THE PROCESS IN MOTION

What does caffeine do once it is drunk?

Start the scene or choose a stage. The animation simplifies the mechanism; what follows gives the details and the available references.

▶ Play the process01From the digestive tract to the blood02Adenosine, the signal of fatigue03Caffeine takes the place04The liver undoes it05Half-life: four to five hours

From the digestive tract to the blood

Caffeine is absorbed entirely by the digestive tract and reaches its blood peak in thirty to sixty minutes. Tea releases the same molecule more slowly, which explains the different sensation at equal dose.

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1. From the digestive tract to the bloodCaffeine is absorbed entirely by the digestive tract and reaches its blood peak in thirty to sixty minutes. Tea releases the same molecule more slowly, which explains the different sensation at equal dose.
2. Adenosine, the signal of fatigueDuring waking hours, adenosine accumulates in the brain. By binding to its receptors, proteins in the membrane of neurons that recognize it, it signals fatigue. There are four subtypes, A1, A2A, A2B and A3; on the A1 receptors located on the transmitting side of the neuron, it slows the release of chemical messengers.
3. Caffeine takes the placeCaffeine crosses the blood-brain barrier, the filter that protects the brain, and occupies the same receptors. It does not make adenosine disappear: it blocks the receptor that should read it. Blocking A2A produces wakefulness; blocking A1 lifts the brake on dopamine, norepinephrine and acetylcholine. Nothing is given back of what has been spent: the doorbell is disconnected, the visitor is still there.
4. The liver undoes itIt is eliminated almost entirely in the liver, by the enzyme CYP1A2, which removes a methyl group from the molecule. It produces mostly paraxanthine, about 80% of the metabolites, then theobromine and theophylline, themselves active on the same receptors.
5. Half-life: four to five hoursIn a healthy adult, half the dose remains after four to five hours, a quarter after twice that: the 8 o'clock cup is still measurable at midnight. This time lengthens in the newborn (several days), on oral contraceptives, in liver insufficiency and during pregnancy, when the enzyme loses a third of its activity in the first trimester and half in the second. It shortens in the smoker: quitting smoking suddenly makes one more sensitive to coffee that one drank without effect.

To remember: Caffeine does not erase fatigue: it masks the receptor that reads it, until the liver undoes it, four to five hours for half.

Read the full article and its sources ↓

Caffeine is absorbed entirely by the digestive tract and reaches its blood peak in thirty to sixty minutes; tea releases the same molecule more slowly, which explains the different sensation at equal dose. It crosses the blood-brain barrier and occupies the four adenosine receptor subtypes (A1, A2A, A2B, A3). Adenosine accumulates in the brain during waking hours and signals fatigue; caffeine does not make this accumulation disappear, it blocks the receptor that should read it.

Blocking A2A produces wakefulness, blocking presynaptic A1 lifts the brake on the release of dopamine, norepinephrine and acetylcholine. Nothing is given back of what has been spent: the doorbell is disconnected, the visitor is still there.

StatPearls, Caffeine (NCBI Bookshelf, NBK519490); Jacobson et al., Adenosine A2A receptor antagonists: from caffeine to selective non-xanthines, Br. J. Pharmacol. (2022); professional training document, Caffeine and decaffeination processes, p. 2; LC ch. 19

The half-life in a healthy adult is four to five hours: half the dose remains after that time, a quarter after twice that, and the 8 o'clock cup is still measurable at midnight. Elimination takes place almost entirely in the liver, by the enzyme CYP1A2, which removes a methyl group and produces paraxanthine (about 80% of the metabolites), then theobromine and theophylline, themselves active on the same receptors.

The half-life lengthens in the newborn (several days), on oral contraceptives, in liver insufficiency, and during pregnancy: the enzyme's activity falls by a third in the first trimester and by half in the second, so that the same cup exposes the pregnant woman substantially longer than before. It shortens in the smoker, whose tobacco induces CYP1A2: quitting smoking suddenly makes one more sensitive to coffee that one drank without effect.

StatPearls, Caffeine (NBK519490); Del Coso (ed.), Coffee and Caffeine Consumption for Human Health, MDPI (2020), pp. 11, 229-233; LC ch. 19