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.
