The Cocoa Trial That Found the Wrong Benefit
The largest cocoa flavanol trial ever run tested blood pressure and total cardiovascular events — and found benefit for neither. What it did find, almost by accident, was a real drop in cardiovascular death specifically.
The COSMOS trial enrolled 21,442 older adults specifically to test whether cocoa flavanols could prevent cardiovascular disease. The headline result wasn't what anyone was testing for.
Key Findings
- 21,442-person trial, 500mg/day cocoa flavanols, median 3.6-year follow-up
- No significant effect on the primary outcome: total cardiovascular events
- No significant effect on blood pressure in a dedicated hypertension analysis
- 27% reduction in cardiovascular death specifically (HR 0.73, 95% CI 0.54–0.98)
What the trial was actually testing
COSMOS (COcoa Supplement and Multivitamin Outcomes Study) gave 12,666 women aged 65+ and 8,776 men aged 60+ daily capsules containing 500mg/day cocoa flavanols (including 80mg/day epicatechin) or placebo, tracking total cardiovascular events as the primary outcome.1 After a median of 3.6 years, there was no statistically significant effect on that primary outcome.2
Blood pressure came up empty too
A dedicated long-term analysis of incident hypertension found cocoa extract had no significant effect: 7.1 events per 100 person-years in the cocoa group versus 7.4 in placebo.3 Smaller, earlier trials had suggested a blood-pressure benefit from cocoa flavanols — the large-scale trial didn't confirm it.
The result nobody was specifically looking for
Despite missing on both the primary outcome and blood pressure, cocoa flavanol supplementation significantly reduced cardiovascular death by 27% (hazard ratio 0.73), while other individual cardiovascular outcomes showed no significant risk reduction.2 Per-protocol analysis — looking specifically at participants who reliably took the active supplement — showed the total-event benefit strengthen further among the highly compliant.
The practical takeaway
- Cocoa flavanols did not lower blood pressure or reduce total cardiovascular events in the largest trial ever run on the question
- They did reduce cardiovascular death specifically by 27% — a real, statistically significant, but mechanistically unexplained finding
- This is a case where the "why" isn't settled yet — worth watching for follow-up research, not a reason to expect a blood-pressure fix from chocolate
Sources
- "COcoa Supplement and Multivitamin Outcomes Study (COSMOS) Study Protocol." ClinicalTrials.gov. clinicaltrials.gov
- "Effect of cocoa flavanol supplementation for the prevention of cardiovascular disease events: the COSMOS randomized clinical trial." PMC. ncbi.nlm.nih.gov
- "Long-Term Effect of Cocoa Extract Supplementation on Incident Hypertension." Hypertension. ahajournals.org
