♪♪ -There's one ritual that happens in every corner of the world.

Chances are, you did it today.

♪♪ Over millennia... humans have turned eating into culture... ♪♪ ...a way to celebrate our roots... ♪♪ ...to bring us together.

♪♪ But at the end of the day, when you strip away everything else... ♪♪ food is really just one thing -- fuel.

♪♪ It's where you get the power to live your life.

Even when you're sitting completely still, your body needs a lot of energy to stay on.

♪♪ But nowhere is the need for fuel more apparent... than when you push yourself to the absolute limit.

♪♪ ♪♪ ♪♪ -As a Navajo distance runner, we run for very specific things.

It's the celebration of life.

It's a form of prayer.

It's a teacher.

It shows us how to overcome obstacles in life.

And finally, it's a form of medicine.

It helps heal us.

♪♪ This is Navajo tea.

It's very hydrating.

♪♪ On a cold morning like this, it's the perfect way to start a cold, long run.

♪♪ Running in the Navajo culture and the way that I was raised, it's a part of life.

♪♪ Whoo!

We get up, and we run every morning to the East to meet the Creator, at that innocent time of the day, the birth of that new day.

As we begin our run, we start with a shout.

When we feel the connection to the earth and to the sky and to the Creator, we yell... Whoo!

...and announce that we're thankful for all those deities.

[ Slow-motion panting ] Our breath is breathing in Father Sky.

The act of running is a prayer.

I'm Shaun Martin, I'm Navajo, and I'm an ultra-marathon runner.

[ Gunshot ] [ Bells ringing, crowd cheering ] -The marathon is considered the ultimate test of human endurance.

♪♪ But an ultra-marathon is in a whole other league -- often upwards of 100 miles, or four standard marathons back to back to back to back.

♪♪ These feats are only for an elite class of athletes... ♪♪ ...and they send the body to the edge of collapse.

♪♪ See, the human body is an amazing machine, capable of feats that seem impossible to most of us.

But like all machines, it requires fuel.

Every one of our nearly 40 trillion cells needs the juice to power on.

And for us, it comes in one form -- glucose.

It's a sugar that allows cells to create energy.

♪♪ And it's locked inside every plant and seed on earth.

♪♪ The trick is extracting it.

♪♪ Like how crude oil has to be refined to make the gas that we pump... our bodies contain a factory that takes all the raw material from food and extracts glucose.

♪♪ You know it as the digestive system, or more commonly, the gut.

It's a series of connected organs from your mouth, through your intestines, and all the way to the other end.

-When I think of the gut, I think of three things -- I think of an engine, I think of a tunnel, and I think of a strainer.

It's the engine because it really is where the food comes in, and it gets processed and digested, and then it goes out to feed all the other parts of the body.

It's a tunnel because when you think about it, it's a hollow tube that divides the body, so when something is in your gut, it's excluded from the rest of the body.

And I like to think of it as a strainer.

You dump something into a strainer, and the liquid goes through into your little pot, whatever you're collecting it in.

And then all the debris that's left gets dumped out.

-The liquid that goes through the strainer is glucose.

And for an ultra-marathon runner getting that fuel is all about timing.

♪♪ -Running, especially long-distance, is a really tricky challenge when it comes to taking in the right amount of food and water to make sure you have the sugar needed to keep moving without eating too much to the point where you start cramping because you can't properly digest.

♪♪ -So, this is what we call blue corn mush.

In Navajo, it's called Tóshchíín.

It's like a porridge.

The corn itself is really easy on the stomach, and then we add juniper ash to the roasted corn, and then with the ash mixed in, it is a really good source of energy that'll last for quite a long time.

♪♪ -Starches like corn break down into glucose in the gut, and getting that sugar before a run is crucial for Shaun.

But it doesn't happen instantly.

Luckily, a while ago, humans discovered how to jump-start the breakdown before we ever take a bite.

♪♪ -We don't know exactly when hominids started using fire.

You know, it could have been as long ago as 2 million years ago, 2 1/2 million years ago, perhaps longer than that.

