Caffeine (metabolism)

Purine alkaloid (C₈H₁₀N₄O₂) metabolised in the liver by CYP1A2. Average half-life 5-6 hours (2-12h depending on genotype). Blocks adenosine A₁ and A₂A receptors, increasing alertness. Safe moderate dose: 400mg/day (healthy adult). Transfer to breast milk: ~1% of ingested dose.

Background & Context

Caffeine is metabolised almost entirely in the liver by the cytochrome P450 1A2 (CYP1A2) enzyme system. The primary metabolic pathway converts caffeine to paraxanthine (80%), theobromine (11%), and theophylline (4%) — all of which have distinct physiological effects. CYP1A2 activity is highly variable: genetic polymorphisms divide the population into "fast metabolisers" (half-life ~3–4 hours) and "slow metabolisers" (half-life up to 9–10 hours). This genetic variation explains why some individuals can drink an espresso at 10pm without sleep disruption while others experience insomnia from a single mid-morning cup. Pregnancy, oral contraceptives, and some antibiotics (notably ciprofloxacin) inhibit CYP1A2, slowing caffeine clearance significantly. The adenosine receptor blockade that produces caffeine's alertness effect is complete within 45–60 minutes of ingestion.

Practical Use

Understanding caffeine metabolism has direct practical applications for coffee timing. For most people, caffeine's half-life means that half of a morning espresso's caffeine load is still circulating 4–5 hours later. Delaying the first coffee until 90–120 minutes after waking (allowing cortisol levels to peak naturally) is supported by research suggesting it produces more effective alertness than an immediate post-waking dose. Decaffeination removes 97–99.9% of caffeine depending on method (Swiss Water, CO₂ supercritical, solvent): a Swiss Water decaf espresso contains roughly 2–5mg caffeine versus 60–70mg in a standard double shot — meaningful for slow metabolisers or those with caffeine sensitivity. Caffeine content also varies by species and roast: a Robusta-blend espresso may contain 90–110mg per double shot vs. 60–70mg for Arabica single-origin. Light roasts retain marginally more caffeine than dark roasts (contrary to popular belief — roasting destroys only ~5–10% of caffeine) because darker roasts require lower dose weights to achieve the same strength. For precise caffeine management, tracking both species blend and dose weight is more reliable than relying on roast colour as a proxy.

Related Terms

Related terms: Caffeine, Robusta, Swiss Water process, Espresso.