Drinking tea or coffee may help preserve brain function and reduce dementia risk, a large US study has suggested.

Researchers said those who had the highest daily intake of caffeinated coffee had an 18% lower risk of developing dementia compared to those with the lowest such intake.

People who drank the most tea appeared to have a 16% reduced risk.

The study, published in the Journal of the American Medical Association (JAMA), ​also found that those who consumed the most tea and coffee also showed a slower rate of cognitive decline compared to those who drank the least.

But decaffeinated coffee was not associated with lower dementia risk or better cognitive performance.

Experts from Harvard University examined data on 131,000 health workers in the US, who were tracked for an average of 43 years.

“We observed the most favourable associations were at moderate intake levels – the strongest associations were seen at about two to three cups per day of caffeinated coffee and about one to two cups per day of tea,” study leader Dr Daniel Wang of Harvard Medical School said ​in a statement.

But the study does not prove caffeine ‌helps protect the brain.

Researchers also concluded the effect of caffeine, if any, was small, and there are ‌other better ways to protect cognitive function as people age.

Lifestyle factors linked with lower risks of dementia include physical exercise, a healthy diet and adequate sleep, according to previous research.

Study has ‘important limitations’

Commenting on the study, Dr Susan Kohlhaas from Alzheimer’s Research UK, said the research “doesn’t prove that coffee or tea protect the brain”.

Professor Tara Spires-Jones, at the UK Dementia Research Institute, agreed there were “important limitations”, adding that “other factors related to coffee and tea-drinking habits could be responsible”.

Further research is needed to validate the factors and mechanisms responsible ‍for the findings, the researchers said.

They noted that bioactive ingredients in coffee and tea such as caffeine and polyphenols have emerged as possible factors that reduce nerve cell inflammation and damage while protecting against cognitive decline.

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Other experts offered a mixed, but broadly cautious, response.

Dr Mohammad Talaei, from Queen Mary University of London, said there was “a robust association” but warned that “reverse causation is a concern” and that dietary substitutions may complicate findings.

Reverse causation could apply in this case by creating an illusion that caffeine lowers dementia risk, when it is also possible that people with early cognitive decline associated with dementia may change their tea or coffee drinking habits, potentially reducing their caffeine intake.

Professor Jules Griffin, from the University of Aberdeen, described “compelling evidence” of lower risk but pointed out “the reduction in risk is quite a small decrease”, while Professor Naveed Sattar at the University of Glasgow said he would interpret the findings with “considerable caution”, pointing out they were “suggestive” only.

Meanwhile, Dr Simon White at the University of Cambridge stressed the study was not sufficient to recommend changing people’s behaviours.