Participants will be prescribed a calorie target between 1200 and 1800 depending upon baseline weight and personal preference. Participants will be asked to monitor their dietary intake using MyFitnessPal and to submit food logs weekly. Calorie reduction content will be based primarily on the Diabetes Prevention Program protocol and adapted from existing behavioral treatment protocols. Individuals will receive general guidelines on eating a healthy diet, including reducing saturated fat, processed foods, refined sugars, and refined carbohydrates, and increasing intake of lean proteins, whole grains, and fruits and vegetables, but will be encouraged to flexibly decide which areas of their diet to modify to reduce calorie intake.

Participants will be prescribed a whole food plant-based (WFPB) low-fat vegan diet promoting intake of fruits, vegetables, starches, legumes, and whole grains. Participants will be encouraged to avoid processed foods, refined oils, and animal products (meat, fish, eggs, and dairy products), and to minimize consumption of high-fat plant-based foods. An adapted traffic-light diet chart will outline foods to eat daily, limit, and avoid. Participants will be advised to eat until satiation and not restrict energy intake. Participants will be asked to consume 50 μg vitamin B12 (methylcobalamin) daily. WFPB nutrition counseling content will be based upon existing successful programs. Each week, participants will complete a simplified dietary self-monitoring food frequency questionnaire aligned to correspond with the traffic light diet chart.

Quality of life will be assessed using the 36-item Short-Form General Health Survey (SF-36; Ware, Kosinski, & Keller, 1994; Ware & Sherbourne, 1992), which assesses eight domains: physical functioning, limits due to physical health, pain, general health, energy/fatigue, social functioning, emotional wellbeing, and mental health. Scores will be converted to a 0 to 100 scale, with higher scores indicating better functioning. This scale produces both physical health (PH) and mental health (MH) component summaries.

Intentions to continue adhering to dietary prescription will be assessed with a 3-item self-report measure ("Future Intentions Questionnaire"). On a Likert scale ranging from 1 (Not at all) to 5 (Completely), participants will respond to face-valid questions asking about plans to continue adhering to the prescribed program diet (sample item: "I plan to follow the diet from this program moving forward") and the feasibility of the program diet (sample item: "The diet in this program is straightforward to follow moving forward"). Accompanying the measure, participants will receive a brief reminder of their dietary prescription (i.e., the traffic light diet chart in the whole food plant-based diet condition, or the calorie goal and daily calorie tracking requirement in the balanced calorie deficit diet condition). Items will be averaged to create a measure of intentions to adhere to the prescribed diet; higher scores indicate greater adherence.

To assess depressive symptoms, the Beck Depression Inventory-II (BDI-II; Dozois et al., 1998) will be administered-a 21-item measure of somatic (e.g., appetite), affective (e.g., sadness) and cognitive (e.g., self-criticalness) dimensions of depressive symptomology. Participants will indicate how they have been feeling during the past week and make responses on a Likert scale ranging from 0 (e.g., I don't cry any more than I used to) to 3 (e.g., I feel like crying but I can't), with higher scores indicating greater levels of depressive symptoms.

Overall program acceptability will be assessed post-treatment with a questionnaire adapted from prior work (Bradley et al., 2017) that will ask participants to report how satisfied they were with the program, the degree to which they found the program helpful, and the likelihood they would recommend it to family or friends using a Likert scale ranging from 1 (Not at all) to 5 (Very much). Scores will be averaged across conditions to create mean program acceptability scores. Higher scores will indicate greater program acceptability.

Participants will indicate the degree to which they found their diet to be acceptable by indicating, on a Likert scale, how difficult they found the diet to follow (1-Very Easy - 5-Very difficult) with regards to: following the diet as a whole, cooking, dietary tracking, staying full, and managing cravings. Participants will also indicate how time-consuming they found their diet (1-Not at all time-consuming - 5-Very time-consuming), and how confident they felt in their ability to follow their diet (1-Not at all - 5-Very much so). Scores will be averaged across items to create mean dietary acceptability scores.

To assess social support vs. social sabotage, participants will complete the Social Support and Sabotage Scale from Ball & Crawford (2006). On this measure, participants separately rate the degree to which friends and family act in ways that support or hinder their healthy eating efforts with a Likert scale ranging from 1 (Almost never) to 5 (Almost always). Scores will be averaged for each subscale to compute: 1) support for health behaviors from friends, 2) support for health behaviors from family, 3) sabotage for health behaviors from friends, and 4) sabotage for health behaviors from family.

Self-reported fatigue will be measured using the Functional Assessment of Chronic Illness Therapy - Fatigue Scale (FACIT Fatigue), a 13-item self-report questionnaire that assesses level of fatigue during an individual's usual daily activities over the past week on a Likert scale ranging from 4 (not at all fatigued) to 0 (very much fatigued) (Webster et al., 2003). Scores will be reversed such that higher scores indicate greater levels of fatigue.