In obesity, pictures of high-calorie food elicit dopamine release equally when hungry or full.

What is it about?

Obesity involves continued eating when eating is not healthy. One theory is that the brain's reward system has gone awry such that high-calorie foods appear desirable even when one has already eaten an adequate meal. We set out to test one part of that theory by measuring dopamine release under several conditions in the striatum (a dopamine-rich part of the brain that influences the cortex). Dopamine is involved in the brain's reward system. We tested 10 people with obesity either during fasting, when hunger and interest in food are adaptive, and after a full meal, when they are not. In each case we measured striatal dopamine release using a brain imaging technique called positron emission tomography (PET) with a tracer molecule called [C-11]raclopride that binds loosely to certain dopamine receptors. The approach we chose measures baseline dopamine receptor availability as well as the change induced by dopamine release when experimental conditions are switched halfway through the scan. Specifically, for the first part of the scan, people saw sequential images of low-calorie food (like vegetables), and for the second part they saw images of high-calorie foods (like hamburgers or pie). Consistent with our hypothesis, neither baseline dopamine receptor availability nor dopamine release when switching from viewing low- to high-calorie foods differed between the fed and fasting conditions.

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Why is it important?

Previous studies by other groups have shown that showing lean people pictures of highly palatable foods after fasting stimulates striatal dopamine release. Our results indicate that this healthy response continues in people with obesity even after eating to satiety, when eating is no longer healthy. This result identifies a possible mechanism subserving eating after reaching satiety in obesity. The main limitation of our study is that we carried out this test only in prediabetic people with obesity. Although others' results in lean people differed, we need to use the same methods as in this report in a lean comparison group to confirm that the two groups in fact differ, and to estimate by how much.