Is dark chocolate really good for you?

6 min read

Dark chocolate is often called "a healthy treat." Rich in polyphenols. Lowers blood pressure. Good for the heart. Search for it and you'll find these claims everywhere.

So how much has actually been confirmed in research on people?

The short answer: for blood vessel function and blood pressure, randomized controlled trials (RCTs) show small but consistent improvements. But whether it reduces "real events" like heart attacks and strokes is still unsettled.

And there are easy-to-overlook caveats: heavy metals, and sugar and calories.

The key isn't "chocolate" but "cocoa flavanols"

The component thought to be behind the effects is the flavanols (flavan-3-ols) in cacao. What matters here is that much of the research used flavanol-rich cocoa products or cocoa extracts, not "chocolate."

Flavanol content varies a lot with how cacao is processed and the type of product [2]. Even if it says "70% cacao," the flavanol content isn't necessarily the same.

Vessel function: fairly consistent improvement

The function of the endothelium, the inner lining of blood vessels, is measured by FMD (flow-mediated dilation). A 2012 meta-analysis combining 42 RCTs of chocolate, cocoa, and flavanols (1,297 people in total, up to 18 weeks) found FMD improved as follows [1].

In the same analysis, a measure of insulin resistance (HOMA-IR) also improved (−0.67). But this was framed as "a promising result not previously reported" [1].

The authors state clearly that larger, longer trials independent of funders are needed [1]. Much cacao and chocolate research has industry funding, which should be kept in mind when reading it.

Blood pressure: a small effect

A 2017 Cochrane review pooled the effects of flavanol-rich cocoa products and chocolate on blood pressure [2].

So there's almost no effect in people with normal blood pressure, and a somewhat larger one, about 4 mmHg, in people with high blood pressure. Even so, that's not big enough to replace medication.

One more point: non-blinded trials tended to show larger drops in blood pressure (not statistically significant) [2]. Cocoa is recognizable by taste and color, so a placebo is hard to make. It's worth remembering this limitation.

Cardiovascular events: not yet settled

Vessel function and blood pressure are only "surrogate markers." What we really want to know is whether it reduces heart attacks, strokes, and cardiovascular death.

The largest trial looking at this is the US COSMOS trial. Older adults took a cocoa extract (500 mg/day of flavanols, including 80 mg of epicatechin) or a placebo for an average of about 3.6 years [3].

Because the primary outcome wasn't significant, we can't say "cocoa reduced cardiovascular events." The 27% drop in cardiovascular death was a secondary outcome, one of many comparisons. The trial report says an analysis of people who took it as directed supported a possible reduction in events, but the researchers themselves concluded that "further research is needed" [3].

And one more thing: COSMOS wasn't a chocolate trial. The researchers themselves said the dose in this trial isn't something you could realistically get from chocolate.

If we assume the flavanol content of commercial dark chocolate is about 0.5%, getting 500 mg would take about 100 g. That's around 600 kcal. This is a rough estimate, and products vary enormously.

Caveat 1: heavy metals

The US consumer organization Consumer Reports tested 28 commercial dark chocolates. In 23 of them, one ounce (about 28 g) exceeded California's maximum allowable daily levels (lead 0.5 µg, cadmium 4.1 µg). Five were within the limits [4].

The same report found that cadmium tended to be higher in products with higher cacao content, and organic didn't necessarily mean lower [4].

Three things to keep in mind:

• This is a consumer organization's testing report, not a peer-reviewed paper.
• It covered products sold in the US.
• The limits are California's, and differ from other jurisdictions' standards.

Even so, the risk of "eating a lot every day" can't be ignored.

Caveat 2: sugar and calories

Dark chocolate is high in calories too. It also contains sugar and fat. As a rough guide, 25 g of a chocolate bar is about 140 kcal, though it varies by product.

If you lower your blood pressure by a few mmHg but gain weight, you've defeated the purpose.

How much should you eat?

The optimal amount can't be determined from the research.

Realistically, swapping some of your other sweets for a small amount of dark chocolate is consistent with what the research covers. The research gives no reason to eat a lot of it "for your health."

What to look for on the package in the store is in how to choose dark chocolate.

So what's actually special about dark chocolate?

Summary

Dark chocolate isn't "a magic treat that makes you healthy if you eat it."

Still, multiple RCTs consistently show that flavanol-rich cacao products slightly improve blood vessel function and lower blood pressure a little. On the other hand, whether it reduces cardiovascular events didn't reach significance even in the largest trial.

So the view most in line with current research is not "eat dark chocolate for your health," but "if you eat it, have a small amount in place of other sweets, and choose the product."

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