♪♪ But one of the major things that it did for us was to allow us to cook our food and to unlock more nutrients.

-Cooking releases about 80% more energy than raw food.

♪♪ That's great news for an endurance runner, especially because exercise actually makes digestion slow down.

♪♪ -Running and digestion are both really active processes in the sense that they both require a lot of blood flow.

So somebody who's an endurance runner, a lot of blood flow gets diverted away from other parts of the body and, in particular, the digestive system and gets diverted to the large muscles in the body.

♪♪ -One of the things I had to learn really early in running these distances was to be able to eat enough to have enough energy and fuel during the long runs, but at the same time, not too much to feel heavy and bogged down.

Scoop in there.

♪♪ When I have blue corn mush before a morning run, I like to have it at least an hour before just so my body has time to digest a little bit and absorb some of the nutrients.

Yep.

That's what I was raised on.

♪♪ -Digestion really consists of six distinct phases.

The first is ingestion -- just taking the food in.

♪♪ -When you eat, mouth saliva breaks food down and binds it into a single mass, ready to be swallowed into the esophagus.

♪♪ Through a process of relaxing and contracting muscles, the esophagus pushes the food on its way.

The pulses are so powerful that you could hang upside-down and still move food against gravity into the stomach.

♪♪ The stomach is basically a vat of acid, so strong it can dissolve metal.

The juices kill off dangerous bacteria, and enzymes turn the solid mass into liquid, and then it's onto the small intestine.

♪♪ It's here where glucose is finally extracted and then absorbed into the gut's lining, where it then enters the bloodstream.

♪♪ Now it is ready to refuel the body.

But glucose can't reach hungry cells all by itself.

It needs to be carried through the blood by a hormone called insulin.

♪♪ -Insulin is hugely important in glucose metabolism.

What do we mean?

When you're eating, you need insulin to come out and ferry that sugar and those carbohydrates where they're needed to convert it to energy.

Most of us know someone or have heard of someone with diabetes.

Those are people who have problems with their insulin system, so they have trouble controlling their sugar levels.

♪♪ -But how does glucose actually become fuel?

Well, your cells have doors, and insulin opens those doors.

♪♪ Once the glucose is inside, it burns into raw energy and heat.

Usually that requires oxygen.

But some cells, like muscles, can do it without... ♪♪ ...which tells us that this chemical process evolved even before the atmosphere had oxygen.

♪♪ Under normal circumstances, insulin keeps the amount of sugar in your blood in range, and levels drop as glucose gets burned for gas.

But for a marathon runner, things work a little differently.

♪♪ -During exercise, there is the suppression of insulin which lowers blood sugar so that we see in people who are doing intense activity, their sugar levels can rise to almost diabetic levels, because their bodies know that they need that sugar to do the intense activity.

-And on the flip side, the cells of elite endurance athletes are also much more sensitive to insulin -- three times more, in fact.

♪♪ Glucose floods in so an ultra-marathoner's body can get more raw fuel to cells faster.

During an ultra is one of those times muscles have to burn glucose without oxygen.

♪♪ When you're out of breath, you can bet your cells are, too.

The upshot is that you can keep running, but there are byproducts.

As you push on, lactic acid builds up, which means soreness.

♪♪ At a certain point, even the most efficient bodies start to break down.

-As I'm running, I'm paying attention to overall effort, how my body's feeling inside, what the muscles are feeling like, little aches and pains, tightness all over the body.

As you go further in an ultra-marathon, those little, tiny aches and pains have the potential to turn into something bigger that could ultimately end your race early.

♪♪ -All runners have felt muscle aches and exhaustion.

[ Slow-motion panting ] But marathoners have to face hitting the wall.

♪♪ It usually happens at about mile 20.

That's when the body's natural glucose reserves are totally dried up.

The gas tank is empty.

♪♪ A fog settles in, and focusing becomes increasingly difficult.

Some runners get cramps, and others just can't continue the race... ♪♪ ...unless they refuel.

♪♪ -About every 15 to 20 minutes, I like to take a gel or blue corn mush.

