For how we live.
A small double-blind trial found a single 5.5 mg/kg dose lifted sprint power for hours, alongside higher noradrenaline and lower perceived effort — but it was 12 people, mostly men.
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.
A meta-analysis of 22 crossover trials with overnight polysomnography found consistent losses in total sleep time and efficiency. The age and dose subgroup differences everyone wants were not statistically significant.