“Two cups of coffee a day can halve risk of suicide,” The Daily Telegraph reports, while the Daily Mail suggests that coffee may also help prevent depression.
These reports are based on a study that combined the results of three large health and lifestyle studies of US health professionals. The results were then examined to see whether there is an association between coffee consumption and risk of suicide.
The main finding that captured the media’s imagination was people who drank more than two or three cups of coffee per day had decreased risk of suicide compared to people drinking less than one cup a week.
However, there are a number of limitations to the study that need addressing.
Suicide was rare in the groups studied – accounting for 0.1% of the total study population. And when further splitting these suicides up according to reported coffee consumption, the numbers became even smaller.
Also, any study relying on small numbers has a high possibility that any associations found will be due to chance.
In addition, there is the possibility that coffee consumption is not directly reducing risk of suicide but that any link is being influenced by other unmeasured confounding factors. One example, given by the researchers, is that people who feel anxious may avoid drinking coffee as it makes their symptoms worse. So the apparent association between coffee and mental health outcomes could be a “symptom” rather than a “cause”.
Overall the findings do not support a recommendation to increase coffee consumption in an effort to benefit mental health.
Where did the story come from?
The study was carried out by researchers from Harvard School of Public Health, Boston.
The three cohorts in this study were all funded by the US National Institutes of Health (though the analysis of data used in this particular study has received no direct funding).
The study was published in the peer-reviewed journal The World Journal of Biological Psychiatry.
Overall, the media has exaggerated the findings from this study.
The study has analysed data collected from three large cohorts, which were not set up to examine the effects of coffee consumption on risk of suicide. The results have numerous limitations meaning that we cannot have confidence that there is any direct association.
What kind of research was this?
This study combined data from three large US cohort studies examining the association between coffee and caffeine consumption and suicide risk.
The researchers say that coffee is widely used as a stimulant to reduce fatigue and improve vigilance and performance. The effects of caffeine upon neurotransmitters in the brain, such as serotonin, have led to speculation that caffeine could have antidepressant effects.
Previous research has observed depression and suicide rates decline with increasing consumption of caffeinated coffee.
The current research combined data from the three cohorts to examine this supposed association in greater detail.
Limitations to a study such as this include:
- the possibilities of inaccurate recall of coffee consumption
- the potential of confounding from various health, lifestyle and socioeconomic factors that could be involved
- the low number of suicides that occur, which increases the risk that any associations may be due to chance
Also, though the research has combined the results of three US cohorts, several other research studies have investigated if there is an association between coffee consumption and mental illness. So perhaps a systematic review combining the findings of all observational research would have been a preferable study design.
What did the research involve?
The researchers combined data from three cohorts:
- the Health Professionals Follow-up Study (HPFS), which recruited 51,529 male US health professionals aged 40 to 75 years in 1986
- the Nurses’ Health Study (NHS), which recruited 121,700 female US registered nurses aged 30 to 55 years in 1976
- the Nurses’ Health Study-II (NHS II), which recruited 116,671 female US registered nurses aged 25 to 42 years in 1989
People in all three studies were followed up with health and lifestyle questionnaires every two years (including diet questions every four years). They excluded people with cardiovascular disease or cancer at baseline. After exclusions, data from 43,599 HPFS, 73,820 NHS and 91,005 NHS II participants were available for analysis.
Similar food frequency questionnaires were used in the three studies. They included questions on coffee (“coffee with caffeine” and “decaffeinated coffee”), tea (non-herbal), carbonated soft drinks (with or without caffeine), and chocolate. They were asked how often they drank a specified amount of the drink (such as one cup or one glass) with nine response options ranging from never, to six or more per day.
The researchers said they assumed that caffeine content in one cup of coffee was 137mg.
Deaths were identified by searching the National Death Index, and 98% of all deaths in the study participants could be identified. The outcome of interest was deaths coded as due to suicide or self-inflicted injury.
