Munching on walnuts may help control hunger pangs, according to a study which found that the snack activates an area in the brain associated with regulating appetite.
"It was pretty surprising to see evidence of activity changing in the brain related to food cues, and by extension what people were eating and how hungry they feel," said Olivia M Farr from Beth Israel Deaconess Medical Center in the US.
Researchers used functional magnetic resonance imaging (fMRI) to observe how consuming walnuts changes activity in the brain. They recruited 10 volunteers with obesity for two five- day sessions.
A controlled environment allowed researchers to keep tabs on the volunteers' exact nutritional intake, rather than depend on volunteers' often unreliable food records.
During one five-day session, volunteers consumed daily smoothies containing 48 grammes of walnuts - the serving recommended by the American Diabetes Association dietary guidelines.
During their other stay in the clinic, they received a walnut-free but nutritionally comparable placebo smoothie, flavoured to taste exactly the same as the walnut-containing smoothie.
Researchers noted that participants reported feeling less hungry during the week they consumed walnut-containing smoothies than during the week they were given the placebo smoothies.
While in the machine, participants were shown images of desirable foods like hamburgers and desserts, neutral objects like flowers and rocks, and less desirable foods like vegetables.
When participants were shown pictures of highly desirable foods, fMRI imaging revealed increased activity in a part of the brain called the right insula after participants had consumed the five-day walnut-rich diet compared to when they had not.
"This is a powerful measure. We know there is no ambiguity in terms of study results. When participants eat walnuts, this part of their brain lights up, and we know that is connected with what they are telling us about feeling less hungry or more full," said Christos Mantzoros, professor of medicine at Harvard University in the US.
This area of the insula is likely involved in cognitive control and salience, meaning that participants were paying more attention to food choices and selecting the less desirable or healthier options over the highly desirable or less healthy options, researchers said.
The study was published in the journal Diabetes.