Background: To understand the pathophysiology of obesity, the role of hedonic eating
needs to be considered. Neuroimaging research has suggested that activity in brain
reward systems is modulated by the rewarding properties of food, nutritional state, and
individual differences in eating behaviour. The interaction between these factors and the
influence of gut hormones, such as the stomach-derived orexigenic hormone ghrelin, on
these systems remains unclear.
Methods: In the first study, 20 non-obese subjects were either fasted or ate a filling
breakfast of their choice (fed). In the second study, 18 non-obese subjects were either
fasted or given a fixed breakfast and received a subcutaneous saline or ghrelin injection.
Subjects underwent functional MRI scanning while viewing and rating the appeal of food
and object pictures and completed appetite and psychological questionnaires.
Results: The first fMRI study showed that fasting increased activation to pictures of highcalorie
over low-calorie foods in the ventral striatum, amygdala, anterior insula, medial
and lateral orbitofrontal cortex (OFC). Fasting also biased the appeal towards high-calorie
foods. Dietary restraint positively correlated with activation in executive systems when
viewing high-calorie compared to low-calorie foods, but negatively correlated with
activation in affective systems. Reward drive positively correlated with activation in the
ventral striatum to high-calorie foods.
In the second fMRI study activation to high-calorie foods in the insula and/or OFC
when fasted and fed was shown to correlate with hunger and predict subsequent food
intake. Administration of ghrelin was shown to mimic fasting to increase the appeal of
high-calorie, especially sweet foods, and OFC activation to high-calorie foods.
Conclusion: The results suggest an interaction between homeostatic and hedonic aspects
of eating behaviour, with fasting and ghrelin biasing brain reward systems towards highcalorie
foods.