The causes of Alzheimer’s and other dementias are only partially understood. However, it seems clear that a healthy lifestyle can reduce the risk of dementia. However, it is controversial whether coffee and black or green tea are included, and studies are contradictory. Now an evaluation of data from two long-term studies with a total of more than 130,000 participants provides new evidence. Accordingly, drinking two to three cups of caffeinated coffee or tea could reduce the risk of dementia and slow down cognitive decline. However: The effect is only small and the studies only show a correlation, not a causal connection. Whether coffee and tea or another lifestyle-related factor is really responsible remains an open question.
There is currently no real cure for Alzheimer’s and other forms of dementia; current therapies can only slow down mental decline, not stop or reverse it. The causes have only been partially clarified so far. Nevertheless, studies suggest that lifestyle and diet also play a role and can at least delay and possibly even prevent the onset of dementia. But so far it is only partially known which lifestyle factors are crucial. In addition to nutrition, plant ingredients, such as those found in coffee or black tea, are also considered possible prevention aids. “Coffee contains bioactive ingredients such as polyphenols and caffeine that may have neuroprotective effects by reducing oxidative stress and neurological inflammation,” explain Yu Zhang from Harvard University in Boston and his colleagues. Laboratory studies suggest that caffeine can slow down the accumulation of harmful amyloid and tau proteins in brain tissue.
Two to three cups of coffee are optimal
However, it is unclear whether these laboratory effects can be transferred to everyday life. Previous studies have produced contradictory results. “Among other things, the dose-response relationship between caffeine intake and dementia risk varies between studies: some indicate an increased risk with high doses of caffeine, while others show protective effects that stabilize at higher doses,” report Zhang and his colleagues. In addition, many previous studies only examined their participants over a relatively short period of time, so there was hardly any data about long-term effects. That’s why researchers have now evaluated the data from two long-term studies that have been running for over 40 years, the Nurses’ Health Study (NHS) and the Health Professionals Follow-Up Study (HPFS). These include the medical background and lifestyle factors of nurses and other healthcare workers in the United States. Zhang’s team used data from a total of 131,821 participants to examine whether caffeinated coffee, tea and decaffeinated coffee affected individuals’ risk of dementia and cognitive health.
The evaluations provided evidence of an effect of caffeinated coffee or tea: “After adjusting for potentially confounding effects of other risk factors, higher caffeine intake was significantly associated with a reduced risk of dementia,” reports the team. Calculated per 100,000 people, 141 heavy coffee drinkers developed dementia over the course of the study period, while there were 330 cases among those who drank little caffeine. The participants with high caffeine consumption also achieved a slightly higher score in tests of their mental performance. However, with a difference of only 0.02 percentage points, this was not significant, as Zhang and his colleagues report. Further analysis showed that there appears to be a satiety effect in the daily amount of coffee and tea: The cognitive benefits were most pronounced in participants who consumed two to three cups of caffeinated coffee or one to two cups of tea daily. However, a higher dose did not improve the effect any further.
Positive effect with important limitations
“Our results are consistent with previous studies that have also observed neuroprotective effects of caffeine and coffee consumption,” write the researchers. Since decaffeinated coffee did not show the effects they found, they believe that caffeine or other ingredients removed during decaffeination are responsible for these effects. The leveling off of the effect at higher doses is also plausible, explain Zhang and his team. The absorption, transport and metabolism of caffeine and other bioactive substances from coffee or tea have physiological limits. Therefore, more does not automatically help more.
However, there are two key limitations. The first: “Although our results are encouraging, it is important to remember that the effect is small and there are many important ways to protect cognitive performance as we age,” emphasizes senior author Daniel Wang from Brigham and Women’s Hospital and Harvard Medical School. “However, our study suggests that consumption of caffeinated coffee or tea may be one piece of the puzzle.” The second, important limitation concerns the type of study: the long-term studies evaluated are observational studies, some of which are based on the participants’ self-reports. This type of study can reveal two correlations, but cannot prove causal relationships. This is also pointed out by the doctor Naveed Sattar from the University of Glasgow, who was not involved in the study.
Therefore, there could be unidentified factors that are responsible for this effect and that coincidentally coincide with moderate coffee consumption. “For example, people who drink moderate amounts of coffee or tea might lead a healthier, more balanced life overall – and this, not the coffee itself, then contributes to their better brain health.” Sleep disorders or other factors that are not recorded can also be behind it. “The results should therefore be viewed with reservations. The fact is that the most important protective factors for the brain remain unchanged: a healthy diet, physical exercise, little alcohol, a healthy cardiovascular system and regular mental stimulation,” says Sattar.
Source: Yu Zhang (Harvard University, Boston) et al., JAMA, doi: 10.1001/jama.2025.27259