“Two chocolate bars a day can SLASH the risk of heart disease and stroke,” the Daily Mirror reports.
The headline is prompted by the results from a large study involving Norfolk residents, investigating how chocolate is linked to cardiovascular diseases. These are diseases that affect the heart and blood vessels, such as coronary heart disease and stroke.
By comparing the highest chocolate consumers with complete chocolate abstainers, they found that chocolate was linked to a lower risk of stroke and cardiovascular disease. However, the risk for coronary heart disease was not statistically significant, so the aforementioned results could have been down to chance.
The biggest caution in taking these results at face value is the possibility that some of the benefits linked to chocolate are actually linked to the person being generally healthier overall.
There were signs of this in the study. For example, the researchers found that higher chocolate consumption was linked to some healthy qualities and behaviours, such as being more physically active.
It is also important not to overlook the large amounts of fat and sugar in chocolate that can contribute to weight gain. If you are overweight or obese, by definition, your weight is probably damaging your health and eating lots of chocolate will make the problem worse.
Read more about how to maintain a healthy weight.
Where did the story come from?
The study was carried out by researchers from the University of Aberdeen and was funded by the Medical Research Council and Cancer Research UK.
The study was published in the peer-reviewed medical journal Heart.
The story was very widely reported by the UK media. Generally, the study facts were reported accurately, but the wider implications and inherent limitations of the study were not fully explained. For example, most coverage correctly said that study participants reporting higher chocolate consumption were generally healthier in many other ways, but did not explain how this makes it particularly hard to attribute any health benefits to chocolate on its own.
BBC News provided a useful quote from an independent expert, Dr Tim Chico: "The message I take from this study is that if you are a healthy weight, then eating chocolate (in moderation) does not detectably 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."
What kind of research was this?
This was a prospective cohort study looking at the effect of eating chocolate on cardiovascular disease.
Cardiovascular disease is a general term that describes a disease of the heart or blood vessels, and is one of the UK's largest causes of death.
There are four main types of cardiovascular disease. They are:
- coronary heart disease – when the flow of oxygen-rich blood to the heart is blocked
- stroke – when blood supply to the brain is blocked
- peripheral arterial disease – when bloodflow to your limbs, usually your legs, are blocked
- aortic disease – problems with the aorta, the largest blood vessel in the body, which takes blood from your heart to the rest of your body, which may need to be treated with an aortic valve replacement
Chocolate, more so the dark variety, contains flavonoids. These are plant chemicals that have antioxidant properties, that many speculate give it health-promoting properties, including keeping hearts and blood vessels healthy.
Small experimental and observational studies, the Aberdeen researchers report, indicate that chocolate might be good for heart and blood vessel health, but the picture is not clear, as these studies have design limitations. This research group wanted to use a large group of people, tracked over a long period of time, to improve the evidence base and better understand if chocolate might be affecting cardiovascular disease risk in real life.
What did the research involve?
The researchers analysed data from a cohort study, which assessed chocolate consumption at baseline and then followed people over an average of 11 years to see who developed cardiovascular disease. They then supplemented this research with a systematic review and meta-analysis of literature.
Researchers analysed data from 20,951 adult men and women taking part in the EPIC-Norfolk study, a large UK-based cohort study started in the 1990s to look at the connection between diet, lifestyle factors and disease. Average chocolate intake was measured once at the start of the study, before people were tracked over decades, to see if they developed or died from cardiovascular disease. The main analysis looked at how chocolate consumption affected the risk of developing or dying from cardiovascular disease, taking into account a range of other known risk factors, like smoking and alcohol consumption.
EPIC-Norfolk cohort participants are men and women who were aged between 40 and 79 when they joined the study, and who lived in Norwich and the surrounding towns and rural areas. They have been contributing information about their diet, lifestyle and health through questionnaires and health checks over two decades.
Chocolate consumption was measured at a single point in time at the start of the study (1993 to 1997). They were asked to indicate which foods they ate from a large list and how often.
Three questions from the food questionnaire related to chocolate consumption:
- “Chocolates singles or squares” (average portion size of 8g)
- “Chocolate snack bars – for example, Mars, Crunchie” (average portion size of 50g)
- “Cocoa, hot chocolate (cup)” (average portion size of 12g powder weight; the liquid to make up the beverage was not included)
Frequency categories were multiplied by the portion size to get the amount of chocolate eaten daily (g/day). The sum of the weights of these food items consumed, rather than their flavonoid or cocoa content, were the important measure in this study.
This average daily chocolate consumption was divided into five equal groups, from highest consumption to lowest. The lowest group didn’t eat or drink any chocolate at all and acted as the comparison group.
After the food questionnaire, participants were tracked for a mean average of 11.3 years to see if they developed or died from cardiovascular disease, coronary heart disease or stroke.
Both admission to hospital and deaths due to these conditions were included in the analysis.
After some people were excluded because of missing data, extreme chocolate intake (thought to be an error), or pre-existing cardiovascular disease, it left 20,951 people for the analysis.
