Tuesday March 3 2015
Three to five cups a day seemed to be the optimal 'dose'
"Three coffees a day cuts the risk of heart disease and strokes," the Daily Mirror reports.
A large study of 25,000 adults from South Korea has found that people who drink between three and five cups of coffee per day were less likely to have the first signs of coronary heart disease.
This is a condition where atherosclerosis (hardening of the arteries) restricts the supply of blood to the heart. In some cases atherosclerosis can cause a blood clot to develop, which can trigger a heart attack.
The participants had a CT scan in which the level of calcium deposits in the coronary arteries was measured. Calcium deposits are one of the first signs of atherosclerosis.
They also completed a food frequency questionnaire to estimate their average food and drink consumption over the previous year.
People who drank between three and five cups of coffee were 19% less likely to have calcium deposits than people who did not drink coffee.
Despite media reports, as the study only looked at data from one point in time, it does not prove that drinking this amount of coffee each day is good for the heart.
Where did the story come from?
The study was carried out by researchers from Kangbuk Samsung Hospital in South Korea and there was no external funding.
The study was published in the peer-reviewed medical journal Heart.
In general, the UK media reported on the study accurately, but they did not explain that the only statistically significant result was for people drinking between three and five cups of coffee per day compared to those who do not drink coffee.
Also, the claim that the reduction in calcium deposits would help prevent heart attacks in later life, while arguably plausible, is unproven.
What kind of research was this?
This was a cross-sectional study that aimed to see if there was an association between coffee consumption and the early signs of heart disease. As it was a cross-sectional study it looked at data from one point in time. This means that it can only show an association, it cannot prove that coffee causes reduced levels of calcium to be deposited in the coronary arteries.
A randomised controlled trial would ideally be required, though studies randomising people to food or drink items over a long period of time to look at cardiovascular outcomes would have serious feasibility issues; especially regarding compliance. For example, asking a seasoned "coffee addict" not to drink any coffee for the next 10 years probably won’t meet with much success.
What did the research involve?
The researchers used information from a large cohort of 30,485 adults who were taking part in the Kangbuk Samsung Health study, which is an ongoing cohort study organised out of a Korean hospital.
All participants had a full health screen and a CT scan of the heart between March 2011 and April 2013 to measure the level of calcium in the coronary arteries. This was taken as an early indicator of atherosclerosis, hardening of the arteries, which leads to heart disease.
A self-administered 103-item food frequency questionnaire was also completed. The participants were asked to estimate how often, on average, they consumed each type of food or drink in the previous year. This included coffee, but did not discriminate between caffeinated and decaffeinated. The researchers say that decaffeinated coffee is not widespread in South Korea.
The researchers then compared the level of coffee consumption with the amount of calcium in the coronary arteries. They adjusted their results to take into account the following confounders:
- educational level
- physical activity level (inactive, minimally active or "health enhancing physically active")
- smoking status
- body mass index (BMI)
- parental history of heart disease
- alcohol consumption
- total energy consumption
- consumption of fruit and vegetables
- consumption of red and processed meats
- systolic blood pressure
- fasting blood sugar
- cholesterol and triglycerides (levels of fat in the blood)
People were excluded from the study if they already had a history of cardiovascular disease or incomplete information.
What were the basic results?
The final sample consisted of 25,138 adults. The average age was 41 years and 83.7% were male.
After adjusting the results for all the potential confounding factors listed above, compared to people who did not drink coffee:
- people who drank between three and five cups of coffee were 19% less likely to have calcium in the coronary arteries (odds ratio (OR) 0.81, 95% confidence interval (CI) 0.66 to 0.98)
- there were no statistically significant changes in risk for people who drank less than one cup, between one and three cups or five or more cups of coffee
How did the researchers interpret the results?
The researchers concluded that "moderate daily coffee consumption was associated with decreased prevalence of CAC [coronary artery calcium] in a large sample of [symptom-free] adults free of CVD". They say that "further research is warranted to confirm our findings and establish the biological basis of coffee’s potential preventive effects on coronary artery disease".
This large cross-sectional study found that people who reported drinking between three to five cups of coffee per day in the previous year were less likely to have calcium deposits in the coronary arteries than people who did not drink coffee. There was no statistically significant difference for people consuming any other level of coffee compared to those who don’t drink coffee.
This type of study cannot prove that drinking this level of coffee stopped calcium being deposited in the arteries, an early sign of atherosclerosis (hardening of the arteries). It shows there is an association, but does not explain why.
Strengths of the study include the large sample size and extent to which potential confounding factors were taken into account. However, there are some limitations:
- As with many attempts to collect data on dietary consumption, there is the potential for inaccurate estimates and recall bias.
- Most of the participants were male, so the results may not be as robust for women.
- It is not clear how applicable the results would be to the UK population as there may be many unmeasured features of the South Korean diet that could have affected the results. Indeed, South Korea has a lower cardiovascular disease death rate than the UK, though reasons for this are likely to be multifactorial.
- None of the participants had any symptoms of cardiovascular disease. The study provides a snapshot of the level of calcium in their coronary arteries. It does not show how drinking coffee might affect these levels over time.
Though the results of this study are interesting and warrant further investigation, they do not prove that drinking three to five cups of coffee a day is good for the heart.
You can reduce your risk of heart disease by stopping smoking, eating healthily, being physically active, and keeping your blood pressure and cholesterol within normal limits, through lifestyle choices and use of medication, where required.
Read more about reducing your heart disease risk.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.