The researchers examined the association between caffeinated and decaffeinated coffee intake, tea intake and risk of suicide. They left at least a two year interval between assessment of coffee intake and suicide outcomes but only going up to four years after assessment (for example, intake from 1980 to 1994 used to predict suicide in 1996-98 and 1998-2000). The researchers took into account potential confounders of:
- smoking status (and quantity if currently smoking)
- alcohol consumption (daily amount)
- body mass index (BMI)
- physical activity levels
- marital status
- self-reported use of antidepressants and tranquillisers
- in women, menopausal status and use of HRT or oral contraceptives
What were the basic results?
Average daily caffeine consumption was 186mg for men in the HPFS, 218mg for people in the NHS, and 169mg for people in the NHS II study. The most frequent coffee drinkers (four or more cups a day) were more likely than rare coffee drinkers (less than one cup a week) to be smokers, to drink more alcohol and to be less likely to report being in a marriage/partnership.
There were 277 deaths from suicide among the total 208,424 participants (0.1%):
- 164 in HPFS, a rate of 20.6 per 100,000 person years (meaning roughly 21 if you follow 10,000 people for 10 years)
- 47 in NHS, a rate of 4.2 per 100,000 person years
- 66 in NHS II, a rate of 5.3 per 100,000 person years
When looking at the pooled results for the three studies, with full adjustment for all measured confounders, compared to drinking less than one cup of caffeinated coffee per week:
- people who drank two to six cups per week had no difference in risk
- people who drank one cup a day had no difference in risk
- people who drank two to three cups a day were at 45% reduced risk of suicide (relative risk 0.55, 95% confidence interval (CI) 0.38 to 0.78)
- people who drank four or more cups a day were at 53% reduced risk of suicide (relative risk 0.47, 95% CI 0.27 to 0.81)
Though there was a trend for decreased risk with increased coffee consumption after two to three cups, the researchers did not find that each further two cup increase in the amount of coffee consumed each day had any extra significant association with risk of suicide.
Decaffeinated coffee or tea consumption was not associated with suicide risk.
How did the researchers interpret the results?
The researchers conclude that the results from the three cohorts “support an association between caffeine consumption and lower risk of suicide”.
This research used data collected from three large health and lifestyle studies of US health professionals to examine whether there is an association between coffee consumption and risk of suicide.
Though it found that people who drank more than two to three cups of coffee per day had decreased risk of suicide compared to people drinking less than one cup a week, there are several important limitations to this study meaning that it does not provide evidence that drinking more coffee is beneficial to mental health.
- Even when combining the results from three large studies, the numbers of suicides is, as would be expected, very low. Only 0.1% of the entire cohort populations committed suicide. When further splitting these suicides up according to reported coffee consumption, the numbers become very small. For example in the NHS study, only eight of the people who committed suicide drank two to three cups a day and four drank more than four or more, compared with 16 who drank less than one a week. When conducting statistical analyses using such small numbers there is a high possibility there was not a true link, and that any significant associations occurred only due to chance.
- Though the study tried to adjust for several other health and lifestyle factors, there is the possibility that any association between the two is being influenced by other factors and that caffeinated coffee does not have a direct effect on your risk of suicide.
- Though the media implied by association that coffee reduces your risk of depression, the study has not actually assessed the presence of any type of mental illness (other than questioning the use of antidepressants and tranquillisers).
- A reliable method was used to assess outcomes of suicide. However, coffee consumption was assessed by self-report to questions on the amount of coffee consumed. This may introduce inaccuracies, as cup size and strength and type of coffee is likely to vary from person to person.
- The three studies were all conducted in health professionals. Therefore we cannot assume that the findings from this specific group would apply to all people from the general population.
- Whatever effects excess caffeine may or may not have on your general feeling of wellbeing, caffeine is a stimulant and excess amounts can have various unpleasant effects, such as causing trembling, increased breathing rate and heart rate and difficulty relaxing or sleeping. There is also the risk of craving and withdrawal symptoms such as headaches when the person goes without caffeine.
Overall, the findings of this study do not prove that coffee is beneficial to mental health.
If you are troubled by a persistent feeling of low mood and hopelessness and you no longer take pleasure in activities you used to enjoy, you should speak to your GP as soon as possible.
There are also a number of mental health advice lines you can contact for help.