How does pain affect eating and food pleasure?

Sufferers of persistent pain have increased risk for many other health problems, including obesity. The mechanisms have not been well understood, however. In this issue, Geha et al. ask why chronic pain and obesity are so closely linked [8]. Drawing on the rich neu-roscience literature which implicates the ventral striatum and medial prefrontal cortex (mPFC) in determining food pleasure [5], the authors point to evidence that chronic pain disrupts the normal structure and function of this circuitry. For instance, relief related ventral striatal responses to offset of phasic pain were absent in patients with chronic back pain [1]. These patients also showed abnormal ventral striatal opioid signalling during pain [15], and functional connectivity between the ventral striatum and mPFC appears to play a role in the transition from acute to chronic pain [2]. Geha and colleagues tested 18 patients with chronic lower back pain (CLBP) and compared their responses to sweet and fatty foods with a matched control group. The average body mass index for each group indicated moderate overweight. Patients with CLBP showed a selective decrease in pleasantness ratings of fatty puddings. Importantly, they also displayed a disconnection between calories ingested during ad libitum food consumption and ratings of hunger and food pleasantness. That is, unlike for controls, there was no relationship between how much they liked the food and how much they ate. Strikingly, eating more also failed to predict a larger drop in hunger in this group. Geha et al. propose that pain-related hedonic blunting (a reduced ability to experience pleasure) caused by disruptions in VS-mPFC brain circuitry may link chronic pain to obesity. This neu-roscientific explanation is compelling, and future studies should address how pain affects the neural mechanisms of hedonic eating and relief of hunger. The link between pain and pleasure is complex, however. One early study of patients with chronic facial or back pain suggested that anhedonia was more closely related to the patients' levels of depression rather than to the experience of pain itself [14]. Other conditions that can blunt pleasure in some domains, such as substance abuse (which also disrupts the ventral striatum-mPFC circuitry and opioid signalling), have been linked to increased liking of sweet tastes [9,10]. Pain has also been shown to enhance directly the enjoyment of sweet foods. After undergoing the cold-pressor test, healthy volunteers reported greater pleasure from eating chocolate [3]. In fact, pain offset appeared to increase sensitivity …