Published 20th September 2022

Why your gut is incredible

Consider the word “gut.” Just three little letters. But this simple word belies an incredibly complex system that fills a significant portion of your torso.

Along its twists and turns are multiple associated organs, colonies of microorganisms, immune networks, a “second brain,” and plenty more besides.

Here, we’ll break out some of the most jaw-dropping facts we’ve found about your gut. But let’s start with the basics.

What is your gut?

Despite its short name, it’s a long bit of kit. This uninterrupted tube snakes from your mouth to your anus. 

Once you’ve swallowed your food, it enters your gullet, or esophagus, before setting up shop in your stomach for a few hours. Once it’s left that acidic bag (more on that later), it enters your intestines.

First, it travels along your small intestine, then your large intestine.

Your small intestine is longer than your large intestine, but the hole down the middle, called the lumen, is much wider in the large intestine, hence the name.

The next stop is your rectum, where what's left waits around to be expelled through your anus as poop. What a journey.

Your gut, also called your gastrointestinal tract, is part of the digestive system, which also includes the liver, gallbladder, and pancreas. 

Now that we’ve got your digestive geography out of the way, let’s get onto the incredible facts.

1. You contain legions

Inside every one of us there’s a vast microbial civilization. Although estimates vary, it seems that there’s roughly the same number of bacteria living in you as there are cells in your body.

The latest estimate suggests that a “reference man” has 38 trillion bacteria in his body. That’s 38 followed by 12 zeros.

But because bacteria are much smaller than human cells, they only weigh roughly 7 ounces (200 grams).

And everyone’s microbiome is different. For example, even identical twins only share 34% of the same gut microbes at a species level. That’s compared with 30% in unrelated people.

2. Powerful acid

Your stomach lining produces hydrochloric acid strong enough to burn your skin. On average, you produce up to 17 cups (4 liters) of acid daily.

How does your stomach lining avoid being burned? It has a few tricks:

  • It secretes a protective alkaline substance to help neutralize the acid.

  • The stomach lining also produces mucus to protect itself.

  • Your stomach only makes acid when it’s needed — when the food arrives.

3. It’s cool to drool

While we’re on the subject of bodily fluids, let’s give a shout-out to saliva. Several glands in your mouth produce it. And saliva contains enzymes so that digestion can get started even before you swallow.

Saliva also acts as a lubricant to help you swallow and contains antimicrobial compounds to kill off unwelcome microbes.

Each day, you produce an impressive 2–6 cups (0.5–1.5 liters) of saliva.

Even when you’re not eating, you make about 1 teaspoon of saliva every 15 minutes, but much less while you’re asleep. 

4. Competing with your brain

Your gut is a hotbed of nervous activity. The so-called enteric nervous system is a vast network of neurons in your gut. Some experts call it your “second brain.”

In fact, you could consider it your first brain because, evolutionarily speaking, it came before your “head brain.” 

Although the enteric nervous system is influenced by the central nervous system, which is your brain and spinal cord, it can operate independently. 

Incredibly, your gut produces more neurotransmitters — chemical signals used by your nervous system — than your brain. 

Your gut produces a startling 95% of your serotonin, the “happiness hormone,” and 50% of your dopamine. 

5. Gas station

Farts, wind, gas, flatulence — we all experience it, and it’s important. If air is in your digestive tract, it needs to get out.

Some experts suggest that we fart an average of 15 times a day, ranging from a few times to up to 40. Others give an average range of 12–25 toots per 24 hours.

Join our mailing list

Sign up for fresh insights into our scientific discoveries and the latest nutrition updates. No spam, just science.

You might think you break less wind than that, and you might, but remember, you also fart when you’re asleep. And you’re not counting then.

As for volume, people produce around 2.0–8.5 cups (0.5–2.0 liters) every day. Not bad going.

6. A sizable journey

A survey of 100 men and 100 women concluded that the gut is 22–30 feet (6.7–9.2 meters) long. That’s about the length of a school bus or, if you prefer, almost twice as long as a giraffe is tall.

The scientists behind the survey also found that, on average, men had longer guts than women. And the gut is longer in a younger person than in an older adult.

Interestingly, gut length doesn’t correlate with height, but it does with weight — heavier people have longer guts.  

We should also note that it’s difficult to measure the length of the gut accurately, and estimates vary considerably. Suffice it to say: It’s long.

7. From your mouth to the toilet

Now that you know the distance that food travels through your body, it won’t surprise you to learn that it takes a while to get all the way from one end to the other.

ZOE’s iconic Blue Poop Challenge timed the journey of muffins that contained blue dye from the mouth to the toilet bowl.

