It is known that certain foods are addictive. Most people are aware of the addictive nature of drinks containing alcohol and caffeine, but it is often surprising to learn that low quality processed foods containing high amounts of fat and sugar are also addictive. Addiction from food is caused by activation of part of the striatum in the brain. In particular, neuronal activity in the mesolimbic dopaminergic system that sends signals to the nucleus accumbens is integral to the reward and craving effects of foods. It is interesting that obese subjects have fewer dopamine D2 receptors in this part of the brain compared to lean controls, which suggests one of two things. Either overeating low quality foods stimulates the striatal reward pathways and downregulated the receptors, or obese subjects compensate for lower dopamine receptors by overeating low quality foods. It is unclear how low quality foods may cause activation of the reward pathways in the striatum of the brain, but one explanation centres on changes in blood sugar.
Low quality foods that contain refined carbohydrates tend to elicit sharp rises in blood sugar. Such high glycaemic foods are thought now to be primary drivers of disease because they overload the liver with nutrients and this leads to the production of fatty acids which are a likely cause of insulin resistance. High glycaemic index foods cause rapid increases in blood sugar and insulin release following consumption, but in the late postprandial period the levels of both glucose and insulin drop below their normal levels which may stimulate hunger and be a driver of overeating. Studies have investigated the effects of glycaemic index on brain activity postprandially and found differences in activity when comparing high and low glycaemic index meals. For example, in one study1, researchers fed overweight or obese men either a low or high glycaemic index test meal that had a similar taste, texture and macronutrient distribution. Only the glycaemic index of the meals were different, and this was achieved by altering the ratio of corn starch to corn syrup.
The results of the study showed that two hours after the meal, the plasma glucose response was significantly higher in the subjects consuming the high compared to the low glycaemic index meal. However, at four hours postprandially, the plasma glucose levels were lower in the group consuming the high glycaemic index meal as they experienced rebound hypoglycaemia. These results are supportive of previous studies showing exaggerated blood sugar responses following high glycaemic index foods. At four hours postprandially, the low glycaemic index meal also elicited a greater activation of the right nucleus accumbens of the striatum, that subsequently spread to other areas of the striatum and the olfactory area. Mechanistically then, high glycaemic index meals may cause changes in blood sugar and insulin, and these changes may drive feeding behaviour in the postprandial period. As insulin is one of the major activators of satiety, it is not surprising the rebound hypoglycaemia causes activation of pathways that may increase food intake.
Dr Robert Barrington’s Nutritional Recommendation: The evidence is now well established to show that high glycaemic index foods can be detrimental to the health if consumed over long periods. However, just because a food is of a low glycaemic index does not mean it is healthy. Fructose has a very low glycaemic index but is thought to be a driver of obesity. The trick is to eat unrefined and unprocessed sources of carbohydrates, which means only choosing whole grains, whole fruits and whole vegetable products.
RdB
