The women were all free of dementia when they joined the study between 1995 and 1999. Their cognition – memory and other thinking abilities – was assessed annually in person, until 2007, and then by telephone. For women who showed evidence of cognitive decline over subsequent years, more information was obtained from someone who knew the woman well.
A panel of specialist physicians who reviewed all the information agreed on whether women had developed probable dementia. Caffeine intake, which included tea, cola and other sources of caffeine, was based on questionnaires the women completed.
To isolate the effect of caffeine, the analyses accounted for other reasons that might affect rates of dementia. These were age, education, use of HRT, weight and height, sleep, blood pressure, heart disease, diabetes, smoking, alcohol use and how well the women performed cognitively at their first visit.
What were the results?
Of the 6,467 women in WHIMS, 209 developed dementia and 388 developed some cognitive impairment. Greater levels of caffeine intake were associated with a lower incidence of dementia or cognitive impairment.
Researchers divided the women into two halves – the first half contained those who consumed higher amounts of and the second, lower. The average amount of caffeine intake in the lower group was 64 mg per day (roughly under one standard cup of coffee); while in the higher group, it was 261 mg (roughly over three cups).
The women in the upper group had 26% (not 36% as reported by the Daily Mail) less chance of developing dementia than those in the lower. This is a statistically significant difference. But when the women’s level of cognition at recruitment was taken into account, the risk reduction was only 20%, which was no longer statistically significant.
The researchers concede several reasons for caution. For a start, this study only looked at older women. They mention another study from France that found coffee had a protective effect on women but not men. And a third study found European men who drank three cups of coffee per day had the lowest rate of cognitive decline over ten years.
Why else should we be sceptical?
The women in this study weren’t representative of women generally. They were better educated than average and just the fact they had survived to 65 to 80 years at the time they entered the study, and then lived even longer to allow follow-up, means they may have been a healthier group. This is called the survivor bias, which can lead to false conclusions.
Nor was there a clear dose-response outlined with the number of cups of caffeinated coffee per day. This means the actual amount of caffeine was not measured and blood levels were not checked. Plus, people make their coffee at different strengths, and because the basis for the analysis is what the women reported, their ideas around caffeine intake measurements could be unreliable.
Further, the diagnoses of dementia were not based on a clinical assessment. Telephone assessments are more prone to error and this introduces some noise.