For how we live.
In a driving study, 200 mg of caffeine followed by a nap under 15 minutes cut simulator incidents to 9% of placebo levels, against 34% for caffeine alone. The order is the point.
Pooled randomised trials show caffeine pushes blood pressure up sharply in the hour after a dose. But when the caffeine arrives as coffee, and over weeks rather than minutes, the effect shrinks toward nothing.
In a two-week trial, 100 adults drank coffee or avoided caffeine on random days. Coffee did not raise the upper-chamber skipped beats it was blamed for — but it nudged up lower-chamber ones and cut sleep.
Across nearly 1.1 million people, each extra daily cup tracks with lower diabetes risk in a clear dose-response. It is consistent and large — and still only observational.
The human intestinal-permeability literature lists endurance exercise, NSAIDs, pregnancy and surfactants as stressors. Coffee is not on that list, and alcohol's case is largely inferred.
Two meta-analyses tie an extra two cups a day to roughly a third lower risk of both liver cancer and cirrhosis, with decaf showing a smaller effect. It is all observational, and the evidence quality is graded very low.
The FDA's threshold traces to a Health Canada assessment and a 2017 systematic review of 381 studies. It describes a population, not a person, and the variation between people is large.
Pooled data across 38 studies found higher coffee intake linked to a higher risk of heart attack — but a lower risk of stroke and irregular heartbeat, with a separate benefit for people with high blood pressure.
A 131,821-person cohort followed for up to 43 years and a dose-response meta-analysis of 450,000 people converge on roughly two to three cups — and diverge sharply above it.