May 16, 2012

coffee intake and risk of death

Title, Date of Publication & Journal

‘Association of Coffee Drinking with Total and Cause-Specific Mortality’, by Neal D. Freedman et al. published 16th May 2012 in New England Journal of Medicine.

Claim supported by evidence?

The paper supports the claim that reduced death rate was seen with increasing coffee intake, but notes that a causal relationship cannot be determined.

Summary

Large, well-designed and well-conducted observational study
A statistically significant inverse association was observed between coffee intake and death rate. Further studies would be needed to investigate any causal relationship.

Study Conclusions

The authors conclude that people with an increased level of coffee consumption have a lowered risk of death. This conclusion is strongly supported by the study results.

The observational nature of the study means that a causal relationship between coffee consumption and mortality cannot be established. In other words, we cannot conclude that increased consumption of coffee had an effect on the death rate. This limitation of the study was recognised and clearly stated by the authors.

However the biological plausibility of coffee intake affecting “deaths from all causes”, which includes deaths due to injuries and accidents, is not discussed.

Strengths/Limitations

Strengths:

A relatively large sample size (more than 400,000 subjects and 52,000 deaths) means that the study is well powered to detect differences in death rate
The analysis appears to have been properly conducted, and appropriate statistical methods have been used
The results provide convincing evidence that those patients with higher coffee intake also had a reduced death rate compared to patients who were coffee-free

Limitations:

There may have been confounding factors involved. Baseline characteristics were adjusted for in the analysis, but these were limited to those collected in the HIH-AARP Diet and Health Study, which were predominantly dietary and health factors; there may be other confounders (e.g. socioeconomic) at work that were not accounted for because the data was not available.
The study results showed that increased coffee intake was associated with reduction in the rate of all manners of death – including accidents – which is biologically implausible. This is suggestive of confounding factors.
Coffee consumption was only recorded at the start of the study. It would have been better to collect coffee consumption habits from each subject several times over the course of the study.
The overall linear relationship between coffee consumption and death rate was not quantified, e.g. the following type of statement would have been helpful: “an increase in coffee consumption of one cup per day was associated with a decrease in death rate of x%”
A randomised trial would be a preferable study design to provide evidence of causality between coffee (or its active ingredients) and mortality

Glossary

Observational study: A study where the assignment of subjects into a treated group versus a control group is outside the control of the investigator
Randomised trial: A study where each subject is randomly assigned to a treated group or a control group before the start of the treatment.

Any specific expertise relevant to studied paper (beyond statistical)?

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