‘Liking’ and ‘wanting’ food rewards: Brain substrates and roles in eating disorders
Introduction
Obesity, bulimia, anorexia, and related eating disorders have become more prominent in recent decades, leading to increased concern about how to deal with them. Can improved knowledge about brain reward systems help guide thinking about eating disorders and devising better treatments?
Basic brain systems for food reward carry potential implications for understanding normal eating and eating disorders. It is important to note first that brain reward systems are active participants, not just passive conduits, in the act of eating. The pleasure of sweetness arises within the brain, generated actively by neural systems that paint the pleasure onto the sensation to generate a ‘liking’ reaction — as a sort of ‘pleasure gloss’. We may be used to thinking of sweet tastes as innately pleasant, but their pleasure is not contained in the intrinsic detail of their sensation but rather in their evolved ability to act as keys that unlock activation of brain ‘liking’ systems [1], [2], [3]. This is evident by considering that if the ability to unlock hedonic brain systems is lost, a sweet taste loses its pleasure while remaining sweet as ever. For example, a particular sweet taste can become perceived as disgusting rather than nice when an individual learns a taste aversion for it after pairing with visceral illness [3], [4], [5], [6]. Conversely, bitterness activates brain systems of aversion and disgust to be innately aversive, but tastes of cranberries, coffee, beer, gin, or opiates can become pleasant for many individuals when experience makes them into keys for hedonic brain systems.
Understanding brain substrates of pleasure and aversion may open an avenue to understanding the impact of food rewards on eating behavior. Pleasure must be translated into motivation or ‘wanting’ in order for food reward to influence actual eating behavior, as the motivation to eat involves brain mechanisms of its own.
What brain systems paint a pleasure gloss onto sensation? And what brain systems convert pleasure into a desire to eat? Answers to these questions will be described that have come primarily from animal experiments involving brain and pharmacological manipulation, supported also by human neuroimaging experiments and related studies of eating.
Section snippets
Understanding brain reward systems for food ‘wanting’ versus ‘liking’
To find brain mechanisms for ‘wanting’ and ‘liking’ first requires that those psychological components of reward be recognizable in measurable ways. This section turns to some issues regarding the brain systems that help generate ‘liking’ and ‘wanting’ for food rewards. ‘Liking’ is essentially a hedonic reaction to the pleasure of a reward. It is nearly what most people mean when they say reward. ‘Wanting’ on the other hand is not pleasure, even though also a component of reward, and may be
‘Wanting’ without ‘liking’
Quite different from hotspots for ‘liking’ generation has been the revelation that some previously-thought hedonic brain mechanisms actually fail to live up to that role, and turn out instead to mediate only the motivational ‘wanting’ to eat, without mediating hedonic ‘liking’ for the same food at all. One example was already described: the opioid sea of pure wanting in nucleus accumbens outside the hedonic hotspot. There are now many other examples as well. The phenomenon of ‘wanting’ without
Potential roles of brain reward systems in eating disorders
So how do ‘wanting’ and ‘liking’ relate to human eating disorders? At least in principle, we can sketch several alternative possibilities in principle for how brain reward systems might function in any particular eating disorder. Comparison of these alternatives may help to frame the issue.
The idea of food addiction will not be adequately discussed here (and will only briefly touch upon potential brain mechanisms). Researchers who deal with eating disorders are best positioned to judge whether
Conclusion
For most people, eating patterns and body weights remain within normally prescribed bounds. Perhaps it is the prevalence of normal body weights, rather than obesity, that should be most surprising in affluent modern societies where tasty foods abound. As is often pointed out, brain mechanisms for food reward and appetite evolved under pressures to protect us from scarcity. As a result, overeating in the face of present abundance could be an understandable overshoot inherited from our
Acknowledgements
I thank Eric Jackson and anonymous reviewers for helpful comments on an earlier version of the manuscript. This paper is based on a Mars Lecture given to the Society for the Study of Ingestive Behavior Conference in Paris in July 2008 and some parts have been adapted from a previous chapter. I am grateful to the SSIB organizers and the editors of this issue for the invitation to contribute. Research from my laboratory described here was supported by grants from the NIH (DA015188 and MH63649).
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