Keeping up with all the research on the possible risks and benefits of coffee might require one to, well, drink lots of coffee. 

The online magazine Slate, for example, calculated that as of 2010 there had been more than 500 studies on the subject.

And as fast as you can say double-double, now comes another report, this time from the International Agency for Research on Cancer. For those keeping track, back in 1991 it was the IARC that found that coffee is "possibly carcinogenic to the human urinary bladder."

Happily for coffee lovers, the agency has now downgraded the threat, finding that coffee poses an "unclassifiable" risk because it hasn't been studied sufficiently in humans — that substantial body of research notwithstanding.

"We can't say that it's completely safe because proving a negative is very difficult, but it has moved down a step in terms of the hierarchy of concern," Dana Loomis, deputy head of the IARC program that classifies carcinogens, told CBC News.

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It can be very difficult to devise reliable nutrition science studies, making it harder to assess the risks and benefits of coffee. (Getty Images/Hero Images)

IARC's current concern is not with the beverage itself but with its temperature. It is really hot coffee or tea that may be damaging, they warn.

This may be reassuring to devoted coffee drinkers, but it provides little guidance for those who are still weighing the possible health effects — yet another headline in a multitude that have either sung coffee's praises or warned of its risks. 

'The flip-flopping of nutrition science'

In the 1970s and '80s, some studies found that drinking coffee could increase the risk of heart problems and high blood pressure. But more recently, researchers have said that coffee may help fight heart disease, protect against liver cancer and ease or reverse the effects of Alzheimer's. 

"I think what's interesting about the coffee story is that it's yet another example of what the public perceives as the flip-flopping of nutrition science," says Timothy Caulfield, Canada Research Chair in Health Law and Policy at the University of Alberta. "This food is good for you, [and then] this food is bad for you."

Caulfield, a self-admitted coffee devotee, points out that the same back and forth happened with red wine. "One worries that people will just stop listening — that they'll just think it's useless information, that you scientists just can't make up your minds."

The problem, says Caulfield, is that it can be very difficult to arrive at reliable conclusions in nutritional science, since findings are often based on observational studies — ones looking at how humans behave in uncontrolled environments. This means researchers can't necessarily prove that it was the act of drinking coffee that made someone more susceptible to cancer rather than some other factor.

"You can't do a randomized control trial on coffee, and have a population that drinks coffee and a population that doesn't drink coffee, and follow them for 20 years," Caulfield explains.

"There are so many variables associated with this. Humans are complicated biological units, we live in complicated environments, we make decisions throughout the day that impact our health. To isolate one food, one substance, isn't easy."

That was the issue with a famous New England Journal of Medicine study from 1981, says Dr. Donald Hensrud, director of the Mayo Clinic's Healthy Living Progam. That study, which found a link between coffee consumption and an increased risk of pancreatic cancer, was flawed because it failed to adequately take into account smoking — a behaviour often associated with coffee drinking.

Hensrud says that study, and other questionable research published at the time, helped blight coffee's reputation for years.

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Most recent studies have shown that coffee drinking is likely beneficial, researchers say. (Getty Images)

"If you look at the data, it's actually quite clear that coffee is beneficial," says Hensrud. (Full disclosure: he also likes a good cup of joe). The evidence, according to Hensrud, shows that coffee can protect against Parkinson's, liver disease, liver cancer and Type 2 diabetes. It may help prevent multiple sclerosis and dementia, in addition to Alzheimer's.

Coffee improves mood, decreases the risk of depression and may boost overall longevity rates, he adds.

However, Hensrud notes that there are some potential downsides to coffee consumption, including decreased iron absorption, insomnia, heart palpitations and, in large amounts, an increased risk of miscarriage.

"If you don't enjoy it, don't drink it," Hensrud advises. "If you enjoy it, watch what you put in it for the calories. And if you don't suffer from the side effects, I have a tough time telling people to cut down on coffee."

Focus on the bigger picture

Caulfield says people need to be careful of over-emphasizing both the positive and the negative effects described in many studies, and shouldn't be concerned about the small extent to which something like coffee may increase cancer.

Instead, they should focus on whether they are eating enough fruits and vegetables, avoiding junk food and getting enough exercise.

"For most people the important thing is to focus on that big picture — eating a healthy diet — and not worry about these marginal risks," he said.

And as for coffee, he has one message: "If you look at the body of evidence out there about coffee, it's probably not bad for you, and may be good for you. So drink up."