Today I found out why spicy foods can cause your nose to run.

In most spicy foods, this is thanks to the same chemicals that cause the burning sensation on your tongue, namely capsaicin or allyl isothiocyanate.  Capsaicin is chemical found in fruits of the genus Capsicum, which includes peppers. It is present, usually in relatively high amounts, in the placental tissue that holds the seeds of the peppers, as well as in lower concentrations in other parts of the fruit.

The capsaicin works as a deterrent to stop various animals, particularly mammals that would crunch the seeds, from eating the fruits or otherwise harming the plants or seeds. It also functions as an anti-fungal agent, which further protects the plants.

Allyl isothiocyanate, on the other hand, is a colorless oil that can be found in things like mustard, radishes, and wasabi.   Like capsaicin, it serves as a defense for the plant against various animals, as well as works as an anti-fungal agent.

These chemicals end up not only causing a “hot” sensation on your tongue, but also irritate the mucous membranes in your nose, causing them to become inflamed.  This triggers those membranes to produce extra amounts of mucous as a defense mechanism to try to keep out whatever unwanted substance or particles are causing the irritation.

This same type of irritation is why your eyes may also become watery when you eat very spicy foods.  The capsaicin or allyl isothiocyanate can irritate the membranes in your eyes, causing your tear ducts to kick into overdrive trying to wash the irritant away.  This can make your nose even more runny as some of the tears drain into your sinuses.

Capsaicin and allyl isothiocyanate also irritate various tissues inside your body, such as your intestines.  This causes your body to react by trying to flush the irritant out.  This is why after eating spicy foods you sometimes have the dreaded “liquid fire” poop.

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Bonus Facts:

• This “runny nose” effect can be used to your advantage when you are nasally congested.  At those times, eat some very spicy foods and your sinuses will clear out in no time.
• Capsaicin also has the effect of dilating blood vessels.  This can result in people becoming flushed as they eat very spicy foods.
• Birds do not have the necessary receptors which bind to the capsaicin causing the burning effect.  As a result, they can eat the pepper seeds with no ill effects.  These seeds then pass through their system and eventually are pooped out, which ultimately gives a nice environment for the seed to eventually germinate wherever it is pooped.
• Capsaicin is fat soluble and, thus, water will be of no use in countering the burning sensation, other than the fact that if it is cold water it will temporarily overpower the capsaicin’s effect on the nerve receptors and tell your brain you are feeling a cold sensation.  But once the cold water has gone, the heat will come back straight away.
• Dairy products work best to counteract capsaicin because they contain a protein called casein which binds to the capsaicin, hindering its ability to bind to your nerve receptors, which helps move it through your system faster and without it affecting your body as much.
• A cold sugar water solution is almost as effective as drinking cold milk in terms of hindering the capsaicin from binding to your VR1 receptors, and thus muting the burning sensation.  This works thanks to a chemical reaction between the capsaicin and the table sugar.
• Tarantula venom activates the same neural pathways as capsaicin, so getting bitten by a tarantula will feel much the same as being exposed to very high level of capsaicin.
• Large enough quantities of capsaicin may cause your skin to turn blue-ish, severely inhibit your breathing, cause convulsions, and possible eventual death.  However, the minimal amount of capsaicin in peppers makes it unlikely you’d ever come in contact with enough of this to have this actually happen, unless someone sprayed law enforcement grade pepper spray directly down your throat or the like.
• Capsaicin was originally called “capsicin”, named by Christian Friedrich Bucholz who was the first person to extract it in an impure form from plants in the genus Capsicum.  It was later renamed to capsaicin by John Clough Thresh who was the first person to successfully isolate capsaicin in nearly pure form.