New US study: Can coffee really help against dementia?

Two to three cups of coffee a day may reduce the risk of dementia. This is shown by the results of two extensive long-term US studies. We reveal what the US long-term studies show – and what they do not.

What is the truth? Two to three cups of coffee a day could reduce the risk of dementia – that is what two large-scale, long-term US studies suggest. Black tea might also have a protective effect. The catch: The observed effect is extremely small, and – even more importantly – the studies do not provide proof of a causal relationship, but merely a correlation. Whether the protection actually results from coffee consumption or from another, unaccounted-for contributing factor remains unclear.

The causes of Alzheimer’s disease and other forms of dementia are still only partially understood. However, it seems certain that a healthy lifestyle can reduce the risk of developing dementia. Certain components of coffee, as well as black and green tea, are also considered potential protective factors. Laboratory studies show that, for example, caffeine and some polyphenols can help reduce oxidative stress and inflammation. Furthermore, these bioactive substances may inhibit the formation of harmful amyloid and tau proteins.

Can coffee really help?

Whether these effects observed in the laboratory translate to everyday life is controversial. Previous studies have yielded conflicting results. “Among other things, the dose-response relationship between caffeine intake and dementia risk varies between studies: some indicate an increased risk at high doses of caffeine, while others show protective effects that stabilize at higher doses,” report Yu Zhang of Harvard University and his colleagues.

Another problem is that many of these studies only observed their participants over relatively short periods, so there is little information about long-term effects. To shed more light on this, Zhang’s team analyzed data from approximately 132,000 participants in two long-term US studies that have been running for over four decades. Using this health and lifestyle data, the researchers investigated whether coffee or tea consumption influences dementia risk and mental health.

“Neuroprotective effects of caffeine”

"Our results are consistent with previous studies that have observed neuroprotective effects of caffeine and coffee consumption,” the researchers write. It is suspected that caffeine or other bioactive ingredients could be responsible for these positive effects. This is supported by the finding that decaffeinated coffee did not affect dementia risk, comparable to that of caffeinated coffee.

The decrease in effectiveness at higher doses also seems plausible: “The absorption, transport, and metabolism of caffeine and other bioactive substances from coffee or tea have physiological limits,” explain Zhang and his team. Therefore, a larger quantity does not automatically lead to better results.

The effect is extremely small.

However, there are two significant limitations. First, “While our results are encouraging, it’s important to remember that the effect is small and there are many important ways to protect cognitive performance in old age,” emphasizes senior author Daniel Wang of Harvard Medical School. In tests of cognitive performance, the difference was only 0.02 percentage points—a barely perceptible value.

Secondly, this concerns the type of study: The evaluated long-term studies are based on observations and, in part, on the participants’ self-reported data. The subjects did not drink a fixed amount of coffee; instead, they reported their usual consumption. As a result, the participants differed not only in their coffee consumption but also in numerous other factors.

Other causes could be responsible.

This means that other, unaccounted-for factors could just as easily cause the positive effects. These might occur in coffee drinkers, but be independent of coffee consumption. “For example, people who drink moderate amounts of coffee or tea might lead a healthier, more balanced life overall,” explains physician Naveed Sattar from the University of Glasgow, who was not involved in the study. Sleep problems or other unaccounted-for factors could also influence the risk of dementia.

“The results should therefore be interpreted with caution. The fact remains that the most important protective factors for the brain remain unchanged: a healthy diet, physical activity, little alcohol, a healthy cardiovascular system, and regular mental stimulation,” says Sattar.

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