Spraying doses of insulin up the nose can cause some people to find food less palatable, shows a study published this week in Nature Communications. This effect is accompanied by a decrease of activity in the brain areas that process reward, which are usually involved in the pleasure responses associated with food.
Stefanie Brassen and colleagues asked 48 people who had fasted overnight how hungry they were, then, after administering insulin or a placebo up both nostrils, asked them to rate how palatable they found pictures of food items while undergoing functional magnetic resonance imaging (fMRI) scanning. Each participant was tested both with insulin and the placebo. After receiving insulin, people who were normally sensitive to insulin found the food to be less appealing than they did after receiving the placebo, and also showed less activity in brain areas that process the reward associated with eating and food. However, people who were insulin resistant (but not diabetic) did not show this result: for them, insulin made no difference on either their subjective rating of the food palatability or the reward in the brain.
Taken together, these results suggest that insulin can decrease food palatability by modulating how the brain processes the reward associated with food. The observation that this effect does not happen in insulin-resistant individuals may be one of the reasons that it is harder for them to resist appealing foods.