[carbocation]

To add more information, the intervention was guidance about caffeine intake. From the Methods:

> If allocated to caffeinated coffee consumption, patients were encouraged to drink at least 1 cup of caffeinated coffee (or at least 1 espresso shot) and other caffeine-containing products every day as per their usual lifestyle. It was recommended that patients in the coffee consumption group not intentionally increase or decrease consumption of coffee or other caffeine-containing products.

> If allocated to the abstinence group, patients were encouraged to completely abstain from coffee, including decaffeinated coffee, and other caffeine containing products.

[cluckindan]

> including decaffeinated coffee

Sounds like the cause could also be some other substance than caffeine? Decaf still contains various other alkaloids.