BITTER & BURN: A DOUBLE WARNING SYSTEM?

I was just going over a list of compounds that activate bitter receptors, and noticed that many of these compounds also activate the “hot” receptor TRPV1 and/or the “cold” receptor TRPA1, both of which cause pain when activated. These include compounds that activate the bitter receptor TAS2R38: ethanol, which burns by activating TRPV1; mustard and wasabi, which activate TRPA1, but also TRPV1 to some extent; and limonene in citrus fruits—especially their rind which we use for zest—which activates TRPA1. Limonene is what gives the zesty "bite."

Zesting an orange. Image from Wikipedia.

The structure of TAS2R38 differs considerably from person to person. Some people carry a version that functions well while others carry a version that doesn't function at all. The compound propylthiouracil (PROP) has been used to define who is and who isn't a "supertaster" based on whether a person carries a functioning TAS2R38 or not.

My thought is that even when a person fails to sense the bitterness of a compound, pain from activation of TRPV1 or TRPA1 may stop further consumption.

We think that the capacity to sense bitter tastes developed in order to warn us of potentially poisonous compounds in our foods. Perhaps the simultaneous activation of the bitter and the pain receptors is nature’s double fail-safe warning system.