Friday September 26 2008
A study has found that “a nibble of dark chocolate a day could help prevent killer heart attacks”, reported the Daily Express. It said that eating just a quarter of an ounce, about a third of a small bar, could cut the risk of heart disease by 25% in men and a third in women. The study, in nearly 5,000 people, showed that eating dark chocolate “dramatically slowed the hardening of arteries, which is a major cause of heart attacks”.
Despite what was reported in the news, this study did not directly assess whether chocolate can prevent heart attacks or slow the hardening of arteries. Instead it assessed the link between eating dark chocolate and levels of C-reactive protein (CRP) in the blood. High levels of CRP indicate inflammation and has been independently linked with an increased risk of cardiovascular disease. Although the study found that these markers were lower in chocolate eaters, the study design is of a type that cannot establish that chocolate caused the reduction in CRP levels. Until more robust studies are carried out, a healthy diet and regular exercise are still the best way to reduce the risk of cardiovascular disease.
Where did the story come from?
Dr Romina di Guiseppe and colleagues from the Catholic University and the National Cancer Institute in Italy carried out this study. The research was funded by Pfizer, the Italian Ministry of University and Research and the Instrumentation Laboratory. The study was published in the (peer-reviewed) medical journal: the Journal of Nutrition.
What kind of scientific study was this?
Dark chocolate contains high levels of flavonoids, which are antioxidants and are thought to potentially lower the risk of cardiovascular disease. In this cross-sectional study, the researchers looked at dark chocolate consumption and if it had any effect on levels of CRP in the blood. CRP indicates inflammation and is thought to be an independent indicator of risk for coronary heart disease.
This study included people who were participating in a larger prospective study called the Moli-sani Project. These participants were aged over 35 and randomly selected from towns in Italy. Those who were eligible (not pregnant and willing to participate) were interviewed and asked about their socioeconomic status, physical activity, medical history, risk factors (including smoking), history of cardio vascular disease, tumours and drug use. Dietary habits were recorded using a food frequency questionnaire that assessed their food intake and the daily amounts of different foodstuffs they had eaten over the past 12 months. The questionnaire also had questions about how much chocolate they ate, how frequently they ate it (daily, weekly, monthly), and the type of chocolate (dark, milk, nut, or any). Trained personnel measured blood pressure and height, weight and waist circumference. Blood samples were taken after the participants had fasted overnight and at least six hours after they had smoked, and the level of CRP in the blood was measured. Following this, the researchers excluded anyone with CVD, were currently being treated for hypertension, diabetes or dyslipidemia, had a special diet or had missing information.
The researchers then compared blood concentration levels of CRP in people who ate dark chocolate with those who did not. In total 4,849 people were available for analysis. Of these 1,317 did not eat any chocolate, 824 ate dark chocolate only and the others ate milk or ‘any’ chocolate (2,708 people). Their analyses took into account other factors that could influence the link (confounders), including age, sex, social status, physical activity, nutrient intake and other food intake. Only the consumption or non-consumption of dark chocolate was analysed, and the researchers did not look at the effect of milk chocolate, other chocolate intake or the effect in people who ate both milk and dark chocolate.
What were the results of the study?
The researchers found there to be significantly greater proportion of people with high CRP levels (>3mg/L) in the group that did not consume dark chocolate compared with the group who consumed it (19% v 14%). The inverse relationship between consumption of dark chocolate and levels of CRP was confirmed in all analyses, which took into account the effects of various confounding factors.
Chocolate consumption did not reduce blood pressure when other factors were taken into account. The researchers also looked to see whether there was a dose response, i.e. whether increased chocolate doses were linked to greater benefits. They found that with increased consumption there was an initial decrease in CRP levels but that the reduction levelled out and even reversed in the highest consumption group.
What interpretations did the researchers draw from these results?
The researchers conclude that regular intake of dark chocolate is inversely related to the concentration of CRP in the blood. While consumption of dark chocolate is also associated with younger age, higher social status and lower total physical activity, taking these into account did not remove the effect of the chocolate.
What does the NHS Knowledge Service make of this study?
This cross-sectional study found a link between the consumption of dark chocolate and levels of CRP in the blood. The findings correspond to those of some other studies so this increases our confidence in the results.
There are some important points:
- The study assessed the link between dark chocolate consumption and levels of CRP in the blood. It did not look for a link between eating chocolate and disease events. The researchers only speculate as to what effect changes in CRP could have on cardiovascular risk (rates of heart disease or stroke), based on data from another selected study. They say the difference in CRP levels between consumers of dark chocolate and non-consumers represents a 26% reduction in cardiovascular disease risk for men and a 33% reduction for women. This estimate in itself has little to do with heart attacks. People can have cardiovascular disease but not have a heart attack. The newspaper report may be misleading in that it suggests that dark chocolate is responsible for a 25% and 33% reduction in heart attacks in men and women respectively.
- Studies of this design (cross-sectional) have shortcomings when exploring causal relationships. It is possible that ‘dark chocolate consumption’ is linked to another factor which is itself responsible for the effects on CRP, i.e. that there are possible confounders. The researchers have tried to measure and adjust their analyses for the most obvious of these – physical activity, age, sex, other dietary factors – but the possibility that an unmeasured confounder is responsible for the link cannot be ruled out.
- Although the researchers also collected information from people who consumed all chocolate types or just milk chocolate, they do not report those results.
Prospective studies, preferably randomised controlled trials, would be a more robust way of determining whether chocolate is responsible for these reductions in CRP levels. Such studies could also look at actual disease endpoints, such as heart attack or other cardiovascular events. Until then, these results should be viewed somewhat cautiously, especially considering that the beneficial effects of chocolate on CRP appeared reversed at high doses.
There is strong evidence that a healthy balanced diet and regular exercise are linked to lower CRP levels and a reduced risk of cardiovascular disease. Eating chocolate should not be considered comparable to the health benefits of these.