Does drinking coffee protect against liver cancer?

Liver cancer is the clearest example in which coffee drinking shows a protective effect. Large observational studies and so-called Mendelian randomisation studies both point in the same direction. That second method is more rigorous than an ordinary questionnaire study, because it does a better job of ruling out other lifestyle factors as the cause of the association. Three or more cups per day appears to offer the most protection.

Why is this so striking? For most other cancer types the picture is far less clear. An umbrella analysis of many studies did find that heavy coffee drinkers had on average about 18% lower odds of cancer than people who drink little coffee, but this is an average across dozens of cancer types combined. For liver cancer specifically, the association is one of the most robustly supported in the coffee-and-cancer literature.

There are also points of caution. Coffee drinkers smoke more often, and in studies that do not correct for this adequately, coffee appears to be a risk factor for lung cancer. Whether any association with lung cancer truly remains after correcting for smoking is still unclear. There is also a signal that women who drink a lot of coffee have a slightly higher risk of bone fractures, although this evidence is limited and is not seen in men.

Are you pregnant? Different advice applies in that case. High coffee consumption is associated with a greater chance of low birth weight, preterm birth and miscarriage. Experts recommend limiting caffeine intake during pregnancy to a maximum of 200 mg per day, which amounts to roughly one to two cups.