fMRI tests revealed increased activity in the right insula, the part of the brain that regulates satiety and cravings, after participants consumed walnuts. [Beth Israel Deaconess Medical Center]
fMRI tests revealed increased activity in the right insula, the part of the brain that regulates satiety and cravings, after participants consumed walnuts. [Beth Israel Deaconess Medical Center]

Dieting is the bane of many individuals trying to control their weight for either health or aesthetics reasons. A common theme among dieters—especially those that fail—is that calorie restriction leads to bouts of hunger, which often causes binge-eating episodes or complete abandonment of a diet regimen. Subsequently, there is no shortage of appetite suppressant strategies, ranging from drugs to protein bars, to increased fiber intake. While the results for many of these approaches are subjective, based on participants’ feeling of satiety, few have clinical evidence to support their claims.

Now, investigators at Beth Israel Deaconess Medical Center (BIDMC) have demonstrated in a new study that consuming walnuts activates an area of the brain associated with regulating hunger and cravings. Findings from the study were published recently in Diabetes, Obesity, and Metabolism in an article entitled “Walnut Consumption Increases Activation of the Insula to Highly Desirable Food Cues: A Randomized, Double-Blind, Placebo-Controlled, Cross-Over fMRI Study.”             

“We don't often think about how what we eat impacts the activity in our brain,” explained lead study investigator Olivia Farr, Ph.D., an instructor in medicine in the division of endocrinology, diabetes, and metabolism at BIDMC. “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.”

In an attempt to determine exactly how walnuts quell cravings, the research team used functional magnetic resonance imaging (fMRI) to observe how consuming walnuts changes activity in the brain. The scientists recruited ten volunteers with obesity to live in BIDMC's Clinical Research Center (CRC) for two five-day sessions. The controlled environment of the CRC allowed the researchers to keep tabs on the volunteers' exact nutritional intake, rather than depend on volunteers' often unreliable food records—a drawback to many observational nutrition studies.

During one of the five-day sessions, volunteers consumed daily smoothies containing 48 grams of walnuts—the serving recommended by the American Diabetes Association (ADA) dietary guidelines. During their other stay in the CRC, they received a walnut-free, but nutritionally comparable placebo smoothie flavored to taste exactly the same as the walnut-containing smoothie. The order of the two sessions was random, meaning some participants would consume the walnuts first and others would consume 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 the experiment provided the research team a clear picture as to what was occurring. 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.

Amazingly, 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,” noted senior study investigator Christos Mantzoros, M.D., D.Sc., Ph.D., director of the Human Nutrition Unit at Beth Israel Deaconess Medical Center and professor of medicine at Harvard Medical School. “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.”

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. The researchers are now planning 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.

The researchers were excited by their findings, but urged caution in overinterpretation of the results, due to the small study size. However, similar studies could reveal how other foods and compounds, such as naturally occurring hormones, impact the appetite-control centers in the brain. Future research could eventually lead to new treatments for obesity.  

“From a strategic point of view, we now have a good tool to look into people's brains—and we have a biological readout,” Dr. Mantzoros concluded. “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 their weight down.”

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