The analysis adjusted for a range of common confounders associated with cardiovascular disease, including:
- physical activity
- energy intake
- body mass index (BMI)
- systolic blood pressure
- LDL cholesterol (bad cholesterol)
- HDL cholesterol (good cholesterol)
- having diabetes
- C-reactive protein – a protein associated with inflammation inside the body
To supplement the EPIC-Norfolk-derived results, the researchers also carried out a systematic review and meta-analysis of research related to chocolate and cardiovascular disease.
What were the basic results?
Higher chocolate consumption was associated with lower age, more physical activity and lower prevalence of diabetes mellitus.
Higher chocolate consumption was more common in men and among smokers. Higher chocolate intake was associated with a higher energy intake, with lower contributions from protein and alcohol sources, and higher contribution from fat and carbohydrates.
The percentage of participants with coronary heart disease in the highest and lowest fifth of chocolate consumption was 9.7% and 13.8%, and the respective rates for stroke were 3.1% and 5.4%.
The confounder-adjusted risk of coronary heart disease was 9% less for those in the top quintile of chocolate consumption (16 to 99g/ day) compared with those not consuming chocolate (hazard ratio (HR) 0.91, 95% confidence interval (CI) 0.80 to 1.04). The confidence interval spans 1, meaning this result could be due to chance alone.
By contrast, chocolate consumption in the highest consuming group was associated with significantly less risk for stroke (HR 0.78, 95% CI 0.63 to 0.98) and cardiovascular disease (defined as the sum of coronary heart disease and stroke, HR 0.89, 95% CI 0.79 to 1.00) compared with chocolate abstainers.
The systematic review and meta-analysis included eight studies (seven cohort studies, one randomised control trial). These were combined with results from the EPIC-Norfolk study to get pooled results (total 157,809 participants).
The studies measured chocolate consumption in different ways, adjusted for different confounders, and measured different health outcomes related to cardiovascular disease. Consequently, only similar studies were combined in meta-analyses.
Overall, the different meta-analysis showed that:
- chocolate consumption was linked with significantly lower risk of coronary heart disease across five studies (pooled relative risk (RR) 0.71, 95% CI 0.56 to 0.92)
- the risk of coronary heart disease mortality from one study showed no significant difference with and without chocolate consumption (RR 0.98, 95% CI 0.88 to 1.10).
- for risk of stroke with chocolate consumption, there was significantly lower risk of both stroke incidence (pooled RR 0.79, 95% CI 0.70 to 0.87; five studies) and mortality (RR 0.85, 95% CI 0.74 to 0.98; one study)
- there was a lower risk of any cardiovascular event (pooled RR 0.75, 95% CI 0.54 to 1.05, two studies, not statistically significant) and cardiovascular mortality (pooled RR 0.55, 95% CI 0.36 to 0.83; three studies, statistically significant)
How did the researchers interpret the results?
The research authors said: “Cumulative evidence suggests that higher chocolate intake is associated with a lower risk of future cardiovascular events, although residual confounding cannot be excluded. There does not appear to be any evidence to say that chocolate should be avoided in those who are concerned about cardiovascular risk”.
This study used a large prospective cohort of English residents to estimate the risk chocolate poses to cardiovascular death and disease. In addition, they systematically combed the research literature for other similar studies, combining their results with that of other researchers.
By comparing the highest chocolate consumers with chocolate abstainers, they found that chocolate was linked to a lower risk of stroke and cardiovascular disease. The risk for coronary heart disease was not statistically significant.
Results from the meta-analysis of eight additional studies showed higher chocolate consumption was linked with lower risk of cardiovascular disease, stroke and death from cardiovascular disease. Two studies showed cardiovascular events were not statistically linked with chocolate consumption.
The biggest reservation for believing these results is the possible role of residual confounding, rightly highlighted by the study authors themselves. In the cohort study part, chocolate consumption was linked to a range of healthy qualities and behaviours, such as lower blood pressure and more physical activity. There is a real possibility that some of the benefits linked to chocolate are actually linked to the person being generally healthier in other ways.
The researchers did their best to account for this using standard statistical techniques, but the possibility remains.
This is just one explanation. Another is that the flavonoids in chocolate do benefit heart and blood vessel health. Although plausible, this study cannot prove this. There are far too many other elements in the mix to pinpoint the risk reduction observed for chocolate.
The study has a number of other smaller limitations that make its results a little less reliable. Chocolate consumption was measured at a single point in time at the start of the study. This does not take account of changes in chocolate consumption over the following decades. Chocolate consumption was measured without consideration of the flavonoid content. Not all chocolate contains the same amount of flavonoids – thought to be the potential disease-preventing ingredient – so lumping them together could have clouded the picture.
Overall, though the message seems to be that if you are generally healthy, eating a little chocolate probably won’t do any harm, and may in fact do some good, this is not actually proven in this study.
The issue arises when chocolate affects your weight. We know that chocolate is high in sugar and fat, both of which can contribute to weight gain. Being overweight or obese is bad for your health, including your heart and blood vessels.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Daily Mirror, 16 June 2015
The Guardian, 16 June 2015
Mail Online, 16 June 2015
Sky News, 16 June 2015
The Daily Telegraph, 16 June 2015
ITV News, 16 June 2015
Links to the science
Heart. Published online June 15 2015