Background Cocoa extract and multivitamins have been proposed to reduce the risk of cardiovascular disease (CVD) and cancer, respectively. However, few randomized clinical trials have tested their long-term effects on these outcomes. Methods The COcoa Supplement and Multivitamin Outcomes Study (COSMOS) is a randomized, double-blind, placebo-controlled, 2 × 2 factorial trial of a cocoa extract supplement and a multivitamin supplement to reduce the risk of CVD and cancer. Here we describe the pragmatic, hybrid design of the trial and baseline characteristics of the trial participants. Results The nationwide study population includes 21,442 U.S. women aged ≥65 years and men aged ≥60 years without baseline myocardial infarction (MI), stroke, or a recent (within the past 2 years) cancer diagnosis. Participants were randomized in a 2 × 2 factorial design to one of four groups: (1) cocoa extract (containing 500 mg/d flavanols, including 80 mg (-)-epicatechin) and a multivitamin (Centrum Silver©); (2) cocoa extract and multivitamin placebo; (3) multivitamin and cocoa extract placebo; or (4) both placebos. Randomization successfully distributed baseline demographic, clinical, behavioral, and dietary characteristics across treatment groups. Baseline biospecimens were collected from 6867 participants, with at least one follow-up biospecimen from 2142 participants. The primary outcome for the cocoa extract intervention is total CVD (a composite of MI, stroke, cardiovascular mortality, coronary revascularization, unstable angina requiring hospitalization, carotid artery surgery, and peripheral artery surgery); the primary outcome for the multivitamin intervention is total invasive cancer. Conclusion COSMOS will provide important information on the health effects of cocoa extract and multivitamin supplementation in older U.S. adults. Clinical Trials Registration: clinicaltrials.gov #NCT02422745.
