Two to three cups of coffee or two cups of tea daily can lower the risk of dementia and preserve cognitive function during ageing, medical researchers reported on Monday, corroborating a protective effect first recognised 24 years ago.

The researchers in the US who analysed data from a large study underway for over four decades have found that both men and women with the highest daily caffeine consumption had an average 18 per cent lower risk of dementia.

Heavy tea drinkers also showed similar gains but those who drank decaffeinated coffee did not, a signal that the benefits emerged from caffeine, the researchers said in their study published in the Journal of the American Medical Association on Monday.

“Our results are encouraging, but it is important to remember that the size of the (protective) effect is small and there are other ways to protect cognitive function as we age,” said Dong Wang, assistant professor of medicine at the Harvard TH Chan School of Public Health, who led the study.

Neuroscience researchers at the faculty of medicine in Lisbon, Portugal, had first raised the question whether caffeine intake protects against Alzheimer’s disease in 2002 after a study involving 54 patients and published in the European Journal of
Neurology.

Multiple studies have since then explored this connection but the findings remained inconsistent. Some studies suggested increased risk at higher intake while other studies hinted at protective effects. Most of the earlier studies were limited by their short follow-up periods.

Now, Wang and his colleagues analysed caffeine consumption in over 1,31,821 participants — nurses and healthcare professionals — in the US who had volunteered for a study that has tracked their health for nearly 43 years. Among the participants, 11,033 developed dementia.

The cognitive benefits were most pronounced at “moderate intake levels” — in the participants who consumed two to three cups of caffeinated coffee or one or two cups of tea daily, the researchers said. There were no additional protective effects at higher intake levels.

Contrary to some earlier studies, however, the higher caffeine intake did not result in negative effects, instead, it provided a similar neuroprotective effect as the optimal dose of two to three cups.

“We also compared people with different genetic predispositions to developing dementia and saw the same results — coffee or caffeine is likely equally beneficial for people with high or low genetic risk of developing dementia,” said Yu Zhang, the study’s lead author, in a media release.

The researchers have said “multiple potential mechanisms” might explain caffeine’s neuroprotective effect. Caffeine may help protect the brain by reducing the buildup of harmful proteins and lowering inflammation, both of which can slow cognitive decline.

Coffee and tea also contain compounds called polyphenols, chlorogenic acid and catechins which, the researchers said, could protect the brain by lowering levels of a biological process called oxidative stress, improving blood flow in the brain and promoting resilience of nerve cells.