Controlled portions of nuts such as walnuts are often recommended for patients with obesity and type-2 diabetes because they are thought to discourage overeating by promoting feelings of fullness. How the nuts might do so, however, remains murky.
In a small new brain-imaging study partially sponsored by the California Walnut Commission, researchers at Harvard Medical School and Beth Israel Deaconess Medical Center have found that consuming walnuts activates an area in the brain associated with regulating hunger and cravings.
The findings, published online in the journal Diabetes, Obesity and Metabolism, reveal for the first time the neurocognitive impact these nuts have on the brain.
“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,” said the study’s first author, Olivia Farr, HMS instructor in medicine in the Division of Endocrinology, Diabetes and Metabolism at Beth Israel Deaconess.
Trying to determine exactly how walnuts quell cravings, Farr and colleagues, including senior investigator Christos Mantzoros, HMS professor of medicine and director of the Human Nutrition Unit at Beth Israel Deaconess, used functional magnetic resonance imaging (fMRI) to observe how consuming walnuts changes brain activity.
The scientists recruited 10 obese volunteers to live in Beth Israel Deaconess’ clinical research center for two five-day sessions. The controlled environment allowed the researchers to keep tabs on the volunteers’ precise nutritional intake rather than depending on volunteers’ often unreliable food records—a drawback to many observational nutrition studies.
During one five-day session, the volunteers consumed daily smoothies containing 48 grams of walnuts—the serving recommended by the American Diabetes Association dietary guidelines. During their other stay, they received a walnut-free but nutritionally comparable placebo smoothie, flavored to taste the same as the walnut-containing smoothie.
The order of the two sessions was random, meaning some participants consumed the walnuts first and others consumed the placebo first. Neither the volunteers nor the researchers knew during which session they consumed the nutty 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. fMRI tests administered on the fifth day of each session gave the research 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 Mantzoros. “We know there’s no ambiguity in terms of study results. 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.”
The researchers 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.
Farr and Mantzoros next plan to test different amounts, or dosages, of walnuts to see whether more nuts will lead to more brain activation or if the effect plateaus after a certain amount. This experiment will also allow researchers to test other compounds for their effect on this system.
Similar studies could reveal how other foods and compounds, such as naturally occurring hormones, impact the appetite control centers in the brain.
“From a strategic point of view, we now have a good tool to look into people’s brains—and we have a biological readout,” said Mantzoros. “We plan to use it to understand why people respond differently to food in the environment and, ultimately, to develop new medications to make it easier for people to keep [excess] weight down.”
The study was supported by Harvard Clinical and Translational Science Center grant UL1RR025758 from the National Center for Research Resources at the National Institutes of Health and by NIH grant K24 DK081913. The California Walnut Commission (CWC) supported the study through an investigator-initiated grant. The CWC had no role in study design or conduct; data collection, management, analysis or interpretation; or manuscript preparation, review or approval.
Adapted from a Beth Israel Deaconess news release.