In prior observational studies, subjects who were given walnuts to eat reported a feeling of fullness, ate less and chose healthier foods than those who did not have walnuts as part of their meal.
Now there is physical proof that eating walnuts discourages overeating by decreasing cravings for food, particularly unhealthy food.
IMAGE: fMRI tests revealed increased activity in right insula, the part of the brain that regulates satiety and cravings, after participants consumed walnuts.
Credit: Beth Israel Deaconess Medical Center
To determine exactly how walnuts quell cravings, Farr, the study’s first author, and colleagues, in a study led by Christos Mantzoros, MD, DSc, PhD hc mult, 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.
Volunteer Recruitment & Tracking
The scientists recruited 10 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 five-day session, 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 gave Farr, Mantzoros and 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.
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.