A study found that drinking 2 to 3 cups of coffee a day or 1 to 2 cups of tea reduces the risk of dementia. The amount of caffeine in that much coffee and tea slows cognitive decline and preserves brain function.
A research team led by Daniel Wang of Harvard Medical School said in JAMA on the 9th that "in a more than 40-year longitudinal follow-up, people who consumed a moderate amount of caffeinated beverages had a lower risk of developing dementia than those who consumed little to no caffeine."
◇ Coffee lowers dementia risk by 18%
The study was conducted by researchers at Massachusetts General Brigham Hospital under Harvard Medical School, Harvard T.H. Chan School of Public Health, and the Broad Institute. The analysis covered 131,821 participants in the Nurses' Health Study for women and the Health Professionals Follow-Up Study for men.
Of the participants, who were in their mid-40s to early 50s when the study began, just over 8%, or 11,033, developed dementia during the study period. The analysis found that participants who consumed more caffeinated coffee had an 18% lower risk of dementia than those who consumed little or no caffeinated coffee. Decaffeinated coffee did not show that effect. People who drank at least one cup of caffeinated tea a day also had a 15% lower risk of developing dementia.
Dementia is an acquired disorder in which multiple cognitive functions, such as memory, language, and judgment, decline to the point that daily life is difficult. Alzheimer's disease, a degenerative brain disorder, causes senile dementia and accounts for two-thirds of patients. There is also vascular dementia caused by stroke. Although antibody therapies that clear amyloid beta protein, known as a cause of Alzheimer's, have recently emerged, they are not very effective after symptoms appear.
The researchers said the most certain way to prevent dementia is early prevention, so they examined how diet affects the onset of dementia. Aladdin Shadiab of the University of California, San Diego (UCSD) School of Public Health said, "This paper presents results from a large study rigorously conducted over a long period on both women and men, showing that drinking 2 to 3 cups of coffee a day is associated with a reduced risk of dementia."
The researchers also periodically assessed changes in cognitive function among more than 17,000 women age 70 and older. Subjective cognitive decline—feeling that memory and thinking have worsened—is often an early sign of dementia progression. Wang said, "Participants who consumed more caffeine scored higher on cognitive assessments for their age group, which suggests that their rate of cognitive decline slowed by about seven months."
Coffee and tea contain bioactive compounds such as polyphenols and caffeine. Previous studies found that these substances reduce inflammation and cellular damage and prevent cognitive decline, but follow-up periods were short and results inconsistent. This study, with long-term follow-up, overcame those limitations.
◇ The specific mechanism of action remains unclear
However, drinking more than 2.5 cups of coffee a day did not yield additional benefits in lowering dementia risk or enhancing cognitive function. Wang said, "It may be because the body cannot further process the bioactive compounds in coffee and tea beyond a certain amount," adding, "Further research is needed to verify the mechanism by which caffeine protects nerve cells."
Fang Fang Zhang of Tufts University's Friedman School of Nutrition Science and Policy also published findings last year on the association between caffeine intake through coffee and reduced mortality. She said, "That study also found no additional benefits beyond three cups of coffee," and noted, "Adding milk or sugar negates caffeine's mortality-reducing effect." The dementia study did not track milk or sugar intake.
Wang said this study found no negative effects from high caffeine intake. But Zhang of Tufts said, "Some research suggests that exceeding a moderate amount of caffeine can worsen sleep disorders or anxiety symptoms and harm health." She said people can try caffeine to reduce dementia risk, but if sensitive to caffeine, it is best to start with small amounts.
Tara Spires-Jones, a University of Edinburgh professor and former president of the British Neuroscience Association, said, "This is a well-conducted study analyzing data from a large population over many years, but it has limitations," adding, "Other factors related to tea and coffee consumption habits may have influenced the reduced risk of dementia."
Spires-Jones said, for example, that sleep disorders or cardiovascular disease increase dementia risk, and such conditions can influence choices about caffeine intake. That could be interpreted as a higher dementia risk due to not consuming caffeine. She also noted that people in the early stages of dementia may have difficulty accurately reporting caffeine intake. In other words, people who consumed caffeine but still developed dementia may have been excluded from the statistics.
References
JAMA (2026), DOI: https://doi.org/10.1001/jama.2025.27259
The Journal of Nutrition (2025), DOI: https://doi.org/10.1016/j.tjnut.2025.05.004