Because coffee is consumed by so many people around the world, evidence of even small health effects is potentially important. Previous studies examining the effects of coffee consumption have produced mixed results, depending on the health outcomes studied. To assimilate this information, researchers performed a review of meta-analyses of observational or interventional studies that evaluated the association between coffee consumption and health outcomes in adults worldwide.

A total of 201 meta-analyses of observational research and 17 meta-analyses of randomized controlled trials were identified. Studies most often compared high versus low (or no) coffee consumption; in these comparisons, coffee consumption had statistically significant beneficial associations with 19 health outcomes and harmful associations with six health outcomes. In studies comparing any with no coffee consumption, coffee consumption had significant beneficial associations with 11 outcomes and harmful associations with three. For one extra cup a day, researchers found significant beneficial associations with 11 outcomes and harmful associations with three. Summary estimates indicated that the largest relative risk reduction was associated with the consumption of three to four cups of coffee per day.

The greatest beneficial associations were between coffee consumption and liver conditions in comparison to other health outcomes, according the authors. They also noted beneficial associations between coffee consumption and all-cause mortality, cardiovascular mortality, cancer (both specific types and overall), type 2 diabetes and other metabolic conditions, Parkinson's disease, depression, and Alzheimer's disease. Coffee consumption was consistently associated with harmful pregnancy outcomes, including low-birth-weight neonates, preterm birth, and pregnancy loss. In women-but not men-coffee consumption was associated with a higher risk of bone fracture.

Based on these findings, the authors conclude that robust randomized controlled trials are needed to determine whether the associations between coffee consumption and health outcomes are causal. They note that significant harm to study participants is unlikely in future trials in which the intervention is increasing coffee consumption within a range of three to four cups a day. However, pregnant women and those at increased risk for fracture should be excluded from any such trials.

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