What makes chilies hot? The science of the burn

There’s nothing like the roasting burn of chili pepper on your tongue. Well, there is something like it — an actual burn.

Although chili peppers don’t literally burn you, it certainly feels like it.

Humans have enjoyed hot peppers for thousands of years, and their popularity shows no signs of slowing.

In this article, we’ll explain why they make you feel like your face is on fire and why they’ve evolved to produce such pain.

We’ll also cover what to drink if you’ve eaten too much spicy food, whether chilies might help treat some diseases, and a smattering of other scientific tidbits along the way.

How do chilies ‘burn'?

The compound that gives chilies most of their fiery temperament is capsaicin (pronounced cap-SAY-sin).  

Although many people think the seeds are responsible, that’s not strictly true. Capsaicin is most abundant in the “placenta” — a spongy, white membrane inside the chili where the seeds are attached.

Once you’ve chomped the chili, capsaicin mixes with saliva and binds to TRPV1 receptors.

These receptors help us detect heat. So, when capsaicin binds, the receptor sends a message to your brain, telling it there’s a fire in your mouth.

Rating the burn

The original and most well-known method of measuring a chili’s hotness is the Scoville scale. 

Jalapenos score around 5,000 Scoville Heat Units (SHU). The habanero — a pretty spicy chili — measures in at around 250,000 SHU.

One of the hottest chilies on the planet — the Carolina Reaper — clocks in at a scorching 2,000,000 SHU. That’s in the same ballpark as pepper spray. They are not to be messed with.

The original method of measuring SHU is not an exact science. To score any given chili, you must first dissolve it in alcohol, then add it to a sugar water solution.

Next, a panel of five specially trained tasters sample the liquid. The liquid is diluted repeatedly and re-tasted until at least three of the five tasters can no longer detect any chili heat. 

The score is based on how much dilution is necessary to drown out the burn. There’s a lot of subjectivity involved here, so it’s not ideal. 

Today, high-performance liquid chromatography provides a more precise method of rating chili spice. This more scientific approach directly measures the amount of capsaicin in a chili. 

It’s more accurate, but it doesn’t have quite the same theatre as a band of specially trained chili connoisseurs sweating in lab coats.

Why do chilies do this to us?

Many plants, including chilies, rely on animals to spread their seeds far and wide. 

That’s why they developed fruits in the first place — they make a good snack for lifeforms that are more mobile than them.

For chilies, birds do a great job — they gulp them down, then poop out the seeds over a wide area.

Mammals, however, are not so useful to the chili plant. We chew them up, grinding the seeds into a pulp and destroying them.

So, as a means of self-defense, chilies unleashed a chemical weapon. 

Studies have shown that birds lack the TRPV1 receptor, so they don’t experience the chili burn. On the other hand, rodents and other mammals do have the receptor and, therefore, avoid chilies.

Humans, however, have decided they enjoy the burn, making the chili plant’s evolutionary attack useless.

Although, you could argue that it’s still helping the plant survive, seeing as they are now grown on a huge scale way beyond their original home in South America. 

So, although capsaicin was initially designed to help them spread their seeds by deterring us from eating them, it actually helped them spread their seeds by making us fall in love with the pain. Homo sapiens is an odd species.

Benign masochism

Although not everyone likes the sensation that hot chili peppers bring, some absolutely love it. But why? 

As far as we know, no other species seek out painful experiences and enjoy them.

Scientists don’t know for sure, but it might be down to something psychologists call hedonic reversal or benign masochism.

When your body is tricked into feeling like it’s in danger — but you’re actually safe — it can generate a sense of pleasure.

In the same way that some folks enjoy horror movies or gut-wrenching rollercoasters, we get a certain pleasure from battling our primitive reaction: Our higher brain knows that our body’s fight-or-flight response is unnecessary.

On a more chemical level, a hot chili experience can boost the release of endorphins. These natural pain-killing chemicals can induce a “high.”

Whoops, too much chili!

If you’ve ever accidentally eaten something spicy (or too much of something you knew was spicy), you’ll know the panic that it causes. 

As the buzzing heat takes over your mouth, you reach for a glass of water to put the fire out. It hardly helps.

This is because capsaicin won’t dissolve in water. Like pouring water on a grease fire, water just spreads the capsaicin around your mouth.

Milk, on the other hand, will bring you relief. Milk (and ice cream) contain a protein called casein, which attracts capsaicin, binds to it, and stops it from burning you.

