It seems every other day there's a new report about whether drinking coffee is good for you or not.

Now, a review of more than 200 scientific studies has given weight to the idea that a cup a day — or even three — could actually have health benefits.

Although researchers don't know why.

Coffee can be a controversial beverage for the health conscious, thanks to conflicting studies and health claims. For every person who swears off it for fears it will dehydrate them or give them cancer there's someone else using it to supercharge their naps, mixing it with butter in a bid to lose weight or trying to stave off heart attack and stroke.

The review, published today in the BMJ, aimed to dispel some of that confusion, synthesising the evidence from 218 previous studies and drawing out common themes.

The verdict: researchers found drinking coffee was consistently associated with a lower risk of death from all causes and a lower risk of several cancers, as well as type 2 diabetes, gallstones and gout.

Liver conditions, such as cirrhosis, saw the greatest benefit associated with coffee consumption.

There also seemed to be beneficial associations between coffee consumption and Parkinson's disease, depression and Alzheimer's disease.

So is caffeine the magic ingredient? Probably not; while there was less evidence for the effects of drinking decaf, it did have similar benefits for several outcomes.

As for how many long blacks you should glug to get these benefits, researchers found three cups a day was the sweet spot when it came to relative risk of death compared with coffee abstainers.

Drinking more than that wouldn't harm you, they found, but the beneficial effect was less pronounced.

But, before you run out to the cafe ...

But before you race out for another foam-topped shot, the researchers found there were some groups of people who should approach coffee with caution.

There was some thin evidence that pregnant women and women at risk of fracture should limit their coffee consumption.

While the review was extensive, it's not the final word on the benefits or otherwise on coffee drinking. Rather, it's a gateway for more research.

The authors called for randomised controlled trials to understand the cause-and-effect of coffee drinking and the health benefits that have been observed.

And not all experts are completely sold on coffee enhancing health.

In an editorial linked to the review, Eliseo Guallar at the Johns Hopkins Bloomberg School of Public Health writes that people shouldn't start drinking coffee simply to prevent disease, given that some people are at higher risk of adverse affects and there is still uncertainty in the data.

The sugary, fatty snacks that go so well with coffee could undo any health benefits conferred by coffee and more besides, he said.

Having said that, "moderate coffee consumption seems remarkably safe, and it can be incorporated as part of a healthy diet by most of the adult population", Dr Guallar said.