Thursday February 19 2009
The study found the reduction in risk was “modest”
“Drinking two cups of coffee a day 'cuts stroke risk by 20%',” reported The Daily Telegraph. It said a study that followed over 80,000 women for more than 20 years showed that those who drank more coffee were less likely to have a clot on the brain. The newspaper said that the results were a “surprise” to researchers, who initially thought that coffee might increase the risk of stroke. It also reported the researchers as stressing that “the protective effect of coffee is only found in those who are already relatively healthy”, and noted that these effects were unlikely to be seen in people with existing heart disease or blood pressure problems.
This study was large and well conducted. However, the results will need confirmation in further studies. In addition, as the authors point out, the reduction in stroke risk is “modest”, and may not apply to all women. Previous studies have suggested that coffee consumption in women with high blood pressure may increase risk of stroke.
Women who want to reduce their risk of stroke would be better off targeting known risk factors for stroke by stopping smoking, eating a healthy diet and taking exercise, rather than trying to drink more coffee.
Where did the story come from?
Dr Esther Lopez-Garcia and colleagues from the Harvard School of Public Health and other universities in the US and Spain carried out this research. The work was funded by the National Institutes of Health. The study was published in the peer-reviewed medical journal Circulation.
What kind of scientific study was this?
This was an analysis of data collected in a prospective cohort study in women. The study, called the Nurses’ Health Study, began in 1976, but data on the women’s diets was only collected from 1980. The current analysis studied data collected from 1980 to look at the effects of coffee consumption on the women’s risk of stroke.
The researchers included the 83,076 women from the Nurses’ Health Study who had no history of stroke, coronary heart disease, diabetes or cancer at the start of the study, and who provided information about their coffee consumption. The women gave information about their health and lifestyles when the study began, and this was updated with questionnaires every two years. Data on the women’s diets was collected by food frequency questionnaires every two to four years during the study. The questionnaires asked how often, on average, women drank coffee and tea in the previous year.
Women’s coffee consumption was classed as less than one cup a month, one cup a month to four cups a week, five to seven cups a week, two to three cups a day, or four cups or more a day. The amount of caffeine in beverages and foodstuffs was estimated based on food composition data from the US Department of Agriculture.
The women were followed up to 2004. Women who reported having a stroke had their medical records assessed by a physician who was blinded to (unaware of) the women’s coffee exposure. The physician classified the woman’s stroke according to standard criteria.
Strokes that were not symptomatic and only identified on imaging were not included. A stroke was classed as “definite” if it was confirmed by imaging techniques, surgery or autopsy, while strokes not meeting these criteria were described as “probable”. The researchers included both non-fatal and fatal strokes in their analyses, including only the first such event for each participant. Deaths were identified by information from next of kin, postal authorities or systematic searches of the National Death Index.
The researchers then used statistical methods to see if there was a relationship between coffee consumption and stroke. Because an individual’s coffee consumption can vary over time, the researchers looked at each two-year follow-up period separately. This let them know if a person had a stroke within a particular two-year period, and the person’s average coffee consumption in the time leading up to that period (rather than an average for the whole period of follow-up).
In their analyses the researchers took into account the women’s ages, smoking, body mass index, alcohol intake, physical activity, menopausal status and use of hormone replacement therapy, aspirin use, and dietary factors that have been associated with risk of stroke or high blood pressure. They adjusted for medical conditions such as high blood pressure, high cholesterol and diabetes. In addition, they looked at the effect of coffee consumption among various subgroups of women, such as those with high blood pressure or smokers. They also looked at whether there was a link between total caffeine consumption and risk of stroke.
What were the results of the study?
During the 24 years of follow-up there were 2,280 strokes among the 83,076 women. Women who drank more coffee were also more likely to smoke and to drink alcohol. They also had differences in their diet compared with women with lower coffee consumption (high potassium, lower glycemic load, lower folate and lower wholegrains).
After taking into account all the potential confounding factors, women who drank two to three cups of caffeinated coffee a week had about a 16% reduction in the risk of stroke compared with women who drank less than one cup of caffeinated coffee a month (relative risk 0.84, 95% confidence interval 0.72 to 0.98).
When researchers looked at specific subgroups, they found that coffee consumption didn't appear to affect risk of stroke in women with hypertension, diabetes or high cholesterol, or women who currently smoked. However, differences between these subgroups of women and other groups of women were not statistically significant.
Increased total caffeine consumption was also associated with a reduced risk of stroke. However, there was no relationship between tea consumption or caffeinated soft drink consumption and risk of stroke. There was a trend towards a lower risk of stroke with increasing decaffeinated coffee consumption, but this trend did not quite reach statistical significance.
What interpretations did the researchers draw from these results?
The researchers conclude that “coffee consumption may modestly reduce risk of stroke” in women. They say the results imply that this could be due to components in coffee other than caffeine, as decaffeinated coffee had a similar effect, but tea and caffeinated soft drinks did not.
What does the NHS Knowledge Service make of this study?
This was a large and well conducted study. Particular strengths include the way that coffee consumption and potential confounders were assessed at multiple time points. The study will no doubt lead to further investigation to identify the “active component” of coffee that may reduce stroke risk. The study has some limitations which should be considered when interpreting its results:
- As with all studies of this type, results can be affected by differences other than the factor of interest between groups. This problem is called confounding. However, the authors did take steps to reduce this possibility by adjusting for potential confounders, and this increases the reliability of their results.
- This study included healthy women, and so the results may not apply to less healthy women, particularly those with existing heart disease, diabetes or previous strokes. In addition, the results suggest that women at higher risk of stroke because of high cholesterol or smoking may also not receive any benefit from coffee consumption.
- The women were asked to recall food and beverage consumption over the past year, and this may have led to some inaccuracies. However, the researchers did find that estimations of coffee and caffeinated drink consumption from these questionnaires showed good levels of agreement with estimates from one-week food diaries in a subset of people who filled these in.
- The authors acknowledge that there may be “reverse causality”, because women who have high blood pressure or other conditions that increase risk of stroke may cut their coffee consumption. However, the researchers did find that coffee consumption reduced stroke risk even when they looked only at women without high blood pressure.
As the authors point out, the reduction in stroke risk is “modest” and may not apply to all women. Previous studies have suggested that coffee consumption in women with high blood pressure may increase risk of stroke. Women who want to reduce their risk of stroke would be better off targeting known risk factors for stroke by stopping smoking, eating a healthy diet and taking exercise, rather than trying to drink more coffee.