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Science Alert: Experts Respond

Chocolate linked to lower heart attack and stroke risk – Expert reaction

Posted in Science Alert: Experts Respond on June 16th, 2015.

A large new study has added weight to claims that there are cardiovascular benefits to eating chocolate. But – before you go on that chocolate binge – independent experts say more research is needed to clarify the link.

SlimChocolate1Eating up to 100 g of chocolate every day is associated with lowered heart disease and stroke risk, according to the new study published online in the journal Heart.

Read more about the study on on

The authors base their findings on almost 21,000 adults taking part in the EPIC-Norfolk study, which is tracking the impact of diet on the long term health of men and women in Norfolk, UK.

Their calculations showed that compared with those who ate no chocolate, chocolate intake was linked to an 11% lower risk of cardiovascular disease and a 25% lower risk of associated death.

It was also associated with a 9% lower risk of hospital admission or death as a result of coronary heart disease, after taking account of dietary factors.

The authors conclude: “There does not appear to be any evidence to say that chocolate should be avoided in those who are concerned about cardiovascular risk.”

The SMC collected the following expert commentary.

Prof Thomas Lumley, Professor of Biostatistics, University of Auckland, comments:

“This research found that people who ate more chocolate were less likely to have strokes or heart disease over the next twelve years. The study is large, with 20000 participants and more than 2000 coronary heart disease events, but the evidence for the association was only moderately strong.

“Since the study is designed to look at many different aspects of diet, it is not at all impossible that any particular interesting correlation is due to chance. Even if it is not due to chance, there are potential explanations other than a beneficial effect of chocolate. For example, people at high risk of heart disease may be avoiding chocolate.

“In some ways, the results contradict previous claims of the dietary benefits of chocolate. Most of the chocolate eaten by study participants was milk chocolate, with low levels of flavonoids. Adjusting for a measure of inflammation (C-reactive protein) had essentially no impact on the correlation between chocolate and heart disease, suggesting that the correlation is not due to an effect of chocolate flavonoids on inflammation.

“The only likely harmful effects of chocolate, however, are due to the calorie content. In this study there was no meaningful correlation between chocolate consumption and BMI or waist:hip ratio, and those who ate more chocolate tended to exercise more. While a single observational study isn’t a good reason to change your diet, it is reassuring that there is no evidence of harm from eating chocolate when it doesn’t lead to increased weight.”

From the UK Science Media Centre:

Prof Aedin Cassidy, Professor of Nutrition at the University of East Anglia (UEA), said:

“Although the findings are not new, they add to the evidence that intake of chocolate may reduce the risk of heart disease and stroke compared to non-consumers.

“When they divided chocolate intake into quintiles they showed that those in the top 20% intake group had a 12% reduction in risk for heart disease and 23 % for stroke compared to the lowest 20% (who didn’t eat chocolate). Intake in the top ranging between 16-99g/d suggesting modest intakes have these health benefits. A couple of squares of chocolate would be around 16g and an average choc bar is around 50g.

“They infer that since most of the chocolate came from milk chocolate that it is unlikely that the flavonoids present in cocoa (called flavan-3-ols) are unlikely to explain their results- however although it is true that dark chocolate in general contain more flavan-3-ols than milk, the levels present are entirely depended on manufacturing processes and some milk chocolate contains high levels of these bioactive compounds too. If these flavonoids are important there are many other sources in the diet, including tea, apples, wine.

“This is an observational study and although thorough to really determine if chocolate is good for your heart we need robust long-term trials and to date most of the trials (which have shown beneficial effects on biomarkers of heart disease including blood pressure, blood flow and cholesterol levels) have been short term (3-4 months in length). Chocolate also contains fat and sugar so only moderate intakes should be recommended as part of a healthy diet rich in fruits and vegetables(an average chocolate bar (50g) provides about 230 kcal and 10% daily energy intake therefore high intake has the potential to affect weight, an important risk factor for heart disease.

“We need long term trials to further understand the importance of chocolate for heart health.”

Dr Tim Chico, Reader in Cardiovascular Medicine and Consultant Cardiologist at the University of Sheffield, said:

“This study adds to the evidence that people who consume chocolate tend to have lower rates of cardiovascular disease, although such studies cannot say whether the chocolate is the cause of this protective effect.

“There is evidence from other studies that have randomised people to be given chocolate that this can have effects that might reduce cardiovascular disease, such as a reduction in blood pressure. “These studies taken together suggest that there might be some health benefits from eating chocolate. However, it is also clear that chocolate has the potential to increase weight, which is unequivocally bad for cardiovascular health.

“The message I take from this study is that if you are a healthy weight, then eating chocolate (in moderation) does not detectibly increase risk of heart disease and may even have some benefit. I would not advise my patients to increase their chocolate intake based on this research, particularly if they are overweight.”

The UK Science Media Centre has also provided a ‘Before The Headlines’ statistical analysis:


Title, Date of Publication & Journal

Habitual chocolate consumption and risk of cardiovascular disease among healthy men and women, 15 June 2015, Heart

Study’s main claims – and are they supported by the data?

The paper provides limited, weak evidence to support a reduction in cardiovascular disease for people who eat more chocolate.

Observational studies like this cannot show a causal affect. The association seen here could have an alternative explanation (some explanations are suggested by the authors).

The top line of the press release could lead people to infer that chocolate has a protective effect against CVD, and the paper does not show this.


The paper shows that people in Norfolk who admitted to consuming more chocolate squares, bars or hot chocolate in a questionnaire administered once in the 1990s were, according to their answers to that questionnaire, younger with lower BMI and blood pressure and less likely to have diabetes or to be physically active and more likely to smoke. During follow-up until 2008, they were also less likely to die from cardiovascular disease (although not less likely to suffer cardiovascular disease).

It is hard to know if the lower risk comes from chocolate or those other factors.  The authors have tried to account for these as far as possible, but the nature of the study means that it is not possible to do that perfectly. Therefore, it is possible that the protective effect might be because of something else – not chocolate.

The study is well conducted observational research, but the limitations of the study design mean that the study can only generate hypotheses for evaluation in further research. The discussion is clear about these limitations (as is the body of the press release):

  • The measurement of chocolate exposure in a questionnaire can be undermined by shy chocolate eaters who underreport their consumption
  • Sometimes people at higher risk cut their consumption of “risky” food – so it could be that people more likely to get CVD are deliberately eating less chocolate, not the other way round
  • It is hard to account for other risk factors (e.g. physical activity) when each is measured only once in a questionnaire. People misreport exercise, just like chocolate consumption, and even an accurate, single answer would not capture activity over the full study period (e.g. 1997-2008). Therefore it is not possible to account perfectly for these risk factors.

The authors were not able to evaluate publication bias.

Publication bias: over-reporting of an association due to statistically significant findings being published and non-significant findings never being submitted or accepted for publication.

Before The Headlines is a service provided to the SMC by volunteer statisticians: members of the Royal Statistical Society (RSS), Statisticians in the Pharmaceutical Industry (PSI) and experienced statisticians in academia and research.  A list of contributors, including affiliations, is available at http://www.sciencemediacentre. org/working-with-us/for- journalists/headlines-for- journalists

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