♪♪ -Every body is different, and some people feel that during the run, they need to take a small amount of carbohydrates or sugar as they're running.

Others get cramps from doing it.

Some people get diarrhea.

It all depends on the body.

-It's not about replacing calories.

It's about getting sugar into the blood as quickly as possible to start reviving those overworked cells.

The trick is to refuel before you run out of gas.

♪♪ Our bodies can be pushed to extremes as long as we know how to support them.

♪♪ The record for longest run stands at 350 continuous miles.

♪♪ -When I finish an ultra, I usually feel really good.

As a Navajo distance runner, it's an enormous amount of connection -- thinking about our loved ones, thinking about our ancestors, and it's very emotional.

♪♪ [ Indistinct conversations ] -Chances are you don't eat with the same precision as an endurance athlete.

Most of us eat when we're hungry, a few times a day.

But is inhaling food every few hours actually what's best for us?

♪♪ -Today, we have access to foods year-round that we wouldn't have had access to through most of our evolutionary past.

♪♪ Our bodies are adapted to dealing with periodic scarcity that we don't encounter now.

-We evolved in a world where sometimes you just didn't catch dinner, or frost killed the plants.

♪♪ And obviously, we survived.

♪♪ Hunger is a part of our DNA.

♪♪ So while long-term starvation is bad, choosing not to eat sometimes might actually be good.

♪♪ ♪♪ -Food is a huge part of my life in every way.

It's sustenance.

You know, it keeps me going, and it keeps me alive.

Food is also a part of my work, and food is also meditation for me.

♪♪ My name is Deepti Sharma.

I am a practicing spiritual Hindu.

♪♪ Growing up, Hinduism was a pretty big part of my life, so I fast as part of my religious practice.

I've tried to incorporate those different beliefs and rituals into my everyday life today.

[ Match strikes ] [ Indistinct conversations, chanting ] Hindus fast for a lot of different reasons.

♪♪ It's a form of discipline.

It's a form of cleansing, to remember family members that have passed away.

I grew up fasting for those reasons, and I continue to because I believe it's just a wonderful tradition.

-Fasting is a test of will.

♪♪ That's probably why it's part of so many religions.

♪♪ But it also has powerful physical effects inside the body.

-Digestion uses up a lot of resources from the body, a lot of blood flow.

So when you fast, you basically allow your digestive tract to take a break.

-And it benefits the rest of the body, too, because of our evolutionary past.

♪♪ -Research actually suggests that fasting might help our cognition, memory, ability to learn, and be better for our bodies.

♪♪ -If you're eating three meals a day, your body relies on glucose for energy.

Whatever isn't immediately used by cells goes to the liver.

It's the body's energy reservoir.

♪♪ When you're not eating, the liver burns that sugar and releases it into the blood.

But when you fast, all that changes.

After about eight hours of no food, the liver runs out of sugar to burn, so you got to tap into a new resource -- fat.

♪♪ Under the skin, around internal organs, and in between muscles are pockets of fat.

♪♪ Fat cells travel to the liver, and it breaks them down into acids called ketones.

♪♪ Ketones can be used in the same way as glucose by the brain, blood, and muscles to power on.

And they're actually a more efficient source of energy.

♪♪ -Fasting seems to promote an increased production of a protein called brain- derived neurotrophic protein, and this seems to be something that increases new cells being laid down, both in the digestive tract, as well as in the brain.

♪♪ -I'm a working mother.

I'm an entrepreneur.

My schedule is all over the place, as in each day is very different for me.

♪♪ So, a day of fasting for me is I usually try to eat one meal a day.

If in between, I feel sluggish or exhausted, I try to consume a piece of fruit, and I drink water throughout the day, because, for me, it's really hard.

-What Deepti does is called intermittent fasting, and it can take a couple different forms.

Some people don't eat for a full 24 hours.

Others just seriously limit their calories once or twice a week.

-I try to adapt to what works for me and my lifestyle, because it's important to take care of my body, my health, my sanity, most importantly.

