Two to Three Cups of Coffee a Day Linked to 35% Lower Dementia Risk in Major Mass General Brigham Study

Drinking two to three cups of caffeinated coffee a day is associated with a 35 percent lower risk of developing dementia, according to a new analysis of more than 130,000 American and Korean adults followed for as long as 43 years, the largest and longest-running study of its kind. The findings, published this week in the journal Alzheimer's & Dementia and summarized Tuesday by ScienceDaily, also point to slower cognitive decline and better preserved memory and executive function in habitual coffee and tea drinkers compared with people who consumed little or no caffeine.

The Mass General Brigham research team, led by Dr. Lloyd Tran of the Brigham and Women's Hospital Channing Division of Network Medicine, drew on three large cohort studies — the Nurses' Health Study, the Health Professionals Follow-Up Study, and the Korean Genome and Epidemiology Study — to assemble a population of 130,000 participants who repeatedly reported on their dietary habits, cognitive symptoms and clinical outcomes for up to four decades. During follow-up, 11,033 participants developed dementia. Compared with people who drank less than half a cup of coffee a day, those who drank two to three cups had a 35 percent lower hazard of any dementia and an 18 percent lower hazard of clinically diagnosed Alzheimer's disease.

The effect was strongest in middle adulthood — between roughly age 45 and 65 — and tapered off in participants older than 75, suggesting that the neuroprotective payoff comes from years of accumulated exposure rather than from short-term consumption late in life. The team also examined caffeinated tea and found a smaller but statistically significant benefit at one to two cups per day. Decaffeinated coffee and sodas containing caffeine showed no such association, suggesting that the bioactive ingredients responsible are most concentrated in caffeinated brewed beverages.

The researchers say several mechanisms may be at work. Caffeine itself appears to dampen inflammatory signaling in the brain and may modulate adenosine receptors implicated in the deposition of beta-amyloid plaques. Coffee is also one of the richest dietary sources of polyphenols, plant compounds that reduce oxidative damage in neurons. Animal studies cited by the authors have shown that long-term caffeine exposure preserves the integrity of the blood-brain barrier and slows the spread of toxic tau protein. "The convergence of human, animal and biochemical evidence is now hard to ignore," Tran said in a Mass General Brigham press release.

The team cautioned that the study is observational and cannot prove causation, and that excessive caffeine carries real risks, including high blood pressure, sleep disruption and anxiety. The American Heart Association advises adults to limit themselves to roughly 400 milligrams a day, which corresponds to about four 8-ounce cups of brewed coffee. Researchers are now planning a randomized trial — funded by the National Institute on Aging — that will give 6,000 adults aged 50 to 70 either a daily caffeine supplement or a placebo for five years to measure cognitive change and biomarker progression directly. Enrollment is scheduled to begin in early 2027 at sites in Boston, Atlanta and Seoul.