Chili peppers leave some people cool, but they can give others the hiccups.
Chili peppers contain a chemical compound called capsaicin, which is part of a group of chemicals that give chilies their individual taste and heat profile. Capsaicin can activate neurons in the diaphragm, which contracts and causes hiccups.
Capsaicin is also an irritant. Chili plants are thought to use capsaicin as a defense mechanism in order to stop animals from eating the plant. In fact, humans are the only species known to enjoy eating spicy food.
So why do we actively seek out food that is an irritant capable of causing painful sensations?
Chili peppers and pain
Capsaicin binds to a pain receptor called TRPV1. When we accidentally expose our skin or eyes to chilies, we activate TRPV1 receptors, thereby causing the pain floodgates open. These receptors in our mouths are the reason for the "mouth-on-fire" feeling we experience when we eat spicy food.
Interestingly, once a pain receptor is stimulated by capsaicin, it goes into a period of rest, which means that the same receptor doesn't transmit pain signals again.
Scientists call this process "defunctionalization." This is why capsaicin is used to treat pain in some patients; it leads to loss of pain perception.
The capsaicin receptor is found throughout the body, including in the nerves that control our diaphragm, which is the muscle that allows our lungs to take in air.
Hiccups are thought to be caused by an involuntary contraction, or spasm, of the diaphragm, followed by contraction of the glottis. Airflow into the windpipe becomes temporarily blocked. When incoming air strikes the glottis, the characteristic hiccup sound occurs.
Scientists are still speculating about the precise mechanism responsible for hiccups, as well as how exactly capsaicin leads to spasms of the diaphragm, which, at this point, remains unclear.
Luckily, the capsaicin molecules that we eat in food don't stick around for very long; they are rapidly absorbed in the stomach and intestine to be broken down.
But why do some of us start to hiccup at the mere sight of a chili, while others don't seem to be bothered by spicy foods at all?
Nature, nurture, and the chili response
A Finnish study showed that our preference for spicy food is partly determined by our genes. Differences in number and distribution of capsaicin receptors in our mouths also contribute to our responsiveness to spicy food.
It is, of course, possible to build up a tolerance to chili peppers. There are no definitive scientific studies on human subjects yet, but we can speculate that the mechanism of tolerance occurs through repeated exposure to capsaicin. This will defunctionalize the TRPV1 receptors in the mouth, allowing us to get used to eating spicier food.