Our scientists found that, on average, it took 28.7 hours for the muffins to make the trip.

However, there was a great deal of variation — we’re all unique, after all. Some people’s muffins exited in under 14 hours, while others spent over 58 hours inside. 

8. It’s not all about length

So, we’ve established that you’ve got a surprisingly long bit of piping coiled up in your abdomen. But the impressive measurements don’t stop there.

If you opened up your gastrointestinal tract and measured its total surface area, it would cover half a badminton court. 

That’s roughly 344 square feet (32 square meters). As a point of reference, the surface area of your skin is only about 22 square feet (2 square meters).

Because the primary job of the intestine is to absorb nutrients from your food, the larger its surface area, the more nutrients it can take in. So, how does your gut lining take up so much space?

To boost its available surface area, the walls of the small intestine are ribbed with folds and tiny finger-like projections called villi

To increase the surface area even further, villi are covered in even smaller finger-like lumps called microvilli.

9. Gut rumbles

When you’re hungry, you might have noticed a rumbling, grumbling sound emanating from your insides. That’s called borborygmic — what an incredible word, right?

It’s caused by peristalsis, wavelike muscular contractions that help push food through your gut.

Although peristalsis is strongest after eating, it also starts up when your intestines have been empty for a couple of hours. It helps clear out all the debris from your last meal. These are called hunger contractions.

The associated grumbling sounds happen often, and they’re louder when there’s no food to muffle them. 

10. Immune links

An incredible 70% of your entire immune system resides in your gut. This makes sense when you consider that the gut constantly deals with the outside world. 

Food — which potentially contains dangerous microbes — is regularly in contact with this part of your body.

So, the immune cells in your gut must be on alert. But, at the same time, they are tightly regulated so they don’t start destroying your “good” gut bacteria. 

In fact, your gut microbiome helps “train” your immune system. It’s an incredibly complex balancing act.

A final fact

We could have continued this list almost indefinitely — your gut is an incredible place — but we had to stop somewhere. 

While researching this, we came across many impressive facts that ended up on the editing room floor. But we simply have to leave you with this wonderful, if slightly random, fact.

The duck-billed platypus has no stomach. That’s right. No stomach at all. Their food goes straight from their gullet to their intestines.

Right, we’ll give you some time to digest that.

Sources

Allergy and the gastrointestinal system. Clinical and Experimental Immunology. (2008). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2515351/ 

Anatomical study of the length of the human intestine. Surgical and Radiologic Anatomy. (2002). https://link.springer.com/article/10.1007/s00276-002-0057-y 

Blue poo: impact of gut transit time on the gut microbiome using a novel marker. Gut. (2021). https://gut.bmj.com/content/70/9/1665.abstract 

Exploring the villus. Gastroenterology and Hepatology from Bed to Bench. (2018). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6040026/ 

Flatulence. (2014). https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/flatulence 

How does the stomach work? (2016). https://www.ncbi.nlm.nih.gov/books/NBK279304/ 

Human skin is the largest epithelial surface for interaction with microbes. Journal of Investigative Dermatology. (2017). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5814118/ 

Immunity in the gut. (n.d.). https://www.immunology.org/public-information/bitesized-immunology/organs-and-tissues/immunity-in-the-gut 

Intestinal gas. (n.d.). https://badgut.org/information-centre/a-z-digestive-topics/intestinal-gas/ 

Interaction between microbiota and immunity in health and disease. Cell Research. (2020). https://www.nature.com/articles/s41422-020-0332-7 

It's a gut feeling: How the gut microbiota affects the state of mind. The Journal of Physiology. (2014). https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/jphysiol.2013.270389 

Loss of genes implicated in gastric function during platypus evolution. Genome Biology. (2008). https://link.springer.com/article/10.1186/gb-2008-9-5-r81 

Peripheral Dopamine Controlled by Gut Microbes Inhibits Invariant Natural Killer T Cell-Mediated Hepatitis. Frontiers in Immunology. (2018). https://www.frontiersin.org/articles/10.3389/fimmu.2018.02398/full 

Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLOS Biology. (2016). https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002533 

Surface area of the digestive tract – revisited. Scandinavian Journal of Gastroenterology. (2014). https://www.tandfonline.com/doi/abs/10.3109/00365521.2014.898326?journalCode=igas20 

Vagal input to the enteric nervous system. Gut. (2000). https://gut.bmj.com/content/47/suppl_4/iv30.full 

Why does your stomach growl when you are hungry? (2002). https://www.scientificamerican.com/article/why-does-your-stomach-gro/

Join our mailing list

Get occasional updates on our latest developments and scientific discoveries. No spam. We promise.