What the Research Says

According to a survey by the California Walnut Commission, nearly nine in ten Americans are worried about consuming dietary fat. However, a growing body of research highlights how plant-based foods such as walnuts may help reduce the risk of chronic disease and be beneficial for overall health, without contributing to weight gain. Continue below for a scientific summary of the research to date showing that walnuts may play a role in achieving ideal body weight, when consumed as part of an overall health diet.

Research Information

A short-term study from researchers at Beth Israel Deaconess Medical Center and Harvard Medical School found that walnuts may increase satiety and sense of fullness.1 Twenty men and women (ages 57-61) with metabolic syndrome participated in this randomized, double blind, cross-over study. For four days, subjects consumed isocaloric diets including a liquid meal containing either 48g of walnuts (approximately 1.7 ounces) or no walnuts. Both meals had similar macronutrient composition with the placebo rich in monounsaturated fats (MUFAs) and the walnut meal being rich in polyunsaturated fats (PUFAs). By day three of the study, subjects on the walnut-containing diet reported feeling more satiated and had a significantly higher rate of feeling full compared to those on the placebo diet. Due to the short study duration, longer-term studies are needed to explore the role of walnuts on these outcomes. 

A study published in Nutrition, showed that healthy adults (ages 18-35) who regularly consume foods that contain PUFAs may experience favorable changes in appetite hormones associated with hunger and satiety.2 Twenty-six participants consumed test meals high in saturated fat at the beginning of the study and then were placed on a seven-day control diet consisting of a typical American eating pattern or a diet high in PUFAs (included whole foods such as walnuts, Alaska salmon, tuna, flaxseed oil, grapeseed oil, canola oil, and fish oil supplements). After the seven-day diet, participants consumed meals high in saturated fat, again. Study participants that consumed a PUFA-rich diet had a significant decrease in fasting ghrelin, a hormone that increases hunger, and a significant increase in peptide YY (PYY), a hormone that increases fullness or satiety. Participants saw increases in PYY while fasting and after consuming a meal. These types of hormone changes could help with better appetite control although it is difficult to discern whether the changes can be attributed to one specific type of PUFA, food source, or a combination of overall dietary factors.

Published research from the Journal of the American Heart Association and Metabolism, found that a diet containing unsaturated fats, such as those found in walnuts and olive oil, may have similar effects on weight loss as compared to a lower fat, higher carbohydrate diet among  overweight and obese women.3,4 Two hundred forty-five women (ages 22-72) were enrolled in a one-year behavioral weight loss intervention and randomly assigned to three different diets: 1) a lower fat, higher carbohydrate diet (excluded nuts), 2) a lower carbohydrate, higher fat diet (excluded nuts), or 3) a walnut-rich (1.5 ounces per day), higher fat, lower carbohydrate diet.

Participants were instructed to reduce energy intake by 500-1000 calories per day and also received dietary guidance from a dietitian. A similar study found that a walnut-enriched, reduced-calorie diet also had similar effects on weight loss compared to a standard reduced-calorie diet among overweight and obese adults.5 One-hundred participants (average age 52-53) were enrolled in a six-month behavioral weight loss intervention and were instructed to reduce their daily caloric intake by 500-1000 calories with additional dietary guidance provided by a dietitian. Those on the walnut-enriched diet consumed 28 – 42 grams (1 – 1.5 ounces) of walnuts per day depending on the caloric restriction. Weight loss was similar across all diet groups, demonstrating that walnuts may play a role in achieving ideal body weight, when consumed as part of an overall healthy diet.

A study from the United States Department of Agriculture (USDA) found that one serving of walnuts (1 ounce) may provide 146 calories, which is 39 calories less, or 21 percent fewer, than the 185 calories listed in the USDA Nutrient Database.6 Eighteen healthy adults were randomly assigned to a sequence of two isocaloric diets: A controlled American diet without walnuts for a three-week period, and a controlled diet with 1.5 servings of walnuts (1.5 ounces) for another three-week period. Bomb calorimetry was used to measure calories and then the data  was used to calculate the metabolizable energy of the walnuts. The study took into account the digestibility of walnut pieces and halves and further research is needed to better understand the results of the study and how this technique for calculating calories could potentially affect the calorie count of other foods.

