Spicy is perceived by the same receptors as the feelings hot and/or pain. But is this just some “trickery” of the receptors or does spicy food do actual damage? And if it doesn’t do actual damage: Why do people sometimes still throw up because of it (if it was too much)?
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The chemical responsible for “spicy” is an oil called capsaicin. Your mouth is full of nerves that are sensitive to heat: when proteins in the nerves warm up, it causes them to bend into a new shape. That change in shape triggers the nerve to fire, which tells your brain that something in your mouth is warm. Some proteins require a higher heat to trigger, and when they do it tells your brain that your mouth isn’t just warm, it’s *hot*. Your brain also looks at how many are triggering: more nerves = more hot.
There are special nerves called *nociceptors* that are tuned to things that are dangerous or damaging, and they tell your brain that there should be *pain*. In this case, there are nociceptors with proteins that only change when they get dangerously hot, which then tells your brain to feel like your mouth hurts. This is meant to protect you because you’ll then spit out whatever you tried to eat that is too hot and will burn your lips, tongue, and throat if you ate it. It also triggers things like making your mouth water in order to cool and protect your mouth, eyes watering to flush out anything getting in there, sweating to cool you off, etc.
Capsaicin interferes with the proteins and nerves, forcing them to trigger regardless of how warm (or not) your mouth is. This tells your brain that your mouth is very hot – *painfully* hot – even though it isn’t. The fact that the food you’re eating might also just be physically very hot will contribute to that painful feeling, but even if the food is cold your brain doesn’t know that. It’s getting a signal from the heat-sensing nerves and nociceptors, so you feel the sensation of heat regardless of how warm it is. The unconscious part of your brain can’t understand that what you’re feeling isn’t real. To it, there’s no difference between your mouth being literally on fire and all of your nerves going off *as if* your mouth is on fire but isn’t. It’s the same sensation either way.
This is *mostly* harmless. Nothing is actually burning, it just *feels* like it’s burning. However, this *can* cause problems because your brain will continue to react as if your mouth is burning and do things that it would need to do if your mouth really was on fire. Some of that response is a lot like an allergic response. That means producing a lot of saliva, which can overwhelm your throat and cause you to choke and cough. Your throat tries to close up to prevent you from swallowing what your brain thinks is dangerously hot. This can make breathing and swallowing difficult, making it more likely that you will choke if you do swallow some food. If the food is spicy enough and you aren’t used to it, this reaction can lead to full on anaphylaxis where your throat swells shut, your heart rate spikes dangerously high, and you may try to vomit to remove the “harmful” stuff. That can be *extremely* dangerous. That’s not to say that eating any spicy food will kill you, just don’t try to swallow a spoon full of straight up raw capsaicin.
Pro tip: capsaicin is an oil, which means it does not mix with water. If you try to use water to flush out your mouth after eating something spicy all it will do is move the capsaicin around your mouth and make it worse. Instead, rinse your mouth with milk. The fats in the milk will grab the capsaicin oil and carry it away. Icecream is also good for the same reason (the fats in the dairy), and although your mouth is not actually hot and on fire your brain will still appreciate the cold of the icecream “putting out” the heat.
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