Is coffee good for you? What the evidence shows

For decades coffee was treated as a vice, blamed for heart problems and cancer. The research has swung around: large studies now link moderate coffee drinking with lower risks of death and several diseases. Most of that evidence comes from watching what people drink rather than assigning them to drink it, so the question of whether coffee itself is responsible is still open.

The broadest summary, a review of more than 200 meta-analyses in the BMJ, found coffee was more often linked with benefit than harm, with pregnancy as the clear exception. A rare randomized trial adds a different kind of evidence, showing what coffee does day to day to activity, sleep and heart rhythm.

What the big coffee review found

The BMJ umbrella review, a study that pools other pooled analyses, identified 201 meta-analyses of observational research with 67 unique health outcomes and 17 meta-analyses of interventional research. Across them, coffee consumption was more often associated with benefit than harm for a range of health outcomes.

The pattern was strongest at moderate intake. Compared with no coffee, three to four cups a day went with about 17% lower risk of death from any cause, 19% lower risk of death from heart disease, and 15% lower risk of heart disease. High coffee intake was also linked with less cancer overall and with lower risks of some liver, neurological and metabolic conditions.

Why coffee’s harms seemed to disappear

Older studies often linked coffee with harm. The review found that harmful associations were largely nullified by adequate adjustment for smoking: for decades, heavy coffee drinkers were also more likely to smoke, and that made coffee look worse than it was.

Two harms remained. In pregnancy, high intake was linked with low birth weight, preterm birth and pregnancy loss. There was also a link with fractures in women but not in men. The authors concluded that coffee seems generally safe within usual levels of intake, outside pregnancy and possibly for women at higher risk of fractures.

Does adding sugar change the coffee findings?

Many studies did not distinguish between black coffee and sweetened coffee. A UK Biobank study of 171,616 adults, published in Annals of Internal Medicine, did. Over about seven years, people drinking unsweetened coffee had lower risk of death than non-drinkers across most intake levels.

Moderate amounts of sugar-sweetened coffee showed a similar pattern, while at the highest intake the link disappeared. The association between artificially sweetened coffee and mortality was less consistent. Like the other studies, this one relied on people’s own reports of what they drank and cannot rule out other differences between coffee drinkers and non-drinkers.

What a randomized coffee trial showed

Randomized trials of coffee are rare. One published in the New England Journal of Medicine gave 100 adults heart monitors, activity trackers and glucose sensors, then used daily text messages to randomly tell them to drink caffeinated coffee or avoid caffeine over two weeks.

On coffee days, people took about 1,000 more steps and slept about 36 minutes less a night. The consumption of caffeinated coffee was associated with 58 daily premature atrial contractions as compared with 53 on caffeine-free days, a difference that could be chance. Extra beats from the heart’s lower chambers were more common on coffee days, 154 against 102. These extra beats are common and usually harmless in healthy people, but some people notice them. Blood sugar did not change.

What health agencies say about caffeine

The FDA takes a middle position on caffeine rather than coffee specifically. For most adults, the FDA has cited 400 milligrams a day, about two to three 12-ounce cups of coffee, as an amount not generally associated with negative effects. It adds that there is wide variation in both how sensitive people are to the effects of caffeine and how fast they clear it.

MedlinePlus lists the effects of too much caffeine, including restlessness and shakiness, insomnia, headaches, dizziness, fast heart rate, dehydration and anxiety. For people who are pregnant, trying to become pregnant or breastfeeding, the FDA recommends a conversation with a health care provider about caffeine.

What coffee research cannot tell us

Almost all of the long-term evidence is observational. Coffee drinkers differ from non-drinkers in income, diet, health and habits, and people who feel unwell may drink less coffee, which can make coffee look protective. The umbrella review’s authors called for robust randomized controlled trials to understand whether the observed associations are causal.

The one randomized trial ran for two weeks in young, healthy adults. It can describe short-term effects on steps, sleep and heartbeats, but not whether coffee changes the risk of heart disease or death over decades.

What the coffee evidence adds up to

The weight of evidence has shifted from coffee as a harm to coffee as generally safe, and possibly beneficial, at usual intakes for most adults. The benefits are associations that may partly reflect who drinks coffee rather than the coffee itself.

Pregnancy stands apart, with links to low birth weight, preterm birth and pregnancy loss. Day to day, coffee means more activity and less sleep for many people, and how much suits any one person depends on their sensitivity, health and medicines.