In exercise and nutrition, the “everybody is different” and “it works for me” arguments can be problematic. They allow us to convince ourselves that something worked for us, despite ample (and more rigorous) evidence that it doesn’t. They are a couple of the worst offenders when it comes to propagating myths and misinformation. However, while these phrases can often be used in damaging circumstances, there are a few instances in which they’re entirely justified. One such case is caffeine. If you and your friend notice different effects from caffeine use, it might boil down to how well you each picked your parents (i.e., it’s in your genes!).

If you ever take a pharmacology class, you’ll become very familiar with the cytochrome P450 system. This is a system of liver enzymes that are responsible for metabolizing, well, just about everything in terms of drugs. That’s an exaggeration, but not a particularly large one. The P450 enzymes are responsible for breaking down a huge variety of drugs. As a result, there are several distinct isozymes that make up the entire family of P450 enzymes. One such isozyme is CYP1A2, which is the main enzyme responsible for caffeine metabolism.

CYP1A2 activity certainly differs between people; while it can be affected by some environmental and behavioral factors, genetics account for over 70% of the variation [1] in CYP1A2 activity! Several studies from multiple lab groups have identified different genotypes for the gene that controls CYP1A2 activity in humans, which affects the speed at which individuals metabolize caffeine. Due to genetic differences, there are some people who are considered “fast” caffeine metabolizers, some who are “slow,” and some who are in the middle.

In the past couple of decades, caffeine research has caused a great deal of frustration among consumers. It feels like every day the story changes; one day caffeine is good for performance, the next it’s bad. One day it’s bad for overall health, the next day it’s supposed to lower your risk of death. There are probably several factors that contribute to this habitual flip-flopping of the evidence, but genetic differences between people most likely play a role.

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