Why are chili peppers spicy?

KohakuWhen you eat something spicy, your mouth feels like it's on fire. It isn't actually hot, though. Isn't that curious?

The answer in 30 seconds

I've summed up what spiciness really is in 30 seconds.

The heat of chili peppers comes from a substance called capsaicin. Capsaicin switches on sensors in the nerves of your mouth that detect heat. So your brain reads it as "hot and painful" even though nothing is actually hot. Spiciness is not a taste like sweet or salty. It is a feeling closer to heat or pain.

Red chili peppers spread out to dry in a round woven basket — Red chili peppers drying in a basket. Their heat comes from a substance called capsaicin.Photo: Basile Morin / CC BY-SA 4.0 / Wikimedia Commons

The heat comes from capsaicin

The spiciness of chili peppers comes from a substance called capsaicin. When capsaicin gets into your mouth, you feel a burning heat and a stinging pain. People have long known that capsaicin acts on nerves to cause pain, but for a long time no one knew how it worked.

A discovery that led to a Nobel Prize

In the late 1990s, American researcher David Julius and his team found a protein in nerves that reacts to capsaicin. This sensor, later named TRPV1, was found to also switch on with heat strong enough to feel painful. For this and other discoveries, Julius was awarded the 2021 Nobel Prize in Physiology or Medicine.

That's why it feels "hot"

TRPV1 is a heat sensor that switches on when it touches something hotter than about 43℃. Capsaicin switches on this sensor even when nothing is hot. So the brain reads it as "something hot is in my mouth," and your mouth feels like it is on fire.

Figure: How spiciness is sensed

The numbers match the text and are based on the paper by Jordt and Julius (Cell).

Birds don't feel the heat

Interestingly, birds do not feel the heat of chili peppers. Experiments have confirmed that the TRPV1 of chickens reacts to heat but hardly reacts to capsaicin at all. Chili peppers are thought to use their heat to keep away mammals, which destroy the seeds when they eat them, while letting birds eat them and carry the seeds far away.

KohakuSo birds can't feel this heat at all, can they?

Want to know more?

Out of millions of DNA fragments, Julius and his team found the one gene that builds the system that reacts to capsaicin.
The chicken sensor still reacts to heat and acid. Birds are thought to feel hot things as hot, too.

KohakuSpiciness is a heat sensor being tricked. A familiar little wonder that led to a Nobel Prize.

Quick quiz: test yourself

What is the heat of chili peppers closest to?

Show the answer

A taste like sweetness
A taste like saltiness
A feeling of heat or pain

A feeling of heat or pain Capsaicin switches on the heat sensors in the nerves of your mouth, so your brain feels "hot and painful" even though nothing is hot.

Sources

Press release: The Nobel Prize in Physiology or Medicine 2021The Nobel Assembly at Karolinska Institutet（ノーベル財団）
Molecular Basis for Species-Specific Sensitivity to "Hot" Chili Peppers (Cell, 2002)Cell（Jordt & Julius, カリフォルニア大学サンフランシスコ校）

This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.