Nova (1974–…): Season 47, Episode 6 - The Truth About Fat - full transcript

The mysteries of fat and its role in hormone production, hunger and pregnancy.

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It's one of the largest organs
in your body

you can't live without.

Yet many people desperately
want to lose their fat.

But fat's been given a bad rap.

We think of fat as being
an evil substance,

but fat is life.

Fat is critical to our health.

It is releasing a
whole host of hormones

important for our bones,
for our brains,

for our reproductive organs.

Fat is actually a very
sophisticated endocrine organ.



For millions of years,

people who were able
to hold on to their fat

had a selective advantage.

And now all of a sudden,

we've asked people
to get rid of their fat.

But our bodies never
evolved to do that.

So, what happens
when we have too much fat?

Diabetes, cholesterol issues,
heart issues.

It just sneaks up on you.

It's like smoking cigarettes.

It kills you slowly,
you don't feel the pain.

I was aware that
I was gaining weight,

but I didn't know
what to do about it.

I followed every single thing
my mom would say.



I exercised for
literally an hour daily.

Do we control our fat?

Or does it control us?

Scientists are finding
hormones and genes

that act on the brain to
influence our size and shape.

And those biological processes

are well beyond willpower.

The implications are huge.

Obesity is the number one
chronic disease

that we're dealing with.

How can science weigh in

on our weight problem?

The pieces are starting
to fall together...

Signals coming from the body,
the brain, and the genes.

It's very complex, but that's
exactly what you would expect

for a system which is
so critical to survival.

"The Truth About Fat"...

Right now, on "NOVA."

♪♪

This is Hiroki.

Weighing in at
around 550 pounds,

he's about to take on Yama,

a two-time winner of the
Sumo World Championship.

Yama is the heaviest
Japanese man in history.

Overeating is part of his job.

Everybody say, "Yama"!

Yama!

While an average man eats
around 2,500 calories a day,

sumo wrestlers consume
up to 10,000.

Because the heavier
you are in sumo,

the harder it is
to take you down.

For years, doctors wondered why
sumo wrestlers, who stay active,

rarely suffer from conditions
triggered by obesity.

Answers would come

as new insights radically
changed our views of fat.

♪♪

Most of us think of
our fat as our enemy.

It's not, it's our friend.

We need our fat to store
any extra calories

so that we can use them
in the future

to keep our heart beating,
to keep our brain working.

And what's we've been learning
over the last 20 years is that

the fat cell isn't just
simply a storage organ.

It's a highly intelligent cell
talking to our brains.

♪♪

So why did body fat
evolve in humans?

To understand the forces
that shaped us,

a team of scientists have
come to Tanzania.

They've set up camp near some
of the last hunter-gatherers

on earth, called the Hadza.

♪♪

The human lineage is
seven million years old.

And for the last
two million years of it,

we've been hunting
and gathering.

That's the key innovation
of the human species.

And the Hadza are
a modern population,

but they keep
their old traditions

still very much alive.

And so, this landscape
and this lifestyle

is a really good window
into our own past.

For over a decade,

Herman Pontzer and Brian Wood
have been studying the lifestyle

and diet of the Hadza people

to see what it might teach us
about modern diseases.

The Hadzas are great models
in public health.

They never get heart disease,

they never get diabetes,
they're not obese or overweight.

And the diseases that
we're most likely to die from

in the U.S. and Europe are
not an issue with the Hadza.

If we look at the Hadzas' level
of physical activity,

it certainly is quite high

when you compare it to
Western, sedentary societies.

They cover more land per day
in their travels.

They spend more of their day
moving around.

And we know that high levels
of physical activity

are certainly protective against
major sources of mortality.

With no livestock or crops,

Hadza men and women
must forage every day.

Bahati digs for tubers,
rich in carbohydrates and fiber,

while other women collect
fruit and baobab seeds.

Boys scramble up trees
to find bees' nests

and harvest their honey.

But the richest source
of calories...

And the hardest to acquire...
Are the meat and fat of animals.

♪♪

Hunting is a risky endeavor.

And that's one of the biggest
gambles that you have to take

in this foraging economy,

is you have to get up in
the morning and go hunting,

knowing full well that
the odds are against you.

Hadza hunters, like Dofu,
use poisoned arrows

to shoot their prey,

but the chase can take hours.

Hunting and gathering
takes energy, a lot of it.

And so, two million years ago,
we start burning more calories,

we get bigger brains.

But the catch is that that

puts us at risk of
starving to death.

'Cause if we're burning our
engines hotter and faster,

then there's that risk that

you're going to run
out of energy.