Researchers have also used magnetic resonance imaging (MRI) to explore possible connections between walnut consumption and important functions in the body. Researchers from the Beth Israel Deaconess Medical Center found that eating walnuts may activate an area in the brain associated with hunger and cravings.7 Ten obese adult participants (ages 48-54) lived at the medical center for two 5-day sessions and were closely monitored for food intake and appetite. Participants reported feeling fuller when they consumed a daily smoothie with 48 grams of walnuts (approximately 1.7 ounces), compared to when they consumed a placebo smoothie with the same macronutrient content but with safflower oil instead of walnuts. Researchers saw increased activity in a part of the brain that is thought to be involved in cognitive control and salience, suggesting that participants paid more attention to food choices after eating walnuts. Longer-term studies in more diverse populations are needed to confirm results.

Another study, published in Circulation, used imaging technology to map body organ fat storage pools in 278 participants (ages 28-69; mostly male and obese) following two types of diet: a Mediterranean, low-carbohydrate diet that included 1 ounce (28g) of walnuts per day and a low-fat diet, with and without moderate exercise.8 After following the diets for 18 months, the Mediterranean, low-carbohydrate diet with walnuts was found to be most effective in reducing fat deposits around the liver, abdomen, and heart. Adding exercise provided additional benefit for visceral fat loss in all groups. Total lean body mass or fat mass measurements were not available from the MRI analysis. Since this intervention involved dietary and physical activity changes, it is difficult to identify the exact factors responsible for the effects.

Information on dietary intake and diet adherence may have been limited in studies where participants were free-living and data was self-reported. Although larger and longer-term studies, as well as studies in more diverse populations, are needed to understand population-wide effects, walnuts can play a role in optimal body weight, when consumed as part of a healthy diet.

Corn Tomato Black Bean Walnut Salad Bowls

Weight Research

View peer-reviewed publications supported by the California Walnut Commission.

1 Brennan AM, Sweeney LL, Liu X, et al. Walnut consumption increases satiation but has no effect on insulin resistance or the metabolic profile over a 4-day period. Obesity (Silver Spring). 2010;18(6):1176-82.

2 Stevenson JL, Paton CM, Cooper JA. Hunger and satiety responses to high-fat meals after high polyunsaturated fat diet: a randomized trial. Nutrition. DOI: http://dx.doi.org/10.1016/j.nut.2017.03.008

3 Rock CL, Flatt SW, Pakiz B, et al. Effects of diet composition on weight loss, metabolic factors and biomarkers in a 1-year weight loss intervention in obese women examined by baseline insulin resistance status. Metabolism. 2016;65(11):1605-13.

4 Le T, Flatt SW, Natarajan L, et al. Effects of diet composition and insulin resistance status on plasma lipid levels in a weight loss intervention in women. J Am Heart Assoc. 2016;25;5(1):e002771.

5Rock CL, Flatt SW, and Barkai HS. Walnut consumption in a weight reduction intervention: effects on body weight, biological measures, blood pressure and satiety. Nutr J. 2017;16(76). doi: 10.1186/s12937-017-0304-z.

6 Baer DJ, Gebauer SK, Novotny JA. Walnuts consumed by healthy adults provide less available energy than predicted by the atwater factors. J Nutr. 2016;146(1):9-13. 

7Farr OM, Tuccinardi D, Upadhyay J, et al. Walnut consumption increases activation of the insula to highly desirable food cues: A randomized, double-blind, placebo-controlled, cross-over fMRI study. Diabetes Obes Metab. 2018;20(1):173-177. doi: 10.1111/dom.13060.

8Gepner Y, Shelef I, Schwarzfuchs D, et al. Effect of Distinct Lifestyle Interventions on Mobilization of Fat Storage Pools: CENTRAL Magnetic Resonance Imaging Randomized Controlled Trial. Circulation. 2018;137(11):1143-1157. doi: 10.1161/CIRCULATIONAHA.117.030501.

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