Walnuts activate brain area that controls appetite, finds study

17 Aug 2017 --- Consuming walnuts activates an area in the brain associated with regulating hunger and cravings. This is according to a first-of-its-kind new brain imaging study by researchers at Beth Israel Deaconess Medical Center (BIDMC), the findings of which, published online in the journal Diabetes, Obesity and Metabolism, reveal for the first time the neurocognitive impact the nuts have on the brain. 

The California Walnut Commission (CWC) supported the study through an investigator-initiated grant.

“We don't often think about how what we eat impacts the activity in our brain,” says the study's first author Olivia M 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.”

Smoothies consumed
To determine exactly how walnuts quell cravings, a study led by Christos Mantzoros, Ph.D., Director of the Human Nutrition Unit at Beth Israel Deaconess Medical Center and Professor of Medicine at Harvard Medical School, used functional magnetic resonance imaging (fMRI) to observe how consuming walnuts changes activity in the brain.

The scientists recruited 10 obese volunteers to live in BIDMC's Clinical Research Center (CRC) for two five-day sessions. The controlled environment of the CRC allowed the researchers to track the volunteers’ exact nutritional intake, rather than depend on volunteers’ often unreliable food records – a drawback to many observational nutrition studies.

During one five-day session, volunteers consumed daily smoothies containing 48 grams of walnuts – the serving recommended by American Diabetes Association (ADA) dietary guidelines. During their other stay in the CRC, they received a walnut-free but nutritionally comparable placebo smoothie, with a flavor that made it taste exactly the same as the walnut-containing smoothie.

The order of the two smoothie-consuming sessions was random, meaning that 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.

Increased activity in a part of the brain
The fMRI tests on the fifth day of the experiment gave Farr, Mantzoros and the team a clear picture as to why the participants felt less hungry. While in the machine, study participants were shown different images: 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,” says 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.”

This area of the insula is likely involved in cognitive control and salience, notes the BIDMC press release, 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. 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 read out,” says 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 their weight down.”

Walnuts are currently a hot ingredient worldwide in new product development. Innova Market Insights reports a global 23 percent compound annual growth rate from 2011 to 2016 for walnut applications.

The study showing walnuts’ beneficial effects on hunger is also just the latest to explore nuts’ general health benefits. A study has found that the risk of colon cancer recurrence was nearly halved in people who ate nuts, and nuts have also been linked to activating the body's own defenses for detoxifying reactive oxygen species. In addition, eating at least 20g of nuts a day has been linked to a reduced risk of heart disease, cancer and other diseases.

By Paul Creasy

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