And so we've evolved fat
as the safety net.

When a man has been lucky enough
to kill a large animal,

the first thing he usually does
is to pinch its skin

and to see how much fat
is on it.

Because that's such a key
motivator for their hunting.

And there is no other species

that extracts fat as efficiently
from the landscape

as we do.

♪♪

Our body breaks down food into
two main sources of energy,

glucose and fat.

Glucose provides immediate fuel

or can be stored as glycogen
in our muscles or liver.

As glycogen is depleted,
the body burns fat,

which yields, per pound,
twice as many calories.

And while humans can only store
about a day's worth of glycogen,

we have enough body fat
to survive for weeks.

So, think of fat as our battery.

One of the big misconceptions
about human evolution

is that we evolved
to be healthy.

And the answer is, "Not really."

The only thing that
natural selection cares about

is how many offspring you have

who survive and then produce
their own offspring.

So, our bodies are beautifully
adapted, exquisitely adapted

to take any excess energy
and turn it into fat.

Not because it makes us healthy,

but because it helps
our reproductive success.

♪♪

Fat has long been revered.

One of the earliest
representations of obesity

is a statue carved
by hunter-gatherers.

Scientists have speculated

that this voluptuous figure
symbolized fertility.

Over the centuries,
we've had a wide range of views

of what the optimal body shape
and the optimal body fatness is,

right, from the
Rubenesque period through

to the flappers of the 1920s and
the heroin chic of the 1980s.

So, across time, our views

of what is an attractive
amount of body fat

has varied enormously.

Today, as we have
stigmatized obesity,

we have demonized fat.

But the ability to be able
to store fuel,

so that you can get away
from your food supply

for extended periods of time,

allows you to engage
in complex behaviors

and the development
of the complex cultures

that humans have.

So, fat is a feat of evolution.

It frees us from the tyranny
of having to eat continuously.

♪♪

Fat cells, called adipocytes,

look like bulbous spheres
beneath the microscope.

Inside are droplets
of triglycerides,

fats that our body can burn to
release vast amounts of energy.

Luckily, fat cells can expand
far beyond their normal size

to safely store
our excess energy.

So, what would happen
if you didn't have any?

Troy Fryer has less
than two percent body fat.

Although he looks extremely fit,

it's the of lack fat that
makes his muscles stand out.

♪♪

With no padding on his face,

his cheeks look sunken
and his eyes are deeply set.

How far are you off that way?

The little bit of fat on my body
is behind my eyeballs

and on my liver.

So walking around
not having fat,

it feels like constant needles

are poking through the
bottom of your shoe.

You know some us stand up
all day, and we're okay with it.

Troy is not.

Because he has no padding
on his feet,

nothing in between his knees.

There's no comfort at all.

It's just unbelievable
to watch him struggle.

Troy was born with
a normal amount of fat.

But by age six,
he began rapidly losing weight,

despite having
a voracious appetite.

I would go through two or three
loaves of bread in one day,

making ten sandwiches at a time.

So, I would keep eating
and eating and eating

until it would hurt.

And if I didn't get that food,

I would get really upset
and really angry.

I would rip doors
off their hinges.

I actually sent him in
for a psych evaluation,

thinking that there
was something wrong

because he couldn't
take the answer "No."

It's like he was starving.

By age nine,
Troy was clearly ill.

Doctors were shocked to find

that his blood was full
of fat and cholesterol,

symptoms typical of obesity.

It made no sense,
until Troy was diagnosed

with a genetic disease
called lipodystrophy.

We always think about
thinness as something good.

But, in generalized
lipodystrophy,

it's beyond thinness.

It's actually absolute

lack of fat under the skin.

So, the excess energy
doesn't have a place to go.

And that's why Troy was sick.

With no fat tissue, excess
calories collected in his liver,

enlarging and inflaming it.

And when it got to that point
of the doctor saying,

"There's nothing else
we could do for you,"

Troy turned to
the doctor and said,

"So how and when
am I going to die?"

Salvation would come
from the discovery of a mouse

that also couldn't stop eating.

But unlike Troy,
this mouse was fat, not thin.

♪♪

A genetic mutant
from a breeding experiment,

it was nicknamed O.B. for obese.

This mouse had a defect
in a single gene.

And the impact of that gene was

a mouse that weighed
three times normal

and had five times as much fat

and that overate voraciously.

And genetics is very powerful,
because what it tells you is

that that obesity has
a biological basis.

What that basis is
required identifying the gene.

Scientists began to hunt

for the mutation
that made the mouse obese,

combing through the 2.5 billion
letters of its genome.

Think of it as an alphabet.

