Packed with nutrients and omega-3 fatty acids linked to better health, walnuts are also thought to discourage overeating by promoting feelings of fullness, or satiety.
Now, in a new brain imaging study, researchers at Beth Israel Deaconess Medical Center have demonstrated that consuming walnuts activates an area in the brain associated with regulating hunger and cravings. The findings reveal for the first time the neurocognitive impact these nuts have on the brain.
The researchers knew that people report feeling fuller after eating walnuts, but were surprised to see activity changing in the brain related to food cues based in what they were eating and how hungry they felt.
About the study
To determine exactly how walnuts quell cravings, the researchers used functional magnetic resonance imaging (fMRI) to observe how consuming walnuts changed activity in the brain.
They recruited 10 obese volunteers to live in Beth Israel Deaconess Medical Center’s Clinical Research Center for two five-day sessions. This controlled environment allowed the researchers to keep tabs on the volunteers' exact nutritional intake, rather than depend on volunteers' food records which can be prone to error.
During one five-day session, volunteers consumed daily smoothies containing 48 grams of walnuts -- the serving recommended by the American Diabetes Association’s dietary guidelines. During their other stay in the research center, participants received a walnut-free but nutritionally comparable smoothie (the placebo), flavored to taste exactly the same as the walnut-containing smoothie. Neither the volunteers nor the researchers knew during which session they consumed the nutty smoothie.
Participants reported feeling less hungry during the week they consumed walnut-containing smoothies than during the week they were given the placebo smoothies. fMRI tests administered on the fifth day of the experiment gave the research team a clear picture as to why.
How walnuts influenced brain activity
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 specific part of the brain, called the insula, after they had consumed the five-day walnut-rich diet compared to when they had not.
When participants ate the walnuts, this part of their brain lit up, a part that's connected with satiety. 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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