What is caffeine?

Caffeine is a naturally occurring alkaloid of the methylxanthine family, produced by coffee, tea, cacao, guarana and kola nut plants. It is the world's most consumed psychoactive substance: it blocks adenosine receptors in the brain, postponing the feeling of tiredness and boosting alertness.

Chemically, caffeine is 1,3,7-trimethylxanthine (C8H10N4O2), a white, bitter, crystalline compound first isolated in 1819 by the German chemist Friedlieb Ferdinand Runge — reportedly on a request from Goethe himself. The plant synthesises it as a natural pesticide: caffeine paralyses leaf-eating insects and also inhibits the germination of rival seedlings around the coffee tree, a phenomenon ecologists call allelopathy.

The pharmacology of caffeine is now well established. Throughout the day, a molecule called adenosine accumulates in the brain and binds to its A1 and A2A receptors, slowing neuronal activity and producing drowsiness. Caffeine mimics the three-dimensional shape of adenosine closely enough to dock onto the same receptors without activating them — a classic case of competitive antagonism. Drowsiness is masked, dopamine and noradrenaline circulate more freely, and alertness climbs. The European Food Safety Authority (EFSA) considers a single dose of up to 200 mg safe for healthy adults and a total daily intake of up to 400 mg as posing no safety concern (EFSA Scientific Opinion, 2015).

Caffeine is almost fully absorbed by the small intestine within 30 to 45 minutes, peaks in the bloodstream around the 45-minute mark, and is broken down by the liver enzyme CYP1A2. The average half-life in adults is 4 to 6 hours, but individual variation is dramatic: slow metabolisers can hold caffeine twice as long as fast ones. This genetic difference is a big reason why one person shrugs off a 5 p.m. espresso while another is still wired at midnight.

How much sits in a cup depends on method and variety. A ristretto-style espresso carries roughly 63 mg, a pour-over around 95 mg, and a French press close to 107 mg (USDA FoodData Central). Arabica beans cap out around 1.2 to 1.5 % caffeine by weight, while Robusta pushes 2.2 to 2.7 %. A specialty espresso pulled in Brussels, Ghent or Antwerp with an 18 g Arabica dose typically lands between 100 and 130 mg — the same amount as the 1.5 litres of cola many office workers sip through the afternoon, delivered in one 30 ml shot.

Caffeine at a glance: key pharmacology