Believe it or not, scientists have investigated the most efficient way to put out a chili blaze. In a small human trial, they tested seven fluids: skim milk, whole milk, seltzer water, Cherry Kool-Aid, non-alcoholic beer, cola, and water.

They found that all helped reduce the inferno, but the most effective three were:

  1. whole milk

  2. skim milk

  3. Cherry Kool-Aid

Seltzer water, non-alcoholic beer, and cola all performed worse than water, but they still improved participants’ pain quicker than drinking nothing.

Kool-Aid might come as a bit of a surprise. According to the authors of the study, other researchers have shown that sucrose (a type of sugar) can reduce chili pain, and Kool-Aid is an incredible 10% sucrose.

Although cola has a similar level of sugar, the scientists explain that fizziness makes it less effective, although they’re not sure why.

Chilies and health

At ZOE, we know that a diet including lots of fresh ingredients is best for good health. As chili peppers are a fruit, they can form part of a healthy, varied diet. 

No single ingredient is the silver bullet to extend your life. 

With that said, one study has linked chilies to a reduced risk of death. The scientists took data from more than 16,000 people and followed them for almost 20 years.

They found that those who regularly ate red hot chili peppers had a 12% lower risk of dying during the follow-up period compared with those who never ate them. 

However, as the authors explain, there could be other explanations for this result. 

For instance, people who eat more red hot chilies might also eat more of other foods that create a healthier diet overall, such as spices and vegetables.

Capsaicin deep dive

Eating whole foods is the key to health, but scientists often study single compounds from foods to get an idea of their potential health benefits.

Over the years, there’s been a fair amount of interest in capsaicin, and there is some evidence — although mostly from animal and laboratory studies — that it might have some health benefits for humans.

Below, we’ll outline just a few of the conditions that scientists are investigating:


There have been a fair few studies into whether capsaicin might help treat cancer. However, the gastrointestinal side effects — including nausea and diarrhea — have been a stumbling block. 

To help get around these issues, some scientists are testing slow-release capsaicin capsules that might reduce the negative effects while still treating cancer.

We should also explain that, while capsaicin is present in safe levels in chilies, when it's in its pure form, it’s dangerous. 

It’s not something you can buy over the counter, and you need a respirator and plastic gloves to handle it safely.


There is growing evidence that capsaicin, or adding chilies to your diet, might help reduce obesity. 

A review on capsaicin, chilies, and obesity concluded that although more research is needed, the evidence looks encouraging.

Because chilies are widely available and not particularly costly, if they do help reduce obesity, it would be a great win.

Gut health

No ZOE article would be complete without mentioning the gut microbiome — the of trillions of microbes living in your intestines. 

Having a diverse collection of gut bugs is associated with good health. And some lab research suggests that capsaicin might increase the diversity of bugs in your gut.

Some scientists think that capsaicin might help kill disease-causing bugs while encouraging the growth of “good” bugs.

Beyond gut bugs, some researchers believe that capsaicin might benefit gut health in a variety of ways, including:

  • relieving the symptoms of gastroesphageal reflux disease (GERD)

  • protecting against some cancers of the gut

  • healing stomach ulcers

  • reducing gut sensitivity in irritable bowel syndrome (IBS)

  • preventing inflammatory bowel disease (IBD)

To reiterate, much of the evidence so far comes from lab research, animal studies, or experiments with small numbers of participants. 

Scientists need to carry out more research before they can make strong claims about capsaicin as a treatment. 

Pain relief

Some researchers have focused on the analgesic — or pain-relieving — powers of capsaicin. In particular, pain caused by nerve damage, which is called neuropathic pain.

Pain is notoriously challenging to treat, so scientists are keen to find a natural substance that might provide relief. According to some, capsaicin might fit the bill nicely.

For instance, a patch that sticks on your skin containing capsaicin appears to help some people with neuropathic pain. 

In fact, in 2013, a cream containing capsaicin was licensed for treating some forms of neuropathic pain in certain circumstances. More research is needed to understand who can safely benefit from this kind of treatment. 

To burn or not to burn?

From their birthplace in South America, chilies have found their way into the hearts and minds of people across the globe.

Overall, there are few risks associated with consuming chilies in moderation, and there might even be some health benefits. 

So, if you enjoy a little chili heat, feel free to add them to your diet. If, however, you don’t like to indulge in benign masochism, feel free to give them a miss.


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