Why does my nose run when eating spicy foods?

July 23, 2007 1:25 PM

What are the biological and/or evolutionary purposes behind a runny nose brought on by spicy foods?

Why does my nose run when eating spicy foods? I'm curious as to what biological purpose it serves. Does it "cool down" the oral/nasal cavities? Or is the spice simply irritating/swelling the sinuses? Just a little something I've always wondered...

posted by afx114 to science & nature (9 comments total)

Not everything is the direct result of evolutionary selection. Some things are side effects.

In this case, anything which irritates the eyes causes an increase in tears to flush whatever-it-is. Excess tears flow into the sinuses (since draining excess tears is the function of the sinuses) yielding a runny nose.

The reason your nose runs when you cut onions is because onions release sulphur dioxide when cut. That reaches your eyes and dissolves, producing sulphurous acid, which stings. Your eyes respond with tears to flush the acid away, which makes your nose runny.
posted by Steven C. Den Beste at 1:34 PM on July 23, 2007

From ask yahoo, not yahoo answers mind you.
posted by jourman2 at 1:40 PM on July 23, 2007

Isn't it more likely a development of the plant rather than of us? The plant developed to produce casaicin in its seed pods to prevent animals from eating it until the seeds and fruit ripened. I don't know of any animals in the wild that will eat a hot pepper.
posted by Pollomacho at 1:54 PM on July 23, 2007

You are laboring under a misunderstanding of evolution. It does not have a "purpose" and it makes no sense to talk about one in a scientific discussion of evolution. It appears that what you are trying to ask is what selective advantage is gained by this behavior. But even that is begging the question. Not every aspect of your physiology is beneficial. And the consequences of bringing yourself into contact with some foreign object frequently cannot be explained by natural selection. You might as well ask what selective advantage is obtained by the fact that humans die when they are shot in the head with a .357 Magnum. You can explain what happens biologically, but handguns did not affect human evolution in any meaningful way.

The chemical that causes spicy foods to taste that way is called capsaicin. In addition to causing nerves to send a signal to the brain that something is burning, it triggers a response in goblet cells that results in the secretion of mucus. It's hard for me to imagine this specific behavior gives a selective advantage to humans. It's more likely to just be a consequence of the way all this machinery already works, which may otherwise be beneficial or not.

Capsaicin almost certainly provides an selective advantage to the plants, because it means their seeds are less likely to be eaten by mammals, and more likely by birds, which are not irritated by capsaicin. The Wikipedia article says that Capsicum seeds can germinate after passing through the avian digestive tract, but not through the mammalian one.
posted by grouse at 1:57 PM on July 23, 2007 [5 favorites]

Much of that extra mucus is going to end up coating your airways and esophagus. This provides an extra layer of protection from the irritants in the food.
posted by chrisamiller at 3:04 PM on July 23, 2007

Pollomacho: there are some birds that lack receptors for casaicin and will quite happily munch on hot peppers.
posted by lazy robot at 8:38 PM on July 23, 2007

Capsaicin does activate nerves that normally sense heat. It's an evolutionarily beneficial small molecule for plants to express to reduce predation.

However, (some) humans enjoy spicey-hot foods. Spicey-hot foods are frequently common in civilizations that reside in very hot or very cold places, but are not as common in temperate climates.

Odd.

Eating hot foods in humans, when the temperature is high, increases sweating. Sweating -> more fluids on the body surface -> more evaporative cooling.

When the temperature is low, eating hot foods increases (perceived?) body temperature, counteracting the colder environment.

Now - some people have very large mucus production responses to capsaicin, other people... not so much.

There are situations where a strong mucus response is a good thing; you're able to get rid of potentially noxious substances from your airways like chrisamiller mentions. Other times, if you can't stand the foods that are common in your culture, you might not be as desireable of a mate. Imagine going on a date with someone who snots up everytime salt or pepper or basil is used to season food or when served potatoes or when served wheat or... (yeah, I know - in this day and age diet can be controlled fairly well [see: celiac disease] but back when humans were breeding or dying?)

How many East Indians or Northern Chinese people do you know who excessively mucose-ate in response to spicey-hot food (well, they've likely been exposed to it since birth, though)? How many Western European-extraction people do you know who snot-up after eating spicy food?

People can get "used to" capsaicin - with repeated exposure many people (not all) will detect spicey-hot capsaicin as a "sweet" taste couple with some heat.
posted by porpoise at 9:15 PM on July 23, 2007

What you're describing is vasomotor rhinitis.

Common symptoms include stuffy/runny nose from any number of non-allergen sources, including spicy foods. The basic idea is that some people are more sensitive to the capsaicin or extreme temperature, which causes irritation to your sensitive sinus tissues, leading eventually to one or more of the classic symptoms.

Some people experience runny noses after consuming very hot (not spicy) foods, for example. Sensitivity may or may not be an evolutionary advantage in this case, but as grouse said, capsaicin in particular is an advantage for plants. You should be able to "train" yourself to tolerate it with more constant exposure, as porpoise suggests re:other cultures.
posted by Ky at 11:00 PM on July 23, 2007

Other times, if you can't stand the foods that are common in your culture, you might not be as desireable of a mate... How many East Indians or Northern Chinese people do you know who excessively mucose-ate in response to spicey-hot food (well, they've likely been exposed to it since birth, though)?

This answers a different question, which is essentially the opposite of the original question—how is the secretion of mucus in response to capsaicin deleterious? But because most human populations (including those in Asia) were probably not exposed to it until 500 years ago, this is unlikely to have any effect on how basic human physiology.
posted by grouse at 1:46 AM on July 24, 2007