In most places, blood pressure creeps up as people get older. It's common enough that doctors sometimes treat it as an almost unavoidable part of aging. But for the Kuna people, an Indigenous group living on a chain of small islands off the Caribbean coast of Panama, that rise mostly doesn't happen. People in their 60s and 70s often have blood pressure readings that look like a healthy 20-year-old's.
That's unusual enough that it caught the attention of a Harvard Medical School researcher named Norman Hollenberg, who spent years trying to figure out why.
Ruling Out the Obvious Answers
Hollenberg's first guess was genetics. Some populations do carry genetic differences that affect blood pressure, so it seemed like a reasonable place to start.
Genetics didn't hold up. Researchers looked at Kuna people who had moved from the islands to Panama City for work or family reasons, and found that once they were living on the mainland, eating a more typical Panamanian diet, they lost their protection against high blood pressure almost entirely. If the protection were genetic, moving wouldn't have changed anything. Since it did, something about their environment or habits, not their DNA, had to be responsible.
The next obvious guess was salt. Diets low in sodium are linked to lower blood pressure, and this is one of the most well-established relationships in nutrition science. But when researchers checked, island Kuna were eating about as much salt as the average American. That ruled out salt too.
What Was Actually Different
With genetics and salt eliminated, researchers looked at what else was different about daily life on the islands versus the mainland. One habit stood out clearly: cocoa.
Island Kuna drank cocoa daily, often several cups a day, prepared from cacao beans with very little processing. It wasn't an occasional treat, it was closer to how coffee functions in a lot of households, a beverage built into the rhythm of the day. Kuna who had relocated to Panama City drank cocoa far less often, closer to occasionally rather than daily.
What this means: Cocoa consumption was the one habit that changed dramatically between the two groups, and it was also the one thing that tracked most closely with who kept their protection against high blood pressure and who lost it.
The Mechanism Behind It
Correlation isn't the same as proof, so researchers went looking for a biological explanation for why cocoa specifically might matter.
Cacao is naturally rich in a group of plant compounds called flavanols. Flavanols aren't unique to cacao, you'll also find them in tea, apples, grapes, and berries, but cacao is one of the most concentrated natural sources when it hasn't been heavily processed. One flavanol in particular, called epicatechin, turned out to be the one doing the heavy lifting.
Here's the biology in plain terms: the inside of your blood vessels is lined with a thin layer of cells called the endothelium. That layer produces a molecule called nitric oxide, which signals the smooth muscle in vessel walls to relax. When vessels relax properly, blood flows more easily and blood pressure stays lower. When that signaling breaks down, vessels stay tighter than they should, which is part of what drives blood pressure up over time.
Epicatechin appears to boost the body's production of nitric oxide. Researchers tested this directly by measuring urinary nitric oxide levels and found that island Kuna had roughly double the levels of Kuna living on the mainland. A study published in the Proceedings of the National Academy of Sciences (PNAS) went further, giving people flavanol-rich cocoa and then measuring blood vessel function directly, along with testing purified epicatechin on its own. Both approaches pointed to the same mechanism, which is what makes the finding more convincing than a single observation would be on its own.
Beyond the Kuna: What a Larger, Modern Trial Found
Observing a population is useful, but it's not the same as running a controlled experiment. So researchers eventually tested the flavanol hypothesis at a much larger scale, in a trial called COSMOS (the Cocoa Supplement and Multivitamin Outcomes Study), run out of Harvard-affiliated Brigham and Women's Hospital.
Over 21,000 older adults took either a daily cocoa flavanol extract or a placebo for an average of about 3.6 years, and researchers tracked their cardiovascular health. The headline result was modest: a 10% reduction in cardiovascular events in the flavanol group, which did not reach statistical significance on its own. That's an important, honest caveat, and it's worth saying plainly rather than skipping past it.
But two secondary findings were more striking. Among participants who consistently took their supplement as directed, the flavanol group saw a statistically significant 15% reduction in cardiovascular events, and a 27% reduction in cardiovascular deaths specifically. Researchers described this as the first suggestive evidence, from a large randomized trial, that long-term cocoa flavanol intake may favorably affect cardiovascular outcomes, while also being clear that more research is needed to confirm it.
What this means: A single population observation from Panama has now been followed up with a real, large-scale clinical trial, and while the primary result was inconclusive, the trial adds real weight to the idea that flavanols specifically are doing something, not just correlating with something else about Kuna life.
Why Most Chocolate Doesn't Give You Any of This
Here's the part that matters most if you're standing in a grocery aisle: almost none of the chocolate on a typical shelf delivers meaningful flavanol content, even if it's labeled dark.
A lot of cocoa processing includes a step called alkalization, sometimes called "Dutch processing," originally used to make cocoa taste smoother and less bitter. Research published in the Journal of Agricultural and Food Chemistry found that this alkalization step can destroy up to 98% of the epicatechin in the final product. The compound doesn't just disappear, it chemically converts into a related compound called catechin, which research suggests is far less biologically active.
On top of that, most milk chocolate and many "dark" bars dilute whatever cacao they do contain with sugar, milk solids, and fillers, further shrinking the actual flavanol dose per serving. So the honest version of this story isn't "chocolate is good for your heart." It's "minimally processed cacao, eaten regularly, is what the research is actually about," and that's a much narrower, more specific claim.
Why This Isn't a "Chocolate Cures Everything" Story
None of this means any chocolate, in any amount, recreates what researchers found in Panama or in the COSMOS trial. The island Kuna were drinking flavanol-rich cacao daily as a dietary staple, not eating a candy bar loaded with sugar and milk solids a few times a month. The COSMOS trial used a concentrated flavanol extract, not a grocery store candy aisle.
The honest takeaway isn't a promise or a guarantee. It's a reason to pay attention to what's actually in the cacao you're eating, how it was processed, and how often you're eating it, rather than assuming any chocolate labeled "dark" is automatically doing something for you.
The Takeaway
A population that avoided the usual age-related rise in blood pressure. A biological mechanism that held up under closer testing. And, decades later, a large randomized trial that found real, if still-developing, evidence pointing in the same direction.
It's not a guarantee, and it's not a reason to eat any chocolate you can find. But it's a genuinely interesting reason to think about minimally processed cacao as a regular habit worth keeping, rather than an occasional indulgence to feel a little guilty about.
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