When 46 amateur endurance runners ate 50 grams of 70% dark chocolate every day for two weeks, their pulse wave velocity fell by 11.82%. That is a measure of how stiff the large arteries have become, and a lower number means the vessels are more elastic. The finding was published in December 2024 in the journal Sports.
Blood pressure moved in the same direction, but modestly. Systolic pressure at the arm dropped by 2.12% and diastolic by 2.79%, according to the prospective cohort study by Vordos and colleagues at the Aristotle University of Thessaloniki. The men were aged 25 to 55 and kept their normal training through the two weeks.
The daily portion in the trial delivered 1,222.5 mg of total polyphenols and 577.5 mg of catechins. Those are the plant compounds researchers think do most of the vascular work.
Where the artery benefit comes from
Cocoa is unusually rich in flavanols, a group of plant compounds that appear to help the inner lining of blood vessels release more nitric oxide. Nitric oxide relaxes the vessel wall, which is one reason flavanol-heavy foods have been linked to better blood flow and lower pressure over time.
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A separate line of research points in the same direction. In a 2024 Mendelian randomization analysis published in Scientific Reports and covering 64,945 people of European descent, regular dark chocolate intake was linked to a lower risk of essential hypertension. Because the method uses genetic variants as a proxy for intake, it is less prone to the reverse-causation problem that dogs ordinary observational nutrition studies.
The dose that seems to matter
Not every square of dark chocolate carries the same punch. Earlier meta-analyses have pointed to a rough threshold: cocoa products delivering around 900 mg of flavanols, or 100 mg of epicatechin, tend to show the clearest effects on blood flow and blood pressure. Weaker products often show little.
Harvard's Nutrition Source suggests choosing 70% cocoa or higher to get the most flavanols, and points to about one ounce a day as a reasonable amount for people who want the potential heart benefits. That is roughly 28 grams, or a couple of small squares off a standard bar.
The calorie trade-off is real
One ounce of dark chocolate carries 150 to 170 calories, and the bar still contains a moderate amount of saturated fat and sugar. Harvard cautions that dark chocolate can contribute to weight gain if eaten in excess, and that the saturated fat can affect blood lipid levels.
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That matters for the arithmetic. Doubling the portion doubles the calories, and there is no evidence that eating more chocolate keeps pushing arterial stiffness lower in a straight line. The trial that produced the 11.82% pulse wave velocity drop used a fixed 50-gram daily dose over two weeks in healthy trained men, not an open-ended habit in the general population.
What the finding does and does not show
The Greek study was small, ran for two weeks and had no control group taking a placebo bar, which the authors note. It shows that a specific dose of a specific chocolate produced a measurable improvement in arterial function in a specific group of trained men. It does not show that dark chocolate lowers heart attack or stroke rates, and it is not a substitute for treatment if a doctor has diagnosed high blood pressure.
Anyone weighing a daily square against their calorie budget, blood pressure medication or blood sugar should talk it through with their own clinician before making it a habit.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.