The actual study (1) is observational and makes no causal claim, only that there exists a statistical association between caffeine consumption and dementia. Nevertheless, people are apt to misinterpret the finding as “caffeine consumption prevents dementia”:

Caffeine -> Dementia

However, the two variables would be correlated if the causal arrow were reversed and dementia influenced the propensity to consume caffeine:

Caffeine <- Dementia

And we would also observe the correlation if a person's general health influenced both the propensity to consume caffeine and dementia risk:

Caffeine <- General Health -> Dementia

Since caffeine is a stressor, we would expect to see reduced consumption among people with reduced general health. But we would also expect increased dementia among that same group. So the relationships in the diagram immediately above are plausible and would give rise to a spurious correlation between caffeine consumption and dementia risk.

While studies can try to “control for confounding factors,” it’s easy to overlook or misunderstand the true causal relationships in play, causing spurious correlations. In other words, you can create false “causal” relationships through imperfect identification and control of confounding variables.

In short, take this article’s claims with a suitable dose of suspicion.

(1) https://jamanetwork.com/journals/jama/article-abstract/28447...