You spell out letters
of the genome.

There are four letters,
A, G, T, and C.

These spell out...
Indirectly... proteins,

and a single spelling error
can lead to a defective gene.

♪♪

In 1994, after a decade of work,

Friedman and his collaborators
homed in on a gene

only found in fat cells.

♪♪

That was really the
moment of a lifetime.

I pulled out the film
with some vague hope

that maybe this would
reveal something

about the nature
of the O.B. gene.

And I looked at it,
and in that instant, I knew

that we had identified the gene

that makes a hormone and
that plays a very active role

in regulating appetite,
metabolism,

and probably other
biological systems.

So if you injected that hormone

into the blood of an O.B. mouse,
the mouse lost weight.

It would completely cure
the mutation of a O.B. mouse.

And over the course
of a few weeks,

depending on the dose you give,

they’ll look indistinguishable
from a normal mouse.

The hormone was named leptin

from the Greek word leptos,
meaning thin.

Its discovery transformed
our view of fat

and the biological forces
controlling appetite.

The O.B. mouse cannot
see itself in a mirror

and realize that
it's hugely obese.

It thinks it's
starving to death,

because this
very critical hormone

is not being made by the body.

And the idea that
a hormone produced by fat

can control what
you think about food

in a very important way,
this is a very radical notion.

♪♪

Growing up on the
outskirts of Chicago,

Sana Mahmood never
suspected hormones

might be driving her hunger.

♪♪

I just thought I didn't
have enough willpower in me

to control myself from food.

No matter how much I'd eat,
I was starving all the time.

From infancy,
Sana was overweight.

Her parents thought she
would soon lose her baby fat,

but she only grew heavier.

I'd exercise for an hour daily,

doing, like, kickboxing
or treadmill,

or just being as active
as I could.

But I didn't know
what was going on,

and I was getting scared,

and I would pack on like ten
or 20 pounds in a month.

♪♪

As Sana struggled with obesity,

she sought help
from Dr. Lisa Neff.

Okay, come on in, Sana.

Go ahead and have a seat.
Okay.

So this is what we call

an indirect calorimeter.
Mm-hmm.

What it's going to do is measure

how many calories
your body is burning in a day.

Just breathe normally.
Mm-hmm.

Sana had tried all different
kinds of diets,

exercise programs,
medications for weight loss,

and yet she had continued
to gain weight.

So, the very first test
that I did was a leptin level

in her blood,
and it was undetectable.

So, Sana has been fighting
hunger her whole life

and it's because she's lacked
this critical hormone.

Like the O.B. mouse,

Sana has a mutated leptin gene,

found so far in just
a few dozen people.

A normal fat cell
producesleptin,

which travels to the brain
and signals the hypothalamus,

a region that determines
when and how much we eat.

High levels of leptin
tell your brain

you have plenty of fat stored.

But low levels,
or in Sana's case,

no leptin at all,
triggers an alarm to eat.

First of all, you get
the hunger signal.

But secondly, you slow down
your metabolic rate.

And if your leptin levels
fall even further,

your body says you're starving,

and you do all sorts of things
to preserve your life.

You slow down
your immune system.

You turn off reproduction

so that you can survive
this period of starvation.

So the signals coming
from the body

to this bit of the brain
called the hypothalamus

are really so powerful.

Sana is now taking leptin

and has lost 40 pounds
in three months.

After taking the leptin,

I've noticed a huge change
in my hunger.

I can go four to five hours
without eating anything,

and I'll be totally fine.

When I think about
the future now,

I'm seeing clear skies
everywhere.

My confidence levels
have soared.

♪♪

And for those like Troy,

who can't make leptin
because they lack fat,

getting the hormone
would be lifesaving.

The starvation was gone within
three days of him taking leptin.

We saved a lot of money.

Troy went from eating
what three people would eat,

to eating what a normal person
would eat in a day.

You got a little bit
of blood on there.

Knowing that I was full,
I'm like,

"Wow, this is amazing."

Does it hurt?

Leptin can't cure
Troy's disease,

but by curbing his hunger,
it protects his liver.

Leptin does not bring
the fat back.

It just helps to deal
with fat's absence.

Patients finally
can take a deep breath

that they're full,

and they don't have
to worry about eating.

The discovery of leptin
was so exciting.

And the molecule entered into
clinical trials in human beings.

And we awaited the results
of those trials

with great anticipation,
because we thought, "At last

"we have something
that's going to cure obesity

in our patients."

That turned out
not to be the case.

Most obese people produce
plenty of leptin, a lot of it.

And in fact what we found out
is that giving more

to people who have plenty
doesn't have much of an effect.

