Some extreme eating behaviours can be considered a form of addiction, as has been demonstrated in animal studies involving feeding highly palatable foods to rats. In some cases, these eating behaviours can cause brain changes that mirror those seen in cases of drug abuse. It is known that some eating behaviour is regulated by endogenous opiate peptides such as the proopomelanocortins, the prodynorphins and the proenkephalins, which increase feeding in mammals via activation of the mu, delta and kappa receptors. The opioid peptides may specifically regulate feeding behaviour linked to particular micronutrients such as high fat or very sweet foods. Opioids may therefore regulate feeding behaviour by modulating the pleasure response to palatable foods, which is supported by evidence that β-endorphin levels are elevated in those individuals with eating disorders that are characterised by binge eating.
To investigate the effects of opiates on the feeding response to sweet and high fat foods, researchers have used the opiate antagonist naloxone and the opiate agonist butorphanol. For example1, 16 obese and 25 normal weight women were infused with either naloxone, butorphanol or a saline placebo and then asked to taste and rate 20 sweetened diary products and consume unlimited quantities of 8 snack foods preselected as favourite foods. Infusion of naloxone suppressed consumption of high fat and sweet foods in a sub-group of the subjects psychologically assessed and confirmed as suffering from bulimia nervosa. In all subjects (including those not suffering from bulimia nervosa), naloxone suppressed the hedonistic response to food, as measured by how much they liked the sweetened dairy products. However, the naloxone did not alter the food intakes of obese women in terms of the amount consumed.
Naloxone was particularly effective at reducing desire for chocolate containing foods, which tends to be high in sugar and fat (up to 98% in some products). Chocolate may therefore be particularly effective at stimulating the opiate reward centres of the brain, reinforcing behaviour that causes increased intakes, particularly in women. Naloxone is also reported to be effective against alcohol addition, providing further evidence that opiates released in response to food stimulate addiction pathways. The fact that naloxone has not produced weight loss in studies investigating its effects indicate that while it is able to alter the feeding behaviour associated with high fat and sweetened foods, it is not able to regulate appetite as a whole. Interestingly, butorphanol had no effects on the feeding behaviour of the subjects, showing that intravenous opiate agonists are not effective at altering feeding behaviour.
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