While some other studies have considered the impact of coffee drinking on life length, the relationship still remains unclear.
One reason for this is that some studies have found that coffee consumption may either increase longevity or have no effect (1,2,3), while other studies have indicated that coffee consumption may decrease longevity (4,5,6)
This study focused on specifically looking at this relationship and trying to find a clear answer.
The authors were also focused on looking at a non-linear relationship.
That type of relationship might mean that coffee consumption decreased the risk of death up to a certain cut-off (like 4 cups a day), and then after that point, the effect of coffee consumption on risk of death would change.
The Study Itself
The data collection projects were long-term. Because of this, the authors made use of data that represented more than 4.6 million person-years.
Coffee consumption was measured using a food frequency questionnaire that was conducted roughly every 4 years.
The questionnaire asked participants about their frequency of consumption for both caffeinated and decaffeinated coffee.
The authors used a range of approaches to determine the number of people from the study group who died, as well as when they died and the causes of death. In total, there were almost 32,000 deaths across the three groups of people (known as cohorts)
The information gathered was statistically analyzed to see what relationships existed.
Graphically, the connection between coffee and mortality looked like this:
The study used a common approach called risk ratio.
In this case, the risk was being compared to people who did not drink coffee. So, people who didn’t drink coffee were a 1 in terms of risk.
The groups that scored below a 1 (which were those drinking less than 5 cups of coffee per day) had a decreased risk of death than people who did not drink coffee.
As you can see from the graph, people drinking between 1.1 cups and 3 cups of coffee per day had the lowest risk.
This general relationship also remained true when looking at just people who had never smoked. That suggests that the observed results are not the cause of the connection between smoking and coffee.
The authors also looked at coffee consumption and specific causes of death.
They found that there was a connection between coffee consumption and decreased risk of death from heart disease and type 2 diabetes.
Coffee was also connected to an increased risk of respiratory diseases and lung cancer, but this was not true when only people who had never smoked were considered.
Implications and Limitations
This is not the first study to suggest that coffee might help to extend the lifespan, but it is probably the most comprehensive study conducted to date.
The study has a number of key strengths.
For one thing, it uses a very large sample size, with a long follow-up period. There were also a significant number of deaths throughout the data.
These factors helped to ensure that the study had enough statistical power to detect any effects that were present.
The study also focused on trying to distinguish between the effects of smoking and coffee.
That is a really important approach, because there is a strong connection between coffee and smoking, and that does have the potential to bias any potential outcomes.
However, research into nutrition and lifestyle does tend to be a challenging field.
That information is interesting, because it highlights how interconnected our lifestyle habits are. That interconnectedness poses a major problem for research. This is also why Science Alert says that nutrition research is incredibly difficult.
Because of this, one of the biggest limitations of the study was its observation nature.
Observational studies can be powerful because they can look at so much data, as this one did.
However, they are not experimental and they don’t have the ability to detect cause and effect. In theory, this means that the relationship between coffee and mortality might not be caused by coffee itself, but by some other external factor – like is the case for the spurious correlations graphs by Tyler Vigen.
The use of a survey to determine coffee consumption does also introduce some potential for error. However, the survey the authors used had been validated and used in many cases, so this is unlikely to have had a strong overall effect.
At the end of the day, this was a strong observational study suggesting that having less than 5 cups of coffee per day may decrease the risk of death.
The outcomes of the study are yet another indication that coffee really can be part of a healthy lifestyle.
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