Study details and key findings

For the study, researchers analyzed data from 86,606 women who participated in the National Health Service (NHS) survey from 1980 to 2023 and 45,215 men who participated in the Health Professionals Follow-up Study (NPFS) from 1986 to 2023. At baseline, NHS participants had a mean age of 46.2 years while NPFS participants had a mean age of 53.8 years. The participants completed food frequency questionnaires every two to four years.

During the follow-up period, 11,033 participants were diagnosed with dementia, which was confirmed by either physician diagnoses or death records. Overall, the researchers found that people who drank two to three cups of caffeinated coffee a day had an 18% lower risk of dementia compared to those who didn't drink coffee.

There was also a similar pattern with participants who regularly drank tea. Participants who reported moderate tea intake (one to two cups a day) had a 16% lower risk of dementia than those who didn't drink tea.

Notably, caffeine may play an important role in this risk reduction. Decaffeinated coffee was not associated with a reduced risk of dementia. Higher coffee intake was also not associated with reduced risk.

The researchers also found that the likelihood of subjective cognitive decline was lower among moderate coffee and tea drinkers than those who didn't drink either coffee or tea. In the NHS cohort, objective cognitive testing also found small but measurable improvements. The difference between the highest and lowest caffeine intake groups was 0.11 points, which corresponds to roughly 0.6 years of cognitive aging.

"From a clinical standpoint, a small shift in mean performance doesn't translate to an immediate, noticeable change for an individual patient," said Yu Zhang, the study's lead author from Mass General Brigham. "However, at the population level, even modest differences, if sustained, could be meaningful when dementia is common and develops over long periods."

Separately, Kellyann Niotis, a preventive neurologist at Weill Cornell Medicine, said it's difficult to determine whether the benefits of coffee or tea are due to their caffeine content or other compounds.

"It is really a big challenge to disentangle the effects of coffee as a whole from all of the other compounds that are also in coffee — and tea," Niotis said. "It's definitely possible that these other compounds are interacting with each other, or synergistically with the caffeine itself, to exert some benefit."

Aside from caffeine, coffee also contains "hundreds of bioactive compounds that influence inflammation, glucose metabolism, vascular function, and oxidative stress," said Sara Mahdavi, adjunct professor in the department of nutritional sciences at the University of Toronto.

Although coffee and tea may have some health benefits, Courtney Kloske, director for scientific engagement at the Alzheimer's Association, emphasized that people should focus on having a balanced and healthy diet instead of just one drink or ingredient if they want to protect their brain health.

"While the modest cognitive benefits for higher levels of caffeine consumption are intriguing findings, more research is needed to better understand the underlying mechanisms and connections between caffeine and cognition," Kloske said. "Developing healthier habits can improve overall wellness and protect memory and thinking as we get older."