"The older people, over 55, were not affected by these high amounts of coffee," study co-author Dr. Chip Lavie, a cardiologist at the John Ochsner Heart and Vascular Institute in New Orleans, said in a video statement.

Now, these findings left us scratching our heads here at The Salt, where we've reported in the past on many of the health benefits linked with coffee drinking. Turns out, we are not alone.

"This result is surprising," Rob van Dam, an epidemiologist at the Harvard School of Public Health, told me via email, "because results from other cohort studies in U.S. men and women suggest that coffee consumption is associated with a slightly lower risk of premature mortality."

In fact, van Dam's own research has found no increased risk of death from any cause in people who drank up to six eight-ounce cups of coffee per day. And last year, research published in the New England Journal of Medicine suggested that people with a daily coffee habit had a lower risk of dying during the 14-year study period than those who abstained.

What's more, van Dam notes that recent studies have suggested that coffee consumption does not increase the risk of dying from heart disease, stroke or cancers – all major causes of death. That body of research, he tells us, is "reassuring."

So what accounts for the increased mortality seen among heavy coffee drinkers in the new study? The data set used provides clues.

Lavie and his colleagues looked at data for more than 40,000 people, ages 20 to 87, who were enrolled as part of the Aerobics Center Longitudinal Study, a long-term study conducted between 1971 and 2002. The researchers followed up with the participants for 17 years on average. But they were only asked about their coffee consumption once – so, as Lavie himself notes, we don't know how their coffee habits changed over time.

Another limiting factor: smoking. Heavy coffee drinkers in the study were more likely to be smokers – which makes sense, since the data were collected beginning more than 40 years ago. Van Dam thinks the research didn't do enough to control for smoking. In fact, as we've previously reported, lots of studies in the 1980s failed to control for the link between coffee drinking and smoking, which is one big reason why early research appeared to give coffee a bad rep. Evidence suggesting health benefits from coffee began to emerge only as studies separated the two habits.

So, what's the bottom line for coffee drinkers?

Lavie says his findings suggest that sipping two to three cups per day is pretty safe, and possibly beneficial. But Van Dam notes that if you're generally healthy (and not pregnant or nursing), the "totality of the evidence" suggests that four cups of Joe per day shouldn't be harmful.

But of course, don't forget to listen to your body.

"If people think they experience detrimental symptoms related to too much caffeine, such as difficulty sleeping or nervousness," says van Dam, "they should try reducing their intake."

Copyright 2013 NPR.

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How Many Cups of Coffee Per Day Are Too Many?