Two people order the same coffee at three in the afternoon. One is asleep by half ten, while the other is still awake at one in the morning. Neither of them has done anything differently. Crucially though, neither is imagining it.
Caffeine is one of the most studied compounds in nutrition, and it is also one of the clearest examples of nutrigenomics in everyday life. Two genes explain most of the variation between people: CYP1A2, which determines how quickly you clear caffeine from your system, and ADORA2A, which influences how strongly you feel it while it is there. Both of these genes are included in our Core Genetic Report, along with 25 other genes that look at how your body reacts to and handles a range of nutrients and toxins, and you susceptibility to food intolerances.
This article explains what caffeine actually does in the body and what those two genes control. It's worth noting that we will talk a lot about coffee in this article, but the same rules apply to any other caffeinated drinks including black and green tea, mate, energy drinks and a range of soft drinks.
What Caffeine Does in the Body
Caffeine is absorbed rapidly in your system, usually reaching peak levels in the blood within 30 to 60 minutes (technically, it's classed as a stimulant, and is the most commonly consumed drug in the world). It is small and fat-soluble, so it crosses the blood-brain barrier easily and starts acting on the brain quickly. It's this speed that it acts which is why you can feel the effects of a strong coffee very quickly after drinking it.
The main impact of caffeine in your body is on a naturally occurring molecule called adenosine. Adenosine accumulates in the brain throughout the day as a by-product of cellular energy use. As levels rise, it docks into adenosine receptors and signals that you are tired. Caffeine has a similar shape to adenosine, so it occupies those receptors without activating them. The tiredness signal is blocked, and you feel alert.
Two things follow from this. Firstly is that caffeine does not create energy, it postpones the perception of tiredness. Secondly is that the adenosine in your body keeps building while the receptors are occupied, which is why the tiredness can arrive quite abruptly once the caffeine clears. This is the stereotypical '3pm crash', or 'post-coffee comedown', that is familiar to a lot of us.
While that is happening in the brain, the liver is working on removing it. A single enzyme handles around 95% of caffeine breakdown, converting it into three by-products: paraxanthine, theobromine and theophylline. That enzyme is made to the instructions of the CYP1A2 gene.
CYP1A2: How Quickly You Clear Caffeine
On average in adults, the half-life of caffeine averages four to six hours, meaning half of a two o'clock coffee is still circulating at dinner. That average conceals an enormous spread. Some people clear caffeine in two to three hours. Others are still processing it nine or more hours later.
Much of that difference traces back to a single SNP (a variation in our genes, which we explain in detail in the linked article) in the CYP1A2 gene. It sits in a region that governs how readily the gene increases enzyme production, and it comes in three combinations:
- AA: the enzyme responds strongly, and caffeine is cleared quickly. Commonly described as a fast metaboliser, and roughly half of people of European ancestry fall into this group
- AC: an intermediate response, with caffeine remaining active noticeably longer
- CC: the lowest enzyme activity of the three, and the slowest clearance
Because a single copy of the C version is enough to shift the picture, AC and CC are both considered to be slower metabolisers of caffeine.
CYP1A2 is also one of the more responsive genes in the body, which is why two people with identical results can still have different experiences. Smoking is one of the strongest known accelerators of its activity, and cruciferous vegetables and char-grilled foods have a milder effect in the same direction. In the other direction, pregnancy slows caffeine clearance considerably, particularly in the third trimester, and oestrogen-containing contraception and certain medications do the same.
Remember, your genotype (your genetic makeup) sets the baseline, but your individual circumstances move around that.
ADORA2A: How Strongly You Feel It
How fast your body clears caffeine is only half of the story. The ADORA2A gene carries the instructions for the adenosine A2A receptor, which is the receptor most responsible for caffeine's stimulating and anxiety-related effects.
Variations in this gene are associated with how sensitive that receptor is. Certain versions have been linked in research to more pronounced jitteriness, anxiety and disrupted sleep from relatively modest doses, sometimes as little as a single cup, in people who otherwise clear caffeine perfectly well.
The two genes work independently, which produces four broad patterns:
- Faster clearance, lower sensitivity: coffee is unremarkable, and later cups rarely cause problems
- Faster clearance, higher sensitivity: a noticeable hit that fades relatively quickly
- Slower clearance, lower sensitivity: little obvious effect at the time, though sleep quality may quietly suffer
- Slower clearance, higher sensitivity: strong effects that persist for hours, and the group most likely to have worked out they have a problem with coffee long before any test
This is why a result in isolation can be misleading. Read together, the two genes explain considerably more than either does alone.
What This Means in Practice
If you've purchased our Core Genetic Report then your results will tell you whether you have any variations in either of these two genes, and your practitioner will have been able to explain what this means for you in more detail.
If your clearance is slower
Timing matters more than quantity if you clear caffeine slowly from your body. Working back eight to ten hours from bedtime is a reasonable starting point for a cut-off, then adjust based on how you sleep. It is also worth tracking dose rather than cups, since a double-shot flat white and a black tea differ substantially in caffeine content.
If your sensitivity is higher
If you experience jitters after caffeine, this might be you. Consider having smaller amounts of caffeine, earlier in the day. Remember that tea, cola, energy drinks, chocolate and some off-the-shelf cold and headache remedies all add to the daily total.
Decaf coffee and tea is readily available in supermarkets, and can be a great way to still have that evening cup after dinner, without the unwanted side effects.
For general context, European food safety guidance considers up to 400 mg of caffeine a day, roughly four to five cups of home-brewed coffee, to be safe for most healthy adults, with 200 mg a day advised in pregnancy. Remember that this is broad advice offered for a wide population, so you should always make sure that you understand your own body, and work with a nutritional practitioner on your own personal regimen. If you take regular medication, your GP or pharmacist can tell you whether caffeine interacts with it.
Your Underlying Wiring
A genetic result explains your underlying tendency. It cannot tell you how much coffee you can drink today, because your sleep debt, stress load, medications, gut health and habitual intake all sit on top of the genetics. Someone with a slower result who sleeps well and drinks one coffee at eight in the morning may never notice a thing.
What the information does give you is an explanation for something you may have observed for years without being able to account for. That is usually where more useful decisions start, and it is exactly the kind of question worth bringing to your practitioner walkthrough. If you're still considering whether a test is right for you, our guide to what happens after you order your genetic test explains what to expect throughout the journey.
This article is for informational purposes only and does not constitute medical advice. Please consult a qualified healthcare professional for advice specific to your situation.