Press reports have highlighted a study published in the New
England Journal of Medicine on May 17 on the relationship between coffee
drinking and mortality (Freedman et al, Association
of Coffee Drinking with Total and Cause-Specific Mortality. N Eng J Med
2012;366:1891-1904). This study involved
an analysis of data from the NIH-AARP Diet and Health Study. Between 1995 and 1996 over 600,000 AARP
members completed a comprehensive questionnaire assessing diet and
lifestyle. In this study responses to
that questionnaire were correlated with follow-up data – most specifically
death and cause of death – in order to address the association between coffee
drinking and mortality.
The results were quite interesting. Overall men or women who reported drinking four
or more cups of coffee per day, after taking into account age, had a rate of
death which was statistically greater that those who drank no coffee. For example, men who drank 6 or more cups of coffee
per day, after taking into account age, had a rate of death 60% greater than
those who drank no coffee.
However, coffee drinking was found to be associated with
other behaviors which in turn have been associated with increased mortality such as
cigarette smoking, high level of alcohol consumption and red meat consumption. Coffee drinkers also tended to have a lower
level of education, exercised less, and had a lower intake of fruits,
vegetables and white meat.
The investigators statistically adjusted the results to account
for such other factors. This type of
statistical adjustment attempts to correct for these other factors so as to be
able to measure the specific effect of coffee drinking alone on mortality. After this statistical adjustment the results
were quite different. Men who drank one
or more cups of coffee per day, and women who drank two or more cups of coffee
per day, had an adjusted rate of
death which was statistically lower that those who drank no coffee. For example, men who drank 6 or more cups of coffee
per day had an adjusted rate of death
10% lower than those who drank no coffee.
Women who drank 6 or more cups of coffee per day had an adjusted rate of death 15% lower than
those who drank no coffee.
Interestingly, the reduction in the adjusted rate of death
was found to persist over a wide range of causes of death. For example, in both men and women who drank
coffee there was a significant reduction in adjusted rates of death due to heart
disease, respiratory disease, stroke, injuries and accidents, and diabetes. In men there was also a significantly lower adjusted
rate of death due to infection. The one
exception to the pattern was that men who drank coffee had a higher adjusted rate
of death due to cancer.
In summary, after taking other factors into consideration,
these investigators found that adjusted overall death rates were statistically
significantly reduced in men and women who drink coffee; this reduction applied
to a wide range of specific causes of death.
Does this mean that drinking coffee will extend your life? No, not necessarily. As the investigators point out this type of
study cannot differentiate between correlation and causation.
Of course, one possible explanation of the results is that
coffee drinking actually does prolong one’s life. Coffee contains thousands of compounds that
could affect various metabolic processes in a beneficial (or non-beneficial) manner.
Conversely, it could be that individuals with physiological
or behavioral characteristics associated with a longer life span, but which
were not specifically adjusted for in this study, are more likely to be coffee
drinkers.
Or perhaps people, who are more likely to die, avoid drinking
coffee. Maybe coffee is more likely to
make such individuals nauseated. This
explanation is perhaps unlikely in this study because coffee seemed to reduce
death due to such a wide range of causes and many of the deaths occurred many
years after the individuals reported their coffee drinking habits.
Furthermore, while the authors used standard statistical methods
to adjust the death rates to compensate for other factors, the statistical
methods assume a specific mathematical form for the interaction between the
various factors. This mathematical model
might not be applicable and thus the statistical adjustment might produce incorrect
results.
Finally, for a study to be considered completely prospective
all decisions regarding the data analysis have to be made ahead of time in the
complete absence of any knowledge – direct or indirect - of the data collected
in the study. If knowledge of such data
informed any decisions regarding how the data were to be analyzed, the study
would be considered a retrospective analysis which is far less reliable. For example, in this study approximately one
third of the individuals who provided completed questionnaires were excluded
from the analysis for various reasons. From
a statistical perspective, the decisions on exactly which groups to exclude
needed to have been made with absolutely no knowledge of the study data. The selection of which possibly confounding
factors were to be adjusted for would also have to be made similarly with no
knowledge – direct or indirect – of the study data. While there is no evidence in this study to
suggest that any decisions were at all informed by prior knowledge of study data,
it is often very difficult in the process of analyzing data to prevent such
knowledge from, even subconsciously, affecting seemingly minor decisions
regarding data analysis which can end up affecting results.
This study does provide very intriguing results. However, as the investigators state this study
by design could at best demonstrate correlation and not causation. Furthermore, since the beneficial effect of
coffee drinking on survival emerged only after a set of statistical adjustments
were applied, one has to question whether the reported effect of coffee
drinking on survival could be an artifact of the statistical adjustment process
itself.
The study does suggest a hypothesis that coffee drinking
reduces mortality. This hypothesis could
be tested in a prospective randomized interventional trial in which individuals
are randomly assigned to drink or not drink coffee over a prolonged period. This undoubtedly would be a difficult trial
to conduct, but only such a trial could reliably determine whether coffee
drinking actually reduces mortality.
Such prospective randomized interventional clinical trials are the gold standard in
medicine for testing hypotheses regarding new treatments and new diagnostic
methods. It might be worthwhile for Starbucks to fund such a study.
In the meantime, I will continue drinking coffee because it
tastes good and is not fattening.
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