin resistance. Such free radicals are generated by nutrient overload, possibly through fructose feeding, which increases the de novo synthesis of fatty acids. These fatty acids are oxidised in the peripheral tissue and the liver where they generate increased amounts of superoxide radicals by stimulation of the electron transport chain through excessive production of acetyl CoA and electron transport intermediates such as NADH and FADH. In response to the increases superoxide generation, the cell initiates countermeasures to prevent further nutrient uptake, and this includes desensitisation of the insulin receptor to the insulin hormone signal. Antioxidants may be beneficial at improving insulin sensitivity because they are effective at removing the free radicals that may initiate this process. In this regard a number of antioxidant rich foods have been investigated for their insulin sensitising effects.
Flavonoids are phytochemicals with antioxidant properties. They are bioavailable in humans, and they have been shown to exert antioxidant effects. Cocoa is rich in flavonoids of the proanthocyanidin variety (flavanol polymers) and has been investigated for its insulin sensitising effects. In one study1, researchers compared dark and white chocolate in a crossover design study on patients with hypertension. Subjects received 100 grams of dark chocolate per day which contained 88 mg of flavanols, or 90 grams of white chocolate that contained the same amount of energy. The chocolate was consumed for 15 day, followed by a 7 day washout period and then subjects switched to the opposite treatment. The results of the study showed that dark chocolate consumption caused a significant improvement in insulin sensitivity (as measured by a number of clinical tests including HOMA-IR, QUICKI and ISI). In addition, systolic and diastolic blood pressure also fell, as did serum low density lipoprotein (LDL).
Therefore dark chocolate shows beneficial effects on insulin resistance in humans with high blood pressure. Concomitant to these insulin sensitising effects dark chocolate also decreases blood pressure and lowers serum LDL levels. The blood pressure reductions seen in this study resulted from improvements in the nitric oxide induced relaxation of the endothelium. Nitric oxide is synthesised via the enzyme nitric oxide synthase in the endothelium, and free radicals and associated oxidative stress are known inhibitors of this enzyme. Plant antioxidants including flavonoids have been shown previously to improve endothelial dysfunction by increasing the activity of nitric oxide synthase. The flavonols in cocoa therefore may be responsible for the blood pressure lowering effects of the dark chocolate seen in this study through their antioxidant activity. While the subjects in this study consumed chocolate, cocoa powder would likely have a similar effects, but is absent of sugar and so may be a healthier alternative.
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