This study investigates whether walnuts help to promote weight loss, associated with increased meal satiety and satisfaction, in 100 overweight and obese men and women who are participating in a 6-month behavioral weight loss intervention. Participants will be randomly assigned to a walnut-enriched reduced-calorie diet or a standard reduced-calorie diet. Body weight, risk factors for cardiovascular disease, and self-reported feelings relevant to satiety and appetite will be measured at baseline and 3- and 6-month follow-up. Also, the response of gastrointestinal tract hormones following meals with or without walnuts will be measured in a subset of study participants (n=20). Results from this study will contribute to understanding the role of nuts in weight control, including further knowledge of the mechanisms, and will expand knowledge of how nuts in the diet may contribute to the prevention and management of obesity.

Epidemiological studies have linked regular consumption of nuts with lower body mass index and reduced likelihood of weight gain in adulthood. Several mechanisms have been proposed to explain why nuts may facilitate weight management, including their high satiety property which may promote dietary compensation with reduced subsequent energy intake. Although proposed as a possible mediating factor, satiety and satiation in relation to nuts or nut-containing meals have been examined in only a few previous studies. The effects of nut consumption in the context of a weight loss intervention have been examined in only five previously-published randomized studies, which have tested the effects of almonds, pistachios or peanuts (but not walnuts) and have had mixed results.

This project is a study to investigate the effects of a walnut-enriched vs. standard reduced-energy diet on body weight and satiety via pre- and post-meal ratings scales, as well as exploring the response of satiety- and appetite-related gastrointestinal peptides to meals with or without walnuts in a sample subset.

The specific aims of this study are:

1. To compare the effects of a walnut-enriched reduced-energy diet to a standard reduced-energy diet on body weight and cardiovascular disease risk factors in a sample of overweight and obese adults in an intensive 6-month weight loss intervention.
2. To examine whether there is a differential response in satiety- and appetite-related ratings scales in association with a walnut-enriched reduced-energy diet and a standard reduced-energy diet among the participants in this weight-loss study.
3. To examine the response of satiety- and appetite-related gastrointestinal peptides (ghrelin, cholecystokinin, glucagon-like peptide, and peptide YY), to meals with or without walnuts in a sample subset, as an exploratory aim.

The first two aims will be addressed in a randomized controlled study involving 100 overweight or obese men and women assigned to a walnut-enriched reduced-energy diet or a standard reduced-energy diet in the context of a 6-month intensive weight loss intervention. Subjective satiety- and appetite-related ratings will be collected at specific intervals before lunch and dinner using visual analogue scales. Postprandial gastrointestinal peptide response to a meal with or without walnuts, the third and exploratory aim, will be measured in a subset of study participants (n=20) using a within-subject crossover study design.

The investigators hypothesize that participants assigned to the walnut-enriched diet study arm will have greater weight loss and overall better improvements in cardiovascular disease risk factors, and that ratings of hunger, fullness, and anticipated prospective consumption will differ from those of participants assigned to the standard reduced-energy diet. The investigators hypothesize that postprandial satiety- and appetite-related gastrointestinal peptides, which play a role in short-term control of appetite and may be biomarkers of satiety, may differ in response to different isocaloric meal composition. Results from this study will contribute to understanding the role of nuts in weight control, including further knowledge of the explanatory mechanisms, and will expand knowledge of how nuts in the diet may contribute to the prevention and management of obesity.

Inclusion Criteria:

• aged 21 years and older
• BMI >27.0 kg/m2 and <40 kg/m2
• non-smoker
• willing and able to participate in clinic visits, group sessions, telephone, and internet communications at specified intervals
• able to provide data through questionnaires and by telephone
• willing to maintain contact with the investigators for 6 months
• willing to allow blood collections
• no known allergy to tree nuts
• capable of performing a simple test for assessing cardiopulmonary fitness

Exclusion Criteria:

• inability to participate in physical activity because of severe disability
• a history or presence of a comorbid diseases for which diet modification and increased physical activity may be contraindicated
• smoker
• self-reported pregnancy or breastfeeding or planning a pregnancy within the next year
• currently actively involved in another diet intervention study or organized weight loss program
• a history or presence of a significant psychiatric disorder or any other condition that, in the investigator's judgement, would interfere with participation in the trial