Chocolate is officially “good for you”, according to The Guardian. We can now apparently rejoice at the thought that munching our Easter eggs will make us less likely to have a stroke or heart attack. The Daily Telegraph says that eating a bar a day could cut the risks by as much as 39%.
The news is based on research that followed 19,000 people over eight years. Looking at the participants’ chocolate intake at the start of the study, researchers found that higher intake of chocolate was associated with reduced risk of heart attack or stroke. However, the strength of this association was reduced when the influence of the participants’ blood pressure was taken into account. Equally, it cannot be concluded that chocolate influenced the participants’ blood pressure as it was only measured once, at the start of the study. It is also important to note that those in the highest consumption category consumed only 7.5g a day, which is far less than a whole bar of chocolate.
Overall, the question remains as to whether chocolate has any cardiovascular health benefits. It is important to remember is that, regardless of any potential benefits, chocolate is high in fat and calories and should be enjoyed only in moderation. A diet high in fat and calories is known to increase the risk of obesity, heart disease and stroke, rather than decreasing it.
This research was conducted by Dr Brian Buijsse and colleagues of the German Institute of Human Nutrition. The study was given financial support by the German Federal Ministry of Science, the European Union and the German Cancer Aid. The study was published in the peer-reviewed European Heart Journal.
The newspapers generally did not give a balanced summary of the findings and limitations of this research, which can give no firm conclusions on the health benefits of eating chocolate.
This was a cohort study designed to investigate the link between eating chocolate and development of high blood pressure, stroke and heart disease over an eight-year follow-up period.
A cohort study is normally a good way of observing whether a risk factor is linked to an outcome across an extended period of follow-up. However, the researchers must ensure that their cohort of participants is sufficiently large (as it was in this study) and that they account for other factors that could possibly influence their outcomes (confounders) when analysing their results. There can be particular problems with assessing dietary factors through a cohort study, namely that it is often difficult to get an accurate quantification of a person’s consumption of a particular food, and dietary habits are liable to change over time.
The preferred method for studying the effects of a substance like chocolate would be a randomised controlled trial, in which people were assigned to consume chocolate or no chocolate. However, this is likely to be unfeasible due to the large number of people and long duration of follow-up that would be needed to study cardiovascular outcomes like stroke risk. Ideally, the participants would also restrict their chocolate intake to only that which was assigned by the researchers. This seems unlikely to happen over a long study period.
If chocolate contains compounds that reduce the risk of cardiovascular disease, these substances could be extracted and tested against a placebo in randomised controlled trials.
This study used participants drawn from another study called the European Prospective Investigation into Cancer (EPIC). This research gathered data on 19,357 members of the general population, aged between 35 and 65, who took part in enrolment examinations between 1994 and 1998. All were free of cardiovascular disease and were not taking blood pressure medications. The examinations included completing a food frequency questionnaire, an interview about their medical history, lifestyle and socio-demographic details, and measurement of blood pressure and body mass index (BMI).
Chocolate consumption was assessed by how frequently a 50g bar of chocolate was consumed and how many bars of chocolate participants ate each day. In addition, 8% of the sample (1,568 people) participated in a 24-hour dietary recall assessment.
Follow-up assessments were carried out by postal questionnaire sent every two to three years. By 2004-6 (average 8.1 years), the researchers had four complete rounds of follow-up, with an average 90% response rate across all questionnaires. Self-reports of heart attack, stroke or associated symptoms were confirmed by reviewing medical records and death certificates and contacting treating physicians.
In this subsequent study, the researchers analysed the relationship between chocolate intake and cardiovascular outcomes in models adjusted for different groups of possible confounding factors. These factors included total energy intake, age, gender, alcohol intake, employment status, BMI, waist circumference, smoking, physical activity, education, diabetes, and intake of fruit, vegetables, red meat, processed meat, dairy, coffee, tea and cereal fibre.
In total, 92.3% of the sample reported chocolate consumption at the start of the study. Various factors were associated with increased chocolate intake, such as being female and having a lower intake of fruit, vegetables, dairy and alcohol. At the start of the study, reporting higher chocolate consumption was also associated with lower blood pressure (1.0mmHg average difference between the highest and lowest consumption categories). Of those who took part in the 24-hour food recall, 57% ate milk chocolate, 24% dark, 2% white and 17% did not specify chocolate type consumed.
There were 166 cases of heart attack and 136 cases of stroke during the eight-year follow-up. After adjusting for age, sex, lifestyle, BMI, diabetes and other dietary factors, those in the highest category of chocolate consumption (7.5g a day) had a 39% decreased risk of the combined outcome of heart attack or stroke compared to the lowest consumers (1.7g a day) (relative risk 0.61, 95% confidence interval 0.44 to 0.87).
Separate analysis for stroke and heart attack risk revealed significant risk reduction for stroke but not heart attack. However, adjusting for the influence of blood pressure at the start of the study reduced the strength of both associations.
The researchers concluded that “chocolate consumption appears to lower cardiovascular risk, in part through reducing blood pressure”. They say the association appears stronger for stroke than for heart attack.
There are a number of important limitations that must be considered when interpreting these results:
Overall, the limitations of this study mean that it cannot conclusively prove that chocolate was directly responsible for the reduction in heart attacks and strokes. The assumption that running to the shops to eat a bar a day will stop you having a heart attack or stroke is tantalising but fanciful. However, chocolate can be enjoyed in moderation as part of a healthy, balanced diet.
High blood pressure and diabetes are both clearly associated with increased risk of cardiovascular diseases, and being overweight or obese is associated with both these risk factors. Therefore, eating a diet high in fat and calories is likely to increase, rather than decrease, your risk of these diseases.