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.
Where did the story come from?
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.
What kind of research was this?
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.
What did the research involve?
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.
What were the basic results?
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.
How did the researchers interpret the results?
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:
- In this type of study, confounding factors, other than the one being assessed (chocolate consumption in this case), may contribute to the differences between the groups. Although this study took into account a number of potential confounders, it is possible that additional confounders were not measured or were inaccurately quantified. For example, lifestyle and other dietary measures were only assessed in a single measurement at the start of the study and may not reflect the participants’ histories or behaviour during follow-up.
- Although the researchers used a standard food frequency questionnaire and a 24-hour food recall questionnaire in a small sample of participants, there may still be inaccuracies in people’s recollection of their diets. Diet, including chocolate consumption, is likely to vary over a lifetime and a single assessment is unlikely to capture a person’s lifelong habits. It is also difficult to take into account the chocolate that may be included in the diet in the form of biscuits, baked goods and other sources.
- The level of chocolate consumption (estimated based on the 8% of the sample who carried out the 24-hour dietary recall) was low. For instance, those in the highest category reportedly consumed only 7.5g of chocolate a day and those in the lowest category only 1.7g. This is considerably less than the mass of the average chocolate bar, and the difference between the two groups is reported to be the equivalent of less than one small square of a 100g bar. It is unclear where the idea of a “healthy bar a day” in newspapers came from.
- Associations between chocolate and stroke or heart attack were reduced in strength when the researchers adjusted for the influence of blood pressure at the start of the study. Although the research reports that the reduced risk of heart attack and stroke may be due to the effect that chocolate has on reducing blood pressure, chocolate consumption and blood pressure were both measured at the same time in this study. This means that it cannot tell whether chocolate could have contributed to the slightly lower blood pressure at the start of the study, or whether the higher consumption group maintained lower blood pressure during follow-up.
- The researchers note that other studies on chocolate have had mixed findings, with some showing a reduction in cardiovascular disease with increasing chocolate consumption and others showing no association or only weak associations. A systematic review of all relevant studies would give a clearer picture of whether an association exists.
- As the researchers rightly say, the findings would need confirmation in randomised controlled trials. There may be some practical difficulties with this due to the long follow-up needed for measuring cardiovascular outcomes and the large participant numbers required. However, if certain compounds in chocolate (such as flavonoids) are thought to be responsible for potential cardiovascular benefits, it may be more feasible for these to be extracted and tested in randomised controlled trials.
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.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Guardian, 30 March 2010
BBC News, 30 March 2010
The Daily Telegraph, 30 March 2010
Daily Express, 30 March 2010
Links to the science
European Heart Journal, March 30 2010 (published online)