Eating spicy food can spark a variety of reactions: a euphoric buzz, a racing heart, a tingling tongue, gastrointestinal apocalypse, existential dread, and tears of pain/joy. A gullet full of chili-loaded foods can also, unusually, cause a runny nose.

What makes food spicy?

A chemical called capsaicin is usually the culprit when we feel the burn from spicy food. The compound is the active ingredient in chili peppers, which are fruits belonging to the genus Capsicum, including everything from the timid bell pepper to the infamous Carolina Reaper chili. It’s primarily found in the thin membrane that holds the seeds, although it can be found in the flesh of some chili peppers. 

There are some other chemicals that trigger a similar spice response, such as allyl isothiocyanate, a compound found in some cruciferous vegetables that puts a wafty kick in mustard, horseradish, and wasabi.

Why do spicy foods feel “hot”?

These chemicals irritate the body’s mucus membranes, the inner lining of some organs and bodily cavities, such as the stomach, lungs, mouth, eyes, and nostrils. The irritation response is set off by the chemical latching onto certain receptors in the membrane that are stimulated by pain and heat. In the case of capsaicin, this is the TRPV1 receptor.

The body and brain are led to believe they’re under threat, so it instructs the membranes to pump out more mucus to flush out the foreign body. As such, we start to salivate, our eyes well up, and the nose starts streaming with mucus.

Even if you’re not snorting spicy food, your nose can be provoked to react because the sensory nerves in your mouth and throat share pathways with those in your nasal cavity. 

Why do people love spicy foods?

Chili plants produce capsaicin as deterrents to stop animals from eating them. It’s essentially part of their chemical warfare system. 

Interestingly, capsaicin only irritates mammals. Birds, for instance, can eat red-hot chillis with impunity because their TRPV1 receptors are subtly slightly different from ours. 

Humans and treeshrew are the only mammals known to actively seek out the heat of capsaicin. This isn’t because our species or treeshrew are masochistic (at least not in most cases). 

In humans, it’s simply because the firing up of pain receptors in this way can spark a light stress response, which provokes the body to release a rush of “feel-good chemicals” like endorphins and dopamine. Conversely, treeshrews have a genetic mutation in their TRPV1 receptor, making them less sensitive to the “burn” caused by capsaicin.

Capsaicin is not inherently toxic to humans, even though consuming significant amounts can cause a host of unpleasant symptoms, from intense pain to gastrointestinal distress. The “problems” associated with it largely stem from our body’s natural response to it rather than the chemical itself being harmful.

That said, its power shouldn’t be underestimated. Capsaicin and other related capsaicinoids are the active ingredients in some pepper sprays and bear sprays. If a spritz of the chemical is enough to ward off a 200-kilogram (440-pound) grizzly, you should treat it with a side order of respect.