♪♪

While leptin couldn't
cure common obesity,

it revealed that fat was not
just a reserve of calories,

but a complex endocrine organ,
producing dozens of hormones.

And those hormones are important

for our bones, for our brains,

for our reproductive organs,
for our muscles.

They're important
for everything.

And through these hormones, fat
can communicate with our bodies.

It can communicate
with our brains.

So fat has different roles
in our body

at different times of our lives.

Especially at birth, when
a human baby, on average,

has the highest percentage
of body fat of any species.

You know, humans really are
by far the fattest ape.

A typical human baby is
about 15% body fat.

A typical hunter-gatherer male
is about ten to 15% body fat,

a typical a hunter-gather female
is 15% to 25% body fat.

That's leagues
beyond any primate.

♪♪

Not only are we fatter
than other primates,

we also have bigger brains.

And compared to its body size,

a human baby's brain is massive.

And that brain is consuming half

of that baby's calories,
roughly.

That's a lot of energy, right?

And the baby can't
stop feeding its brain.

The brain doesn't hold
onto energy.

It's a constant,
thirsty, demanding organ.

And fat ensures that we always
have energy available

to pay for this thirsty organ.

So, when you see a fat, pudgy
baby, that's a healthy baby.

Because fat is life, right?

If you don't have enough fat,
you're at risk.

As we get older,
fat plays a role again,

because you need a
sufficient amount of fat

to have enough estrogen...
Fat produces estrogen.

In fact, girls have to gain on
average about 13 pounds or so

before they're able
to initiate puberty.

♪♪

Model Hartje Andresen
never thought much about fat

until she confronted
the pressure to be thin.

Will you send me the picture?

The photographer
who discovered me said,

"You know, you're perfect,

"you're beautiful
the way you are.

"But if you get into
the modeling industry,

"they are going to tell you
to lose weight.

So, my word of advice to you
is don't, don't do it."

♪♪

But for many models,
like Robyn Lawley,

that advice was hard to heed.

Women who walked the runway

had to wear size two
or smaller clothing.

It was that heroin chic look
at the time.

So, it would be, girls of
my height, like six foot,

in a size zero.

So, you had to be emaciated
to get that look.

You literally had
to starve yourself.

And if you didn't do what you
were told, you could be cut.

So, I tried,
I lost a bunch of weight,

maintained it all
for two minutes,

and then it all came
back on again.

Hartje lost the weight
an agency had requested,

getting down to 100 pounds.

But slowly, she began to realize

that being thin was
not necessarily healthy.

My hair had really bad quality

and my fingernails
were really brittle.

I would get cracked lips and
sores at the corner of my mouth,

but the point that
made me realize

that maybe I was
getting too thin

was when I started
breaking my ribs.

♪♪

Google "starvation,"
and that's what happens.

Like, you will lose your period,

you will cause
all kinds of diseases

that you never thought
would come to you.

When I was speaking to my doctor

about wanting to have a baby,

he actually warned me
not to be any thinner.

And even though
I was exercising a lot,

I was obviously not healthy.

If you don't have
enough fat stores,

one thing that is
affected significantly,

is that we develop
issues with bone,

which can then lead
to osteoporosis...

So, actual brittle bones

that have a high susceptibility
to breaking.

And so that's something that
people might not think about

as we're looking
for this aesthetics

of getting to that
Twiggy ideal body image.

At the other end of the scale
are sumo wrestlers.

Given their massive size,
how do they avoid diseases

normally associated
with obesity?

The answer lies in
where they store their fat...

Safely underneath their skin,

instead of in the
abdomen or chest,

packed around internal organs.

And so if you do a scan of
an active sumo wrestler,

a CT scan or an MRI scan,
you'll see all that fat,

but it's on the outside,
it's on the buttocks and thighs,

it's on the outside
of the abdomen.

So even though
they have body fat,

they don't contain a lot of fat

in those deleterious places
such as the liver,

around the heart,
around the pancreas,

and even around the kidneys.

So, they're
metabolically healthy.

And the key reason is that sumo
wrestlers exercise rigorously,

up to seven hours a day.

When we exercise,

our fat releases a hormone
called adiponectin,

and adiponectin actually helps
guide fats in the blood

into safe deposits of fat,

so they'll guide it
into subcutaneous fat,

right under the skin.

Interestingly,

when sumo wrestlers come off
their exercise regime,

they get metabolically
unhealthy very quickly.

To maintain a stable weight,

the number of calories you eat

needs to match
the number you burn.

So many assume
that losing weight

is simply a matter
of eating less

and exercising more.

But scