Coffee and cancer – what does the evidence say?

Coffee is one of the world’s most popular drinks, and it’s the way many of us choose to jump-start our day. But is it possible that its beneficial effects go further than a morning pick-me-up?

As you may have already read in the news, the International Agency for Research on Cancer (IARC), a group of international cancer experts convened by the World Health Organisation, has just concluded that there’s no strong evidence that coffee increases your chances of cancer (something that they’ve mooted in the past).

But there may also be some other good news for those of us who are coffee drinkers: there’s also some emerging evidence that coffee could in fact reduce the risk of certain cancers.

In this post we’ll discuss what their announcement means in practice. But what’s worth saying from the outset is, although there is indeed some evidence that coffee may reduce cancer risk, there are still question marks over the finding. And it’s certainly not enough for us to start recommending people should start drinking coffee if they don’t already.

And it’s also worth noting that, in the same report, IARC has also concluded that drinking very hot drinks (over 65 degrees) may increase the risk of oesophageal (food pipe) cancer. But while drinks of this temperature, such as tea and mate, are popular in Middle Eastern countries and South America, it’s much hotter than how we generally prefer our tea in the UK.

Just another headline?

Barely a day passes without the media reporting that this or that food prevents cancer. But all too often these stories are based on only a single study.

As we’ve said before, it’s unlikely that any single study – even a high quality one – would yield strong enough evidence to change our diet.

But IARC reports are worth paying a bit more attention to. Today’s announcement comes after a comprehensive review of the evidence, encompassing around 1000 studies. You can read a summary in the Lancet Oncology.

Before we discuss the findings, we need to briefly look at what IARC’s categorisation system actually means.

IARC scrutinises research on things that might cause cancer, and assigns them a category based on how strong this evidence is. But these categories, illustrated below, don’t reflect how much cancer something is likely to cause. If this sounds confusing, we recommend you read this post about IARC’s decision on processed meat and cancer last year, where we discuss their system in detail.

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So what about coffee? IARC last reviewed all the evidence for coffee and cancer in 1991, and put it in Group 2B – ‘possibly carcinogenic to humans’. This is for things where the limited available evidence isn’t good enough to rule a risk in or out.

IARC’s 1991 ruling was mainly due to a handful of studies suggesting that coffee drinkers appeared to be slightly more likely to develop bladder cancer.  But these studies were relatively small, and didn’t take into account things like smoking (which has a clear effect on bladder cancer risk).

The new report – probably a good thing

Since 1991, a lot more evidence has emerged, leading IARC to reassess the links between coffee and cancer. Today’s report is the result.

As far as bladder cancer goes, larger, more robust studies have since failed to provide any strong evidence that coffee increases bladder cancer risk.

And conversely, some studies have found that certain other cancer types are less common in coffee drinkers, in particular liver cancer. For example, in 2015, the World Cancer Research Fund (WCRF) – a cancer prevention charity – combined the results of 6 studies (a meta-analysis) and found that liver cancer risk seemed to be lower in people who drank coffee.

But again, these studies weren’t always able to take into account known liver cancer risk factors (eg being overweight or drinking alcohol), so it’s not impossible that the effect is due to something other than the coffee itself (for example, perhaps people who don’t drink much coffee tend to drink more alcohol).

There’s also some evidence that coffee reduces womb cancer risk: in 2013, another meta-analysis showed that women who drank a cup of coffee each day had a decreased risk of womb cancer (however this positive effect was only found in the less robust studies).

It’s also worth noting that IARC didn’t comment on any different types of coffee (eg instant vs espresso), or differences in how coffee is served i.e. adding milk and sugar.

All in all, this amounts to some emerging signs that people who regularly drink coffee might have a lower chance of developing some forms of cancer. But it’s far from clear whether there is a real effect. And, is the idea that coffee prevents cancer even biologically plausible?

How might coffee reduce the risk of cancer?

The exact mechanism behind how coffee may reduce the risk of cancer is unclear, and there are several different theories.

There’s some evidence that chemicals in coffee may be able to improve insulin sensitivity and prevent long-term (chronic) inflammation – both things thought to be involved in the development of cancer.

Other researchers have proposed that coffee could protect against DNA damage, slow the growth of tumours or cause damaged cells to die before they develop into cancer.

But at the moment these are just theories – we’ll need more research to understand whether and how coffee could affect cancer growth and development.

Should you start drinking coffee?

So if you already drink coffee regularly, then the reassuring news is that you’re probably not increasing your risk of cancer (although, watch the calories if you’re worried: swap to reduced fat milk and ditch the sugar. And no, sweeteners don’t increase cancer risk, despite what you may have heard.)

And if you don’t drink coffee already? We wouldn’t suggest this news means you need to start. The evidence is still uncertain, and if subsequent research does reveal an effect, it’s probably small – much smaller than any positive benefits gained from the important things, like keeping a healthy weight or cutting down on the amount of alcohol you drink.

All that said, if research did uncover a mechanism by which coffee prevented cancer… well that would be exciting – because exploiting it could lead to a way to turn the world’s favourite morning pick-me-up, into something much more powerful.

Casey