Feel the Burn: Why Spiciness is Connected to Physical Heat
And what science says about why we like spicy food in the first place
My most memorable experience with excessive spiciness was sometime in my early teens. My family took me to a Cleveland Cavaliers game, and I got a helping of nachos with cheese and jalapenos. For some reason, I decided to eat the whole cup of jalapenos in one gulp. I didn’t watch much basketball after that. With my mouth on fire and sweat pouring out of my body, it was hard to pay attention.
One takeaway is that teenagers sometimes do stupid things. Another — familiar to eaters of spicy food everywhere — is that the body’s reaction to hot peppers shares something with its reaction to excessive heat.
In fact, it’s been known for a long time that this reflects a deeper connection. The active ingredient in jalapenos and other peppers, capsaicin, was first isolated in the 19th century. Experiments not long after that showed that it can influence thermoregulation. When applied to the stomach of dogs via a catheter, pepper extracts were shown to lower their rectal temperature. This implied that capsaicin could influence how the dog’s body was maintaining its temperature.
The molecular basis of the connection became clear in the 1990s when researchers isolated the receptor responsible for binding capsaicin. Like cells throughout our body, the sensory neurons in our mouths and digestive system use receptors to sense what is in their environment. A receptor is a protein, often located in the outer membrane of a cell, which is able to detect some stimulus in the environment and pass that information into the cell. The receptor that detects capsaicin is called TRPV1. It can be found in sensory neurons not only in the digestive system, but in many tissues around the body.
One interesting thing about TRPV1 is that it can respond to many kinds of stimuli. One of those is heat, in particular excessive or noxious heat.
Once the gene for TRPV1 was identified, it was possible to study it more carefully in the lab. It was found to be an ion channel. Sensory neurons (and other kinds of cells) keep various ions — atoms with a positive or negative charge — at different concentrations inside and outside the cell. Calcium ions for example are at higher concentration outside. One way to send a signal is to allow these ions to pass through the membrane going from the side with high concentration to the side with low. Researchers found that at normal temperatures, TRPV1 did not allow ions to pass, but when they raised the temperature above 40 Celsius (104 Fahrenheit), the channel opened. That is, at higher temperatures TRPV1 changes its physical shape in such a way as to allow positive ions like calcium to pass the membrane. The resulting influx of ions influences other proteins in the cell, effectively sending a signal.
Temperatures above 40 Celsius roughly correspond to the threshold for pain in humans.
We now know that TRPV1 receptors are part of a pathway for detecting noxious heat. When we touch something that is too hot and which may damage our tissues, signaling from these receptors helps produce the painful burning sensation that prompts us to avoid danger. It turns out capsaicin binds onto this same receptor causing it to change shape and allow ions to pass through the membrane. So, jalapenos and other peppers produce a burning sensation because capsaicin stimulates the same receptor that excessive heat does. That’s why spicy food feels “hot”, even though it may be at room temperature when we consume it. (TRPV1 is also involved in detecting other noxious stimuli, such as high acidity. It can even detect infrared radiation in some species).
Why some people tolerate spicier food
In the years since TRPV1 was discovered, knowledge of this system has also been used for a practical purpose in humans: treating pain. One important characteristic of the TRPV1 system is desensitization. Repeated stimulation of the receptor by capsaicin (or by heat or other stimuli) leads to a long-term reduction in the level of response. This is due to molecular changes which ultimately lead the sensory neurons to take some of their TRPV1 receptors out of the cell membrane. Less receptors means a less intense response.
These kinds of changes are likely to underlie spice tolerance (the fact that people who eat a lot of spicy food aren’t bothered by it as much). They have also been used to treat neuropathic pain. One approach has been to use a skin patch to deliver capsaicin to the skin (this is what the Qutenza system does). The exposure then desensitizes sensory neurons, with the benefit of reducing pain due to other causes. Such approaches have shown some efficacy. However, they also have significant side effects. For example there have been problems with patients getting burns because the system for recognizing noxious heat is impaired.
One thing we might ask about this whole system is, why does capsaicin activate the TRPV1 receptor in the first place?
This may reflect the evolutionary interests of chili plants. For fruiting plants such as chilis, it matters which animals eat their fruits. Having a compound that triggers a pain response in mammals is probably advantageous in deterring seed predators like rodents (who destroy the seeds when they ingest them). From the plants’ perspective, birds are a preferable group of fruit eaters, better able to spread the seeds. Interestingly, the TRPV1 receptor in birds does not respond to capsaicin. It may be that evolution selected for capsaicin production in plants specifically because it triggered aversion in mammals but not in birds. (It’s worth adding that capsaicinoid compounds in general have multiple functions in the fruits of chili plants. Another important one is fighting fungus.)
So why do we love spicy food?
If capsaicin triggers pain in mammals and deters many of them from eating chilis, why do so many humans like eating spicy food?
That is a tricky question. One part of it relates to potential benefits. There are many theories about this, ranging from possible antimicrobial effects to the idea that we might use it to influence our own thermoregulation. Evidence for these ideas is mixed, and none provides the kind of obvious or clear-cut explanation that would allow us to say “this is why people eat spicy food.”
There is, however, a somewhat more procedural answer: Spicy food preference in humans is an acquired taste.
Many studies show that people who are exposed to it over time develop a liking. This is particularly true for those who are exposed in childhood. In this sense it fits with the larger observation that humans are highly flexible in our diet and can develop a taste for all kinds of foods that seem dubious to non-aficionados (haggis, stinky tofu etc.). In food as in many other things, humans are an exquisitely cultural animal.
