For some of us, getting through the day, or even a couple of hours, without a cup of coffee can be hard work.
But despite the ubiquity of coffee drinking – some estimates suggest that up to 40 per cent of the world’s population consumes the beverage every day – it has been uncertain whether it confers health benefits.
While many studies have suggested that coffee can be good for the heart and the liver, there have also been scares over potential harms, including possible cancer risks.
Among the researchers currently interested in the effect of coffee on our health is Dr Jonathan Fallowfield, a medical doctor and senior clinical fellow at the University of Edinburgh in the UK.
Coffee drinkers less prone to liver disease
Dr Fallowfield and other researchers at Edinburgh and the UK's University of Southampton found in a study published last year that drinking coffee could reduce the risk of chronic liver disease by a fifth, with coffee drinkers about half as likely to die from the condition.
All types of coffee were found to be protective against chronic liver disease.
“We did a series of studies looking at epidemiological data showing increased coffee consumption seemed to be associated with lower risk of liver cirrhosis and liver cancer. We even showed it might protect the kidney,” Dr Fallowfield said.
But he said the research he and his colleagues carried out was based on observational data, which meant it could have overestimated any protective effects.
The 2021 paper, as well as other studies that looked at coffee’s health effects, did not involve a randomised controlled trial in which one group was given a certain coffee regime and compared to another that was kept away from coffee.
Such an RCT would provide more definitive evidence of coffee’s benefits or otherwise, and would eliminate the risk that the results were skewed by factors such as coffee drinkers having a different socio-economic status, or people with liver disease going off coffee and drinking less.
Some observational studies on coffee drinking have been shown to be flawed because of these types of effects.
For example, in an online briefing paper, the American Cancer Society says that research once indicated that coffee could be linked to an increased risk of bladder cancer.
It says that further analysis indicated that this was incorrect. The real reason for a higher rate of bladder cancer among coffee drinkers was not the drink itself, but because coffee drinkers were more likely to be smokers.
Despite the caveats associated with observational studies, Dr Fallowfield believes enough data has accumulated for people to be confident that there are health benefits associated with drinking coffee.
“I’m reassured by the scale and reproducibility of the effects across different studies and populations. There’s definitely something in it,” he said.
Health benefits clear, experts say
While the scale of the effect remains uncertain, it is legitimate to suggest coffee drinking to people as good lifestyle advice, he said.
Aside from protecting the liver, other health benefits have also been associated with coffee drinking, but these may depend on the type of coffee consumed.
“There is strong evidence that coffee can modestly reduce risk of diabetes, heart disease and total mortality,” said Walter Willett, a professor of epidemiology and nutrition at Harvard TH Chan School of Public Health in the US.
“However, this evidence refers to filtered coffee, and the same benefits may not be seen for unfiltered coffee because it contains some components that raise blood cholesterol levels."
This year, researchers at Semmelweis University in Budapest, Hungary, and Queen Mary University of London found that drinking moderate amounts of ground coffee reduces the overall mortality rate and the risk of strokes.
This effect was not reported among people who drank instant coffee, which the researchers suggested was because of the production processes and additives involved.
What is the magic ingredient?
Teasing out what it is in coffee that confers health benefits is not easy, because the drink contains many hundreds of substances that are “bioactive”, which means they have some biological effect.
“It’s a real mass of things that could be helpful. People have tried to identify the magic ingredient," Dr Fallowfield said.
Caffeine is, perhaps unsurprisingly, the best studied of coffee’s many bioactive compounds. It is known to block a particular receptor on cells that form scar tissue in the liver and, in doing so, protects against fibrosis, the scarring associated with cirrhosis of the liver.
While earlier studies raised fears of cancer risks associated with coffee, more evidence has accumulated that shows it can lower the risk of the disease.
Nevertheless, in its online briefing document, the American Cancer Society states that “associations with cancer overall or with specific types of cancer are unclear”.
The organisation reports the findings of a 2016 paper that reviewed a vast array of studies and found that drinking coffee did not cause pancreatic, prostate or female breast cancers. The risk of liver and uterine endometrium cancers may also be cut by drinking coffee.
When and how much to drink
So, how much coffee should a person drink a day to maximise the health benefits? Prof Willett suggests that three to five cups a day is sensible.
Dr Fallowfield recommends about three to four cups a day, because the largest reductions in health risks appear to be associated with this amount.
However, he said he “wouldn’t be too prescriptive”.
“People know how much coffee they drink or like to drink,” he said.
Much of this comes down to tolerance of caffeine, which varies considerably between different people.
Dr Fallowfield can happily drink a cup of coffee at 10pm without any ill effects, such as difficulty sleeping, but other people find that they are much more sensitive to the drink.
Genes that influence coffee metabolism play an important role in this variation.
“People who don’t like coffee are very slow metabolisers. People, like me, who like it tend to be very rapid metabolisers,” he said.
While the general advice may be to drink between three and five cups a day, this does not apply to pregnant women.
“Pregnant women should keep intake low or not drink coffee at all because this may lead to low birth weight,” Prof Willett said.
The UK’s National Health Service recommends that pregnant women consume no more than about 200 milligrams of caffeine a day, which is equal to about two cups of instant coffee a day.
For the rest of us, a few cups a day are likely to offer some benefit, especially if we are drinking filtered coffee. So next time you reach for your cafetiere to get a caffeine fix, you do not need to feel guilty.