A cup of coffee can grant one person a surge of alertness for half the day, barely stimulate another, and plunge a third into anxiety. This disparity is caused not by habit, but by genetics and individual bodily characteristics.
Genetics: Metabolism Rate. A key role is played by the CYP1A2 gene, which codes for a liver enzyme responsible for breaking down caffeine. Individuals with the “fast” variant of this gene metabolize caffeine in 2–3 hours, hardly experiencing long-term effects. Holders of the “slow” variant do this over 6–8 hours, so caffeine circulates in the blood longer, intensifying both positive and negative effects (nervousness, tachycardia).
Tolerance and Habituation. With regular consumption, the body adapts: the number of adenosine receptors in the brain increases (which caffeine “presses” by hindering the sensation of fatigue). Therefore, heavy coffee drinkers eventually require a larger dose to achieve the former result. For those who drink coffee infrequently, the receptors are more sensitive—hence a strong reaction even to a small serving.
Interaction with Other Receptors. Caffeine does not only block adenosine. It also influences the release of dopamine (hence mood improvement) and norepinephrine (enhanced focus). The individual sensitivity of these systems varies. In people with anxiety disorders, caffeine might excessively stimulate the norepinephrine system, triggering panic episodes.
Lifestyle Factors
Smoking nearly doubles the rate of caffeine metabolism.
Taking oral contraceptives or pregnancy, conversely, slows down its breakdown.
Medications: Certain antibiotics and antidepressants (like fluvoxamine) inhibit the CYP1A2 enzyme, intensifying and extending caffeine’s impact.
Body Mass: The lower the weight, the stronger the impact from a standard dose might be.
Why Might Coffee Induce Drowsiness?
A paradoxical reaction occurs in individuals with “slow” caffeine metabolism, especially when tired. Caffeine obstructs some adenosine receptors, but not all. When its influence wanes, the accumulated adenosine massively binds to the freed receptors, causing a sharp drop in energy—the “caffeine crash.”
Practical Conclusion: Pay attention to your body. If coffee causes jitters or sleep issues—you are likely a “slow” metabolizer. Try switching to tea (where caffeine is bound to L-theanine, yielding a gentler effect) or reducing the dosage. Your reaction is personal biochemistry, not a weakness.
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