BACKGROUND: Few studies have compared diets to determine whether a program focused on 1 dietary change results in collateral effects on other untargeted healthy diet components. OBJECTIVE: To evaluate a diet focused on increased fiber consumption versus the multicomponent American Heart Association (AHA) dietary guidelines. DESIGN: Randomized, controlled trial from June 2009 to January 2014. (ClinicalTrials.gov: NCT00911885). SETTING: Worcester, Massachusetts. PARTICIPANTS: 240 adults with the metabolic syndrome. INTERVENTION: Participants engaged in individual and group sessions. MEASUREMENTS: Primary outcome was weight change at 12 months. RESULTS: At 12 months, mean change in weight was -2.1 kg (95% CI, -2.9 to -1.3 kg) in the high-fiber diet group versus -2.7 kg (CI, -3.5 to -2.0 kg) in the AHA diet group. The mean between-group difference was 0.6 kg (CI, -0.5 to 1.7 kg). During the trial, 12 (9.9%) and 15 (12.6%) participants dropped out of the high-fiber and AHA diet groups, respectively (P = 0.55). Eight participants developed diabetes (hemoglobin A1c level > /=6.5%) during the trial: 7 in the high-fiber diet group and 1 in the AHA diet group (P = 0.066). LIMITATIONS: Generalizability is unknown. Maintenance of weight loss after cessation of group sessions at 12 months was not assessed. Definitive conclusions cannot be made about dietary equivalence because the study was powered for superiority. CONCLUSION: The more complex AHA diet may result in up to 1.7 kg more weight loss; however, a simplified approach to weight reduction emphasizing only increased fiber intake may be a reasonable alternative for persons with difficulty adhering to more complicated diet regimens. PRIMARY FUNDING SOURCE: National Heart, Lung, and Blood Institute.