Slow metaboliser here. If I drink a coffee at 11am my heart rate rises and doesn't return to normal until the following morning.
There are other genes for it too, like one that affects the degree to which coffee causes anxiety. And plenty of others that we haven't found, presumably.
I love love love love coffee. It took me years to realize that my high blood pressure was controlled almost immediately by cutting out a high-test canned “Draft Latte” drink. I can drink a single 8oz cup every few days. Decaf is fine.
There are other genes for it too, like one that affects the degree to which coffee causes anxiety. And plenty of others that we haven't found, presumably.