This randomized, cross-over trial aims to assess changes in body weight, plasma lipids, insulin sensitivity, and postprandial metabolism with a low-fat, plant-based diet and a Mediterranean diet, both followed for 4 months.

In a 36-week cross-over trial, overweight adults will consume a low-fat vegan diet and a Mediterranean diet in two separate 16-week phases. Changes in body weight will be the primary dependent variable. In addition, plasma lipid concentrations, insulin sensitivity, and postprandial metabolism will also be assessed and changes over time will be compared between the two diets.

Participants will be randomly assigned to start either a low-fat vegan diet or a Mediterranean diet for 16 weeks. Both groups will receive weekly classes and support, and will be asked to make no changes to their exercise patterns for the study period. After a 4-week washout period, participants will cross over to the second dietary intervention for another 16 weeks.

Inclusion Criteria:

• Men and women age ≥18 years of age
• Body mass index 28-40 kg/m2

Exclusion Criteria:

• Diabetes mellitus type 1, history of any endocrine condition that would affect body weight, such as thyroid disease, pituitary abnormality, or Cushing's syndrome
• Smoking during the past six months
• Alcohol consumption of more than 2 drinks per day or the equivalent, episodic increased drinking (e.g., more than 2 drinks per day on weekends), or a history of alcohol abuse or dependency followed by any current use
• Use of recreational drugs in the past 6 months
• Use within the preceding six months of medications that affect appetite or body weight, such as estrogens or other hormones, thyroid medications (unstable dose within the preceding 6 months), systemic steroids, antidepressants (tricyclics, MAOIs, SSRIs), antipsychotics, lithium, anticonvulsants, appetite suppressants or other weight-loss drugs, herbs for weight loss or mood, St. John's wort, ephedra, beta blockers
• Pregnancy or intention to become pregnant during the study period, as verified by self--- - Unstable medical or psychiatric illness
• Evidence of an eating disorder
• Likely to be disruptive in group sessions
• Already following a low-fat vegan diet or Mediterranean diet
• Lack of English fluency
• Inability to maintain current medication regimen
• Inability or unwillingness to participate in all components of the study
• Intention to follow another weight-loss method during the trial