Consuming walnuts activates an area in the brain associated with regulating hunger and craving for food, says a new study.
The findings, published online in the journal Diabetes, Obesity and Metabolism, shed light on how walnuts discourage overeating by promoting feelings of fullness.
"We don't often think about how what we eat impacts the activity in our brain," said the study's first author Olivia Farr from the Beth Israel Deaconess Medical Centre (BIDMC) in Boston in the US.
"We know people report feeling fuller after eating walnuts, but 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," Farr said.
To determine exactly how walnuts quell craving for food, Farr and colleagues used functional magnetic resonance imaging (fMRI) to observe how consuming walnuts changes activity in the brain.
The scientists recruited a small group of volunteers with obesity to live in BIDMC's Clinical Research Centre (CRC) for two five-day sessions.
During one session, volunteers consumed daily smoothies containing 48 grams of walnuts.
During their other stay. they received a walnut-free but nutritionally comparable placebo smoothie, flavoured to taste exactly the same as the walnut-containing smoothie.
As in previous observational studies, participants reported feeling less hungry during the week they consumed walnut-containing smoothies than during the week they were given the placebo smoothies.
Functional MRI tests administered on the fifth day of the experiment gave the team a clear picture as to why.
While in the machine, study 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," said Christos Mantzoros, Professor of Medicine at Harvard Medical School.
"When participants eat walnuts, this part of their brain lights up, and we know that's connected with what they are telling us about feeling less hungry or more full," Mantzoros said.
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.
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