Filtering Through the Coffee Studies

Lopez-Garcia acknowledges this lack of a one-size-fits-all diet, saying that "people with hypertension, insomnia, and anxiety should ask their doctors before starting drinking coffee."

There may be little fault in this study's findings -- perhaps because they have been found in many other studies -- but even this study has its own potential problems.

"One cannot discount the possibility of a bias known as 'reverse causation,'" said Charles Poole, an epidemiologist at the University of North Carolina School of Public Health. "Sick people may be avoiding or cutting down on coffee. This would make coffee look protective."

The lesson is that each study, no matter how well-designed, will have its limitations.

"Epidemiology is attractive to the general public and the media because its findings are directly relevant to free living humans," said Dr. Charles Hennekens, a professor at Florida Atlantic University, who first looked at coffee's impact on health in the 1970s. "It is also true, however, that epidemiology is crude and inexact, as observations of free living humans can never take place under the controlled conditions in a laboratory."

Repeated findings over multiple studies are the key to making decisions about diets, because of the flaws any individual study will have.

"No one study is ever the definitive word on anything," said Blake.

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