Also, if the women drank coffee just before their assessments, the alerting effect may have helped them score better.
Another possible explanation for the findings is that women may have cut back on their coffee just before they enrolled in the study for reasons linked to incipient dementia, also known as mild cognitive impairment. For example, incipient Lewy body dementia can lead to sleep disturbances as its first symptom even before the dementia becomes apparent; so people with symptoms might stop coffee to help sleep better.
What else should we take into account?
Observational studies such as this are not the gold standard. To really assess coffee’s effect on cognitive decline, we would need a planned randomised controlled trial where women are allocated to caffeine or decaffeinated intake and followed for some years. The women and raters would need to be blind as to which group they would be in. Clearly this would be very difficult if not impossible, especially in our café society.
What should readers do? Caffeine is perhaps the most widely used addictive substance in the world and appears safe. People have different reactions to caffeine that may vary with age and health. Some people become more anxious, others find it can improve their performance. I find that as I have grown older, my sleep is more sensitive to caffeine.
There are things you can do that may reduce your risk of dementia, such as eating healthy food, preferably based on the Mediterranean diet of lots of vegetables and fruit and very little saturated fat, and staying physically fit and mentally active.
Don’t get too hung up on this kind of research. Let’s see more evidence over time. – Henry Brodaty
This is an interesting study but I agree there are major issues with its methods and conclusions. It is unclear exactly how caffeine intake was assessed. The paper states caffeine intake was self-reported using a questionnaire asking about coffee, tea, and cola beverages, but it did not specify whether drinks contained caffeine or not. Hence researchers assumed it was all caffeinated.
It is also disappointing the women were only split into two groups: those who drank more coffee, and those who drank less than average. There is a reasonable chance of misclassification bias, meaning some people in the lower caffeine intake group should really be in the upper group, due to limitations in the assessment of caffeine intake. You normally address this by splitting participants into more than two groups, and often four or five.
It is very interesting that those in the highest caffeine intake group were also less likely to have diabetes at baseline. While this fits with a major review of the relationship between risk of type 2 diabetes and coffee consumption, it’s also possible there is some remaining confounding bias due to better overall health of those with higher caffeine intakes that is not accounted for.
I agree that further longitudinal analyses would be of value, especially if they repeated the measure of coffee and other caffeinated beverages, particularly decaffeinated coffee, over a number of time points. It would be even more interesting to look at results where people changed their intakes over time. – Clare Collins