Researchers followed more than 131,000 nurses and healthcare workers for up to 43 years. During that time, about 11,000 participants developed dementia.
The study found that people who regularly drank moderate amounts of caffeinated coffee or tea were less likely to develop the condition.
The strongest results were seen in adults aged 75 and below. Scientists found that consuming around 250mg to 300mg of caffeine per day, equal to about two to three cups of coffee, was linked to a 35% lower risk of dementia.
However, researchers said drinking more caffeine than that did not provide extra protection for the brain.
Scientists believe caffeine may help by blocking a brain chemical called adenosine, which slows down important brain activity. Caffeine may also reduce inflammation and help regulate blood sugar, both of which are linked to brain health.
The study also found that drinking one to two cups of tea daily may help protect the brain. Researchers noted that tea contains antioxidants and other compounds that could support healthy blood vessels and brain function.
At the same time, experts warned that too much caffeine may have negative effects. High caffeine intake can interfere with sleep and increase anxiety, which may harm brain health over time.
Researchers also observed that people who drank more decaffeinated coffee experienced faster memory decline. They suggested this may be because some people switched to decaf after developing health problems already linked to dementia risk.
Although the study focused on healthcare professionals, researchers said similar findings appeared in 38 other studies they reviewed.
Overall, the evidence suggests that moderate caffeine intake, especially from coffee or tea, may help support brain health as people age.
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