'Chocolate good for the heart' claims sadly too good to be true

"Regularly tucking into a bar of chocolate may actually be good for us," reports the Mail Online.

Researchers in Denmark say people who eat chocolate one to six times a week are less likely to get a heart condition called atrial fibrillation than those who eat it hardly at all (less than once a month).

Atrial fibrillation (AF) – an irregular heartbeat – can increase the risk of blood clots forming, and so raises the risk of a stroke.

However, as is often the case with health news that sounds too good to be true, the research is not particularly persuasive. People who ate chocolate less than once a month were also more likely to have diabetes, high blood pressure and cardiovascular disease; all of which are risk factors for atrial fibrillation. So they may have been avoiding chocolate for health reasons.

There is also no evidence from this study that eating chocolate will help with symptoms of atrial fibrillation if you already have it.

If anything, the opposite may be true: regularly overindulging in chocolate could increase your blood pressure and diabetes risk, which could eventually trigger the symptoms of atrial fibrillation.

Where did the story come from?

The study was carried out by researchers from Beth Israel Deaconess Medical Center in Boston, US, Aalborg University Hospital and the Institute of Cancer Epidemiology in Denmark, and Western University in Canada.

It was funded by grants from institutions including the US National Heart, Lung and Blood Institute, the European Research Council, the EU, the Harvard Clinical and Translational Science Centre, the Danish Cancer Society and the Danish Council for Strategic Research.

The study was published in the peer-reviewed journal Heart on an open-access basis, making it free to read online.

The study was covered widely in the UK media. The headlines, as you would expect, provided a simplistic "chocolate can be good for us" slant. But the actual "meat" of the reporting in most papers described the limitations and made it clear the study did not prove cause and effect.

What kind of research was this?

This was a prospective cohort study. Cohort studies are useful for spotting patterns but cannot prove that one thing (in this case chocolate consumption) directly causes another (chance of getting AF).

What did the research involve?

Researchers recruited 55,502 people in Denmark aged 50 to 64. Everyone completed a food questionnaire, had health checks and gave other information about their health and lifestyle.

Participants were followed up for an average 13.5 years. Researchers checked them against a Danish health registry to see if they were treated in hospital for AF. After adjusting for potential confounding factors they looked to see whether chocolate consumption was linked to their chances of getting AF.

The research takes advantage of the Danish National Patient Register, which makes it possible to track large numbers of people over time. Researchers included the following potential confounding factors:

• sex
• body mass index (BMI)
• blood pressure
• total cholesterol
• total calorie intake
• coffee consumption
• smoking
• years of education
• hypertension, diabetes and cardiovascular disease

The researchers analysed figures for men and women both separately and together.

What were the basic results?

Over 13.5 years, there were 3,346 cases of AF among the 55,502 people taking part in the study. People were less likely to have AF if they ate chocolate at least once a month:

• 10% less if they ate chocolate one to three times a month (hazard ratio [HR] 0.9, 95% confidence interval [CI] 0.82 to 0.98)
• 17% less if they ate chocolate once a week (HR 0.83, 95% CI 0.74 to 0.92)
• 20% less if they ate chocolate two to six times a week (HR 0.80, 95% CI 0.71 to 0.91)
• 16% less if they ate chocolate every day – but the numbers of people who ate chocolate daily and had AF were so low that we can't be sure these results were not just down to chance (HR 0.84, 95% CI 0.65 to 1.09)

The separate figures for men and women show men seemed to have the lowest risk if they ate chocolate two to six times a week, and women if they ate it once a week. However, these differences were small and may be down to chance and the fact that fewer women had AF.

How did the researchers interpret the results?

The researchers say higher levels of chocolate consumption "were associated with an 11-20% lower rate of clinically apparent AF among men and women." They say they adjusted the figures using "extensive data" on diet, lifestyle and other illnesses, but that "we cannot preclude the possibility of residual or unmeasured confounding."

They suggest that "antioxidant, anti-inflammatory and antiplatelet properties of cocoa" may be the reason for lower rates of AF among chocolate eaters.

Conclusion

Health stories that suggest eating or drinking something we like, whether it's chocolate or wine, are always popular. But they don't really tell us anything we don't know already. Certain foods may have a small impact on certain types of diseases, but it's the overall diet that counts.

Previous studies have already suggested that the antioxidant properties of cocoa could reduce the risk of cardiovascular disease, so it's surprising that this study focused on one particular cardiovascular disease, atrial fibrillation.

AF is a common condition that affects heart rate, often causing a faster than normal, irregular rhythm. It isn't usually life-threatening, although you may need treatment to reduce risk of linked conditions such as stroke.

While the study had some strengths, such as being very large, using a reliable database and taking account of a number of confounding factors, this type of study cannot show that chocolate actually prevents AF. It's quite possible that other factors could be more important than diet.

A plausible interpretation of this study is not that eating chocolate prevents AF, but that people with AF (or associated risk factors) avoid eating chocolate, possibly on the advice of their doctor.

It's worth remembering that – as well as cocoa – chocolate contains a lot of fat and sugar. In the study, one portion of chocolate was 30g. There's nothing wrong with eating a small amount of chocolate as part of a healthy, balanced diet – but hoping that a single "superfood" such as chocolate will make a big difference to your health is misguided.

Read more about so-called superfood claims and the evidence behind them.