♪♪ As I am spending time with other people talking to them about work, what I do, and what they do, I started a company, FoodtoEat, where we partner with immigrant-, women-, and minority-owned restaurants in New York City.

We love coming into restaurants, getting to know the owners.

Like, where are you guys from?

Like, why did you start this restaurant?

We essentially help these local restaurants be able to book more catering opportunities at large corporate offices.

So, our clients are, like, The Skimm, Microsoft, Warby Parker.

-Wow.

-We love to get them to think about diversity and inclusion.

But before we onboard these restaurants, we taste their food.

-The first dish is a soup dumpling.

-Wow.

Wow.

-Wow.

Look at that.

-That looks amazing.

Those look so good.

But I will not be eating today, because I'm Hindu, and I'm actually fasting.

-Wow.

-Yeah.

♪♪ -The fact that fasting is mostly mind over matter is particularly tough, considering how hardwired that connection is between our stomach and our brain.

-Digestion begins long before the food actually hits the gut.

In fact, the thought of food or the smell of food can actually stimulate the release of digestive enzymes.

♪♪ -When we see something appetizing, receptors in the brain fire signals for the mouth to start producing saliva to help break down that food before we even swallow it.

♪♪ Within our saliva is an enzyme that kick-starts the chemical part of digestion, preparing the body for the task ahead.

♪♪ -So I always bring somebody along with me who gets to enjoy the food more than I do, I guess.

-Give feedback.

-Yeah, give feedback.

-I'm eating for two.

-Okay.

-Where should I start?

-Here you go.

-There you go.

Wow.

♪♪ -I'm so hungry.

I haven't eaten anything.

This is definitely torture.

[ Chuckles ] -The stomach is really fascinating in how it can tell your brain, "I'm full," or, "I'm hungry."

Our brains actually get a signal when our stomach is stretched out saying, "Oh, my gosh.

I'm full.

Don't eat any more."

And on the other hand, when our stomachs are empty, this also triggers our brain in saying, "Eat and fill me up."

Our guts can directly affect our moods and vice-a-versa.

♪♪ -While scientists have connected fasting with an increase in mental flexibility, they've also found an increase in hanger, the irritable, hot-tempered feeling that comes along with hunger.

When we aren't eating, and glucose levels drop, adrenaline and cortisol levels rise.

♪♪ These chemicals in the brain that trigger for hunger are the same ones that trigger for anger, rage, and other emotions.

♪♪ -We refer to the gut as the enteric nervous system, and we have about five times as many neurons in the gut as we have in the spinal cord.

90% of serotonin, the feel-good hormone, is produced in the gut, and we know that many other neurotransmitters are also produced in the gut.

So it really has a whole separate system in there that is helping to regulate mood and feelings, et cetera.

-I can't wait to come back and to eat for real, 'cause that looks great.

When I'm fasting, my body definitely feels it.

There's a difference than when I'm stuffing my face.

[ Chuckles ] My body does feel a little sluggish and a little weaker.

It's a lot of discipline of, like, what you're able to push yourself through.

♪♪ ♪♪ But there is this energy, this power that comes from within.

♪♪ -The fact that a lack of fuel might make you healthier isn't the only irony lurking in the gut.

♪♪ It's also home to a giant population of bacteria... ♪♪ ...that, instead of making you sick, might actually be the key to a strong immune system.

♪♪ See, despite being in the middle of our body, the gut is our most direct connection to the outside world.

Food brings with it pieces of nature, and nature makes itself at home inside us.

There's a symbiotic relationship between our own cells and foreign ones, an alien universe that runs right through us.

♪♪ -The microbiome essentially describes the bacteria living in our gut.

Scientists estimate we have over 10 trillion bacteria in our gut, which are playing a great role.

They help us break down food, reabsorb nutrients and vitamins, and even play a really critical role in keeping out the bad bacteria.

-The gut is a diverse environment where our own cells, food from outside, and the bacteria that live within us all coexist.

♪♪ And because bacteria and human cells are constantly touching, it's a perfect place for the body to learn what's safe... ♪♪ ...and what's dangerous.

-Where is your immune system?

It's in your gut.

-The thin mucus barrier that protects the gut is home to 80% of all the immune cells in the body.

As we digest, these cells learn to tell the difference between friend and foe.

♪♪ They know that what we eat is fuel, even though it comes from the outside.

And anything that can hurt us provokes a response.

♪♪ -An intact intestinal membrane is really essential for protecting us from toxins and keeping us healthy.

-But when these cells malfunction, fuel can suddenly become poison.

♪♪ -Hey, guys.

-Hi.

[ Laughter ] -Okay, I have some cream cheese and carrots, your favorite.

-Oh, yay.

-Do I use this for that?

-Mm-hmm, you can dip it.

It's all allergy-friendly.

Is that good?

[ Laughter ] -The carrots are really hard.

[ Laughs ] -My name is Ava, and I'm 11 years old.

I'm allergic to dairy, wheat, soy, fish, shellfish, nuts, tree nuts, and I don't know if I said egg, but egg.

It is kind of scary to have all these allergies, but, like, as long as you stay away from them, they can't really hurt you.

I can be near dairy.

I can be near nuts.

I just can't eat it or touch it.

♪♪ -For a variety of reasons we don't quite understand, some people are more predisposed to having allergies than others.

♪♪ What happens in these people is the immune system will see certain types of food as dangerous, attach onto it, and trigger this huge cascade.

♪♪ -Whenever I have dairy or some other allergens, then I'll probably start vomiting, and I have to stay home from school, but I'm anaphylactic to nuts, so they could potentially kill me if I didn't get the right treatment time.

♪♪ It's really terrible when you go into anaphylactic shock, 'cause you start, like, grabbing at your throat.

It's like you're underwater, but you can't get out.

♪♪ -The health of the G.I.

tract is one of the main factors that's involved in allergic reactions and why we're seeing so many people who are having food allergies and food sensitivities.

♪♪ -An allergy is basically a case of mistaken identity, where the immune system sees food as toxic.

♪♪ There are a lot of different immune cells involved in an allergic reaction, but there's one special type of cell that starts it.

They're called dendritic cells.

♪♪ And they live on the top layer of the gut's lining.

They're the messengers of the immune system, and their job is to spot threats.

♪♪ When you eat something, dendritic cells eat, too, in a way.

They try a piece of what's passing through to find out whether it's safe or not.

♪♪ Underneath this outer layer, deep in the gut's lining, are other immune cells that can be activated fast.

♪♪ Let's say these cells absorb milk or a nut and decide it's dangerous.

♪♪ They raise the alarm for other cells to go to war.

♪♪ They release chemicals that attack the food.

One of the most common is called Immunoglobulin E. -Now, IgE historically was used to hunt down parasites in our body.

But in some people, for whatever reason, the IgE will see food as foreign, which can cause all the symptoms we see with food allergies.

-You may throw up or break out in hives -- classic signs of the body trying to get rid of poison.

♪♪ When the reaction happens once, it will likely happen again.

Often, it starts early on.

-When Ava was born, she had a lot of feeding issues.

She would vomit every thing that ever went into her mouth.

-We know that one of the contributors to developing food allergies is whether we've been exposed to a lot of germs early on.

♪♪ -From the moment we're born, bacteria starts putting down roots in the gut.

-Breast milk encompasses the exact ratio of protein, carbohydrates, and fat needed to give Baby nutrition, but it also contains antibodies to promote healthy gut bacteria growth and to prevent the growth of some bad bacteria.

-Our infant immune system interacts with these bacteria, and if all goes well, this training makes it easy for us to process fuel.

But if the body doesn't get exposed to enough germs, the immune system may never learn to fight the right stuff.

And instead, the body could end up attacking food.

♪♪ Without the trillions of foreign organisms living inside us, we're more susceptible to disease.

There are other effects, too.

♪♪ The microbiome is also directly responsible for keeping us energized.

-We are completely animated by our microbes.

We are really just the hive, but without our microbes, we are an empty hive.

The microbes are the worker bees that are doing all the work.

They're synthesizing the vitamins.

They're breaking down the toxins.

They are digesting the food.

-They also play a role in fat storage and helping us with our blood sugar.

So any alteration in our microbiome can actually increase our risk of obesity or diabetes, and we can alter our gut bacteria through lifestyle changes, diseases, antibiotics.

Research is still out on how we can go backwards, then, and fix our gut bacteria to make ourselves healthy.

-Alright, so, eat, homework, and then off to basketball practice.

-We kind of live through life feeling like there's this monster out there that could get her.

So in order to make life normal, we make sure that Ava has all the safe alternatives.

Take the flaxseed milk and mix it in with the rice flour.

-Whether allergies can be completely reversed is a question we have yet to answer.

But it's no surprise that the solution might lie in restoring balance to the microbiome.

But let's rewind.

♪♪ Now that we know allergies and the gut are connected, here's another question -- What hurts the gut in the first place?

♪♪ -We're seeing so many people who are having food allergies, and one of the main factors is we're exposed to so many things that really are microbial disruptors, that can affect our G.I.

system and that healthy ecosystem of bacteria.

We are exposed to so many pesticides and chemicals through our food supply.

The average American child will have 18 courses of antibiotics before their 18th birthday.

That's a time when we should be really stuffing our kids full of vegetables and trying to get them to avoid antibiotics.

Our microbiome ages with us as we age, but the good news is that it becomes more resilient.

-Oh, that looks so good!

I hope they make more foods for me, like coconut whipped cream.

It's delicious.

-Oh, that's a big bite.

[ Laughs ] What makes me hopeful is that she's capable.

As much as she's tempted, she won't try food that may make her sick.

-I double-check the labels for all the food and just make all safe stuff.

That way, I'm not hurt at all.

♪♪ -Most broad-spectrum antibiotics will remove 1/3 of your gut bacteria.

♪♪ So it's such a balancing act trying to figure out how can we establish practices that can keep us safe from diseases, but at the same time, not going too far so that we're endangering our health and our microbiome because we have super-sanitized ourselves.

♪♪ -But a sanitized environment doesn't totally decide your gut's fate.

♪♪ We're also learning that it's much more dynamic than other parts of the body.

-Based on a variety of factors, from environmental and lifestyle factors to genetics, our gut microbiome is changing constantly, all day, every day.

-It's like a microbial impression of your past and present, more unique than a fingerprint.

And it's not just bacteria.

There might be upwards of a trillion viruses roaming through our bodies at any given time.

The microbiome is a living, breathing organism, much like a garden, which means that we can cultivate it.

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ -There are definitely some negative processes in the outside world that I think parallel what's going on in our digestive systems.

Just as we're destroying the external environment, we're destroying our internal environment by being over-medicated and not paying attention to the food we're eating.

♪♪ -The microbiome is like a moss that sits on top of the lining that separates the gut from the rest of the body.

♪♪ It's one of the most diverse ecosystems on Earth, controlled by more than 300,000 genes.

♪♪ Human DNA by comparison comes from a pool of only 20,000 genes.

♪♪ But if we deprive our microbes for too long, the moss goes barren... ♪♪ ...and bad bacteria spread.

♪♪ That makes it easier for other toxins to slip into the body.

The same goes for farmland.

When the soil is depleted by deforestation and pesticides, disease creeps in.

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ -So how do we restore a landscape that's been damaged?

♪♪ -The really optimistic message about the microbiome is we can change it.

I like to think about gut health as sort of threefold -- remove, replace, and restore.

Remove all the things that are potentially causing problems, and that would be a lot of medications that are not absolutely necessary.

Replace -- What can we do to replace gut bacteria?

We could eat fermented food.

We can roll in the dirt.

We can get a dog.

We can open a window.

We can be exposed to nature.

And lastly, restore -- How can we restore our microbiome?

We really get most of our gut bacteria from the food we eat.

In conservation ecology, rewilding means reintroducing species into a landscape where they've become extinct, and we can apply those same principles to our gut.

-Whether they're in your gut or in the ground, good bacteria need fuel to thrive.

On Georges' land, that fuel comes from an unlikely source.

-The same fundamental building blocks of the physical world that make up the food that ultimately becomes the energy that fuels our mind and body are also in our waste products.

-All organic matter can be turned into energy.

It's a cycle, which means that waste can become fuel... ♪♪ ...under the right conditions.

♪♪ -For most of us, stool is toxins, waste matter, passing out of the body, and it's mostly dead red blood cells and bacteria.

And it's the dead red blood cells that tend to give it that brownish, nice, beautiful, chocolate color.

-And so what we do at SOIL is we try to think about, how can we use the example of nature to create a system whereby the nutrients that we consume, instead of going into the water, where they pollute aquatic ecosystems, actually get back onto the soil so that they can produce more food?

And we do that by taking human poop and turning it into incredible, fertile soil.

♪♪ -Even though it's organic matter, if it's not treated properly, human waste can be deadly.

[ Car horns blaring ] -99% of human waste in Haiti is going untreated into the environment.

Those pathogens are making their way into the water that people use to drink, and that's why waterborne disease continues to be one of the leading causes of death in Haiti.

But if you know how to treat it well, then you can actually transform it and harvest the amazing nutrients that it has within it.

♪♪ ♪♪ ♪♪ -Human waste on its own would not reach the temperatures to treat the pathogens, but when you mix it with agricultural waste, that's when you get that perfect mixture.

-Over time, the waste breaks down into compounds that can be put back into the ecosystem.

-If you really want a healthy forest environment, the best way to do that is to actually leave the forest alone.

How can we sort of stand back and allow this ecosystem to really blossom on its own?

♪♪ If you want a really healthy gastrointestinal track, the best thing to do is to really limit what you're putting in there and limit it to food.

♪♪ -The health of Georges' crops is actually more connected to your own gut than you might think.

♪♪ Because just like plants need good soil to thrive, the microbiome needs plants.

♪♪ -Healthy bacteria like a lot of plant fiber.

And plant fiber is mostly indigestible, and it's indigestible because it's really not there to feed us.

It's there to feed our gut bacteria.

When the fiber gets broken down by gut bacteria, it's fermented into substances that are really important for the health of the gut.

♪♪ -After they break down fiber, gut bacteria release something called short-chain fatty acids -- basically microbial poop.

Short-chain fatty acids strengthen the lining of the colon.

This not only prevents disease, it helps the departure of our own waste.

So you could say that the gut is doing its own recycling.

-What does a healthy gut look like?

A great example is a bacteria called Faecalibacterium prausnitzii.

It's protective against colon cancer, heart disease, and diabetes.

But you can't just sort of transport them in.

Just like when you're trying to rewild land, you have to figure out, how do I create the right environment to really encourage a growth of these indigenous species?

If you're not feeding the F. prausnitzii a high-fiber plant diet, they're not going to repopulate and recolonize your gut in a sufficient number to be helpful.

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ -I like to sometimes say not we are what we eat, but rather, we are what our cells eat.

I think it would be healthy for us as humans to think about what they need.

When we think about what it takes to be a good gardener -- watering those plants, giving them sun -- we forget that our own bodies are also like a garden.

-[ Chuckles ] ♪♪ -What we know about this system is changing more rapidly than any other.

♪♪ -What excites me about our gut is that it's almost a symphony... ♪♪ ...between a half-dozen organs, a ton of bacteria, hormones, and chemicals.

It's more than just breaking down and digesting food.

If you strip away everything, we all feel the exact same thing.

♪♪ We go through the exact same physiologic struggles.

-It's more than just an engine.

It's a unique footprint and an entire universe.

-It is influenced by so many things -- by the food we eat, by the places we've lived, the experiences we've had.

In fact, we refer to the gut as the second brain.

It reflects everything about us.

It's the hub.

It's the center.

♪♪ ♪♪ ♪♪ -To order "Human: The World Within" on DVD, visit shopPBS.org or call 1-800-PLAY-PBS.

♪♪