To eat is pleasurable and rewarding. It is not surprising therefore that brain centres involved in pleasure and reward are activated when we eat. Fundamental neurobiological mechanisms involved in food reward are of considerable importance for understanding how body weight is regulated, both in health and in disease.
A great deal of obesity research over the past two decades has identified genes and mechanisms that are important for maintaining energy balance. Although body weight is strongly influenced by our genes, it is also influenced by lifestyle and social habits, reflecting a powerful interaction between genes and environment. Food intake, however, is motivated not only by the need to restore energy homeostasis; palatable, rewarding high fat and/or sugar foods such as chocolate can motivate intake even when we are full. Obesity reflects an energy imbalance in which genetically susceptible individuals become increasingly vulnerable to an obesity-promoting environment. Thus, both the palatability and availability of foods in the Western diet play a major role for the development of this disease. An emerging hypothesis concerns the role of the brain’s reward system, that responds to the stimulus provided by rewarding and palatable obesity-promoting foods and appears to override those that serve to keep us in energy balance. Indeed, mismatch between the hedonic/rewarding value attributed to food and energy needs is characteristic of eating disorders, including those that lead to obesity.
Palatable, rewarding high-fat high-sugar foods such as chocolate can motivate intake even when we are full.From an evolutionary perspective, it is easy to understand why eating involves hedonic processes. A positive hedonic experience in association with the consumption of food helps ensure an adequate supply of nutritionally diverse foods from our environment. Whereas man the hunter would have benefited from the hedonic experience of eating, in our modern obesogenic environment, it may be more advantageous for health and survival to suppress it. Indeed, this concept has inspired research and development of anti-obesity drugs that target the reward mechanism. Such agents would be expected to reduce food intake through the suppression of food reward, involving direct or indirect interruption of food-sensitive reward pathways.
When considering how the brains controls appetite it remains difficult to identify what makes Luxury Creamy Dark Chocolate Fudge Cake more rewarding to eat than a bowl of piping hot porridge, so much so that we are prepared to consume a large portion or two at the end of a satiating meal. How do rewarding foods trigger brain responses that reinforce their consumption? Is it the ingredients or even the combination of ingredients that elicit special visual, olfactory and/or oro-sensory experiences that make them more palatable and heighten their rewarding value? Rats, like humans, have a “sweet tooth” and show preference for sweet and/or fatty foods. Indeed, a well-recognised and highly reproducible finding is that animals show increase their food intake (an increased kcal consumed per day) when switched from normal chow to an obesity-promoting diet. The most likely explanation for this over-eating is that the obesity-promoting diet is more rewarding and invites increased consumption. Indeed, sweet taste alone appears to be sufficient to activate the brain’s reward system, reflected by the effects of high sucrose drinks to increase motivated behaviour for food.
What makes Luxury Creamy Dark Chocolate Fudge Cake more rewarding to eat than a bowl of piping hot porridge?Moreover there are indications that the calorie content of sucrose, not the taste, that is rewarding. Sugar and fat are especially effective for inducing motivated behaviour for food in rats, especially in combination. On the other hand, properties of the food such as palatability (i.e. the hedonic evaluation of a flavour stimulus) enhance pleasure and motivational drives that induce further consumption, providing positive reinforcement. In the context of chemical drug reward, reinforcement forms part of the addiction mechanism. For survival, evolutionary pressures have clearly promoted reinforcement of the pleasurable oral experience of eating in order to help maintain energy balance. The problem emerges of how to put on the break during times of food excess.