吃辣明明是痛感，为何有人越吃越上瘾？
The first thing to understand about eating spicy food is that it really isn’t a matter of taste. Capsaicin (辣椒素), the active chemical in capsicum (辣椒) plants that are a key ingredient in anything you’d think of as “spicy”, evolved as an irritant to stop mammals from chewing and destroying plant seeds. It acts on the nervous system directly through receptors in the tongue, throat and skin — no taste buds required — and, in theory, tells our bodies that the thing we’ve just ingested is something to get rid of as soon as possible. The obvious question, then, is: why do some of us like the sensation so much?
Firstly, some people have different variants of the TRPV1 gene, which change how easily it switches on and turns off after activation (interestingly, the gene is present but insensitive to capsaicin in all birds and one species of tree-shrew <树鼩>, letting them eat spicy seeds without harm). But the receptors also become less sensitive over time, so it’s entirely possible to develop a relative immunity to moderately spicy foods — or deliberately cultivate one.
As for why we like it, that might be because of the sensation it causes. “Research is still ongoing into how human brains learn what is and isn’t safe, including which foods are safe for consumption,” says Browne. “Recent work emphasises prediction, context and controllability. The idea is that when you eat something spicy, you get an initial alarm-like ‘heat’ signal. With repeated exposure the body’s outer response grows less sensitive, and the brain learns the cue is safe and under control. That shift in prediction and certainty is a large part of why the experience becomes tolerable — and then rewarding.” The more spice you eat, the theory goes, the less intense the first painful part becomes, and the more relief you get afterwards.