“The key to preventing a stroke? One coffee and four cups of green tea a day, say scientists,” is the advice in the Mail Online website’s headline.
The Mail reports on a large long-term Japanese study that found that both drinks may have a protective effect against stroke.
And while the headlines do reflect the findings of the study, the news is overdramatised.
Researchers found that people who drank four or more cups of green tea a day had a 20% lower stroke risk compared to those who seldom drank it. Those who drank coffee at least once a day were also about 20% less likely to have a stroke than those who seldom drank it.
These findings are of interest, but this study alone cannot prove that either green tea or coffee directly lowers the risk of stroke. While the researchers did try to take into account other factors that can influence stroke risk, it is possible that they may have overlooked something. And as its research was done in Japan, there may be genetic, cultural and environmental differences between the study population (and their habits) and those of us in the UK.
To be more certain that green tea and coffee consumption did lower stroke risk, further research would be needed.
Where did the story come from?
The study was carried out by researchers from several universities in Japan and was funded by the Japanese Ministry of Health, Labor and Welfare. The study was published in the peer-reviewed medical journal, Stroke.
It was covered fairly but uncritically by the Mail Online website and The Daily Telegraph.
But the coverage in the Daily Express is poorer, with its headline stating that green tea or coffee “can slash the risk”.
What kind of research was this?
This was a large prospective cohort study that investigated the association between green tea and coffee consumption and stroke risk in a Japanese population.
This type of study, which enables researchers to follow people for lengthy periods, is useful in looking at associations between lifestyle habits (such as drinking green tea or coffee) and health outcomes (such as stroke). However, on its own, it cannot prove a direct cause and effect (causal) association.
Other factors such as alcohol intake, diet and smoking, may influence the results, even though researchers normally try to take account of these.
To be reasonably certain that a lifestyle habit or exposure directly causes a health outcome usually requires accumulation of a lot of different types of supportive evidence. Arguably the ideal study design to assess the effect of green tea or coffee consumption on stroke risk would be a randomised controlled trial, but expecting people to stick to a green tea-free or coffee-free for many years could be asking a bit much.
However, prospective cohort studies (in which people are followed in real time), are more reliable than a retrospective one (in which participants may have to recall their lifestyle habits over a period of many years).
The authors point out that green tea and coffee are both popular drinks in Japan and while previous research has suggested they may both have health benefits, few prospective studies have examined their association with the incidence of stroke.
What did the research involve?
The researchers used for their data an ongoing cohort study of over 100,000 Japanese adults aged 40 to 69 (47,400 men and 53,538 women). It focused on lifestyle and the risk of cardiovascular disease and cancer.
Participants were divided into two cohorts and asked about their dietary habits using a validated food frequency questionnaire, the first group in 1995 and the second in 1998. The questionnaire also collected information on factors such as height, weight, medical history, lifestyle factors and physical activity.
People who reported having been diagnosed with either cardiovascular disease or cancer; who were lost to follow-up; moved out of the area; or who incompletely answered the questionnaire were excluded from the study. This left 81,978 adults (38,029 men and 43,949 women) who could be included in the analysis.
In the questionnaires, participants were asked in detail about the frequency of their consumption of various foods and drinks. For the current analysis of green tea consumption, researchers looked at whether people drank green tea:
- never; one to two times a week; or three to six times a week, and
- one; two to three; or four or more cups a day
For the analysis of coffee consumption, they looked at whether people drank coffee:
- never; one to two times a week; or three to six times a week, and
- one cup; or two cups or more a day
They did not collect information on whether coffee was caffeinated or not, since decaffeinated coffee is rarely consumed in Japan.
Participants were followed up for a period of 13 years on average.
To find out whether participants suffered cardiovascular disease (CVD), medical records from 54 major hospitals in the area where they lived were reviewed by hospital workers, doctors and researchers. A systematic search of death certificates was undertaken to obtain information on fatalities from CVD overall (including both stroke and coronary heart disease (CHD))
From this data, all strokes were confirmed using nationally accepted criteria. Researchers divided strokes into three stroke subtypes:
- cerebral infarction (in which a stroke is caused by a blood clot)
- intracerebral haemorrhage (in which a blood vessel in the brain bursts)
- subarachnoid haemorrhage (bleeding in and around the brain)
They also collected information on incidence of CHD.
They used standard statistical methods to look at the association between CVD overall, stroke overall, the different types of stroke and CHD. They adjusted their results for confounders such as smoking, alcohol consumption, age, body weight, medication use and history of diabetes.
What were the basic results?
There were 3,425 strokes during the follow-up period (1,964 due to blood clots, 1,001 due to a burst blood vessel in the brain, and 460 subarachnoid haemorrhages) and 910 incidents of coronary heart disease. In their analyses, the researchers found that:
- People who drank two to three cups of green tea a day were 14% less likely to have any type of stroke than those who seldom or never drank it (hazard ratio [HR] 0.86, 95% confidence interval [CI] 0.78 to 0.95).
- People who drank four or more cups of green tea a day were 20% less likely to have any type of stroke than those who seldom or never drank it (HR 0.80, 95% CI 0.73 to 0.89).
- Higher green tea consumption was associated with a lower risk of CVD overall and lower risk of the different types of stroke.
- People who drank coffee three to six times a week were 11% less likely to have any type of stroke compared with those who seldom drank it (HR 0.89, 95% CI 0.80 to 0.99).
- People who drank coffee once a day were 20% less likely to have any type of stroke compared with those who seldom drank it (HR 0.80, 0.72 to 0.90).
- People who drank coffee twice or more daily were 19% less likely to have any type of stroke compared with those who seldom drank it (HR 0.81 95% CI 0.72 to 0.91).
- Coffee consumption was associated with a lower risk of CVD overall and the cerebral infarction type of stroke.
- Higher green tea or coffee consumption together, was associated with reduced risks of CVD and all types of stroke, especially intracerebral haemorrhage.
- No significant association was observed between green tea or coffee consumption and coronary heart disease.
How did the researchers interpret the results?
Higher green tea and coffee consumption were associated with a lower risk of CVD and stroke, say the researchers. Both beverages contain substances that may have antioxidant and other protective effects, they say.
This large study with a long follow-up period has suggested that green tea and coffee consumption may be associated with a reduced risk of stroke. The findings are of interest, but the study has a number of limitations, as the authors point out:
- The information on illness, green tea and coffee consumption was all self-reported, which introduces the possibility of error. For example, although food frequency questionnaires are an accepted way to assess food intake, there may still be errors in people’s estimations of their consumption.
- People’s consumption of green tea and coffee was only measured once, so any changes in consumption over the years was not taken into account by the study. It was not clear when the potential confounders (e.g. smoking) were assessed, and these may also have changed over the course of the study.
- The results may have been affected by other factors (confounders). The researchers did adjust for a number of these, including other dietary factors, but others could be having an effect. In particular, although researchers adjusted their results for whether people took medication for high blood pressure or high cholesterol, they did not take account of the existence of these conditions in participants who were not taking medication.
- The study took place in Japan so its results may not be applicable to the UK or other countries.
It is not possible to say conclusively from this study alone that drinking more coffee or green tea will reduce your stroke risk. To be reasonably certain that a lifestyle habit or exposure directly causes a health outcome requires accumulation of a lot of different types of supportive evidence.
Caffeinated drinks such as coffee should be drunk in moderation. Drinking excessive amounts (more than 600mg a day – or around four cups) can lead to feelings of irritability, anxiety and symptoms of insomnia. Pregnant women shouldn’t drink more than 200mg a day.