Emma and Leo are siblings, but their relationship with coffee couldn't be more different. One afternoon, they both shared a small pot of green tea. Within twenty minutes, Emma was jittery and anxious, while Leo felt absolutely nothing. This isn't a matter of "willpower"—it's biology.

Sensitivity is largely determined by two genetic factors: CYP1A2 and ADORA2A.

The Liver vs. The Brain

The CYP1A2 enzyme in your liver is responsible for breaking down caffeine. About half the population has a "fast" version, while the other half possesses a "slow" version. Emma is likely a slow metabolizer, meaning the caffeine stays in her blood much longer.

The ADORA2A gene determines how sensitive your brain's receptors are to the caffeine that is present. Even if metabolism is fast, some people's brains react more violently to the stimulant.

Which one are you?

• High Sensitivity: You feel anxious or get heart palpitations with less than 100mg. You likely have a variation in ADORA2A or slow liver enzymes.
• Low Sensitivity: You can drink coffee late and still sleep (though your quality of sleep might still suffer). You are a fast metabolizer.

The takeaway: Don't compare yourself to your "Leo" friends. Use our calculator to select between Slow, Normal, and Fast metabolism to see how your unique body handles that late-afternoon cup.