The Frontal Cortex

If you like spicy food - and I love spicy food - then you'll find this report from Harold McGee's blog rather interesting. It concerns the evolution of capsaicin, the pungent chemical that makes chilis so spicy:

Levey, Tewksbury and colleagues tested the theory that capsaicin selectively repels rodents and other grain-eating mammals, which would chew up the chilli's seeds along with the surrounding fruit, while having no deterrent effect on birds, which have no teeth, swallow the fruits whole and defecate the seeds intact. They monitored wild chilli plants in Bolivia and in Arizona with video cameras, and found that only birds ate the fruits, as the theory predicts. Interesting sidelights: in past studies, lab rats frequently fed hot chillis have developed a strong liking for them, just as many humans do. And the wild species that were monitored, Capsicum chacoense and Capsicum annuum, bear fruits with a significant oil content, unlike our domesticated varieties. Oily chillis could be an interesting new ingredient.

Of course, that still doesn't explain why capsaicin tastes "hot," and why spicy food "burns" our tongue. For that, we need to investigate the physiology of taste. It turns out that capsaicin binds to a special class of vanilloid receptor inside our mouth called VR1 receptors. After binding capsaicin, the neuron is depolarized, and it signals the presence of spicy stimuli.

But here's the strange part: VR1 receptors weren't designed to detect capsaicin. They bind spicy food by accident. The real purpose of VR1 receptors is the detection of heat. They are supposed to prevent us from consuming food that is too hot, in the thermal sense. (That's why our VR1 receptors are clustered in our tongue, mouth and skin.) So when they are activated by capsaicin the sensation we experience is that of excessive heat. We start to sweat and get the urge to drink lots of water. But that pain is just an illusory side-effect of our cell receptors. There is nothing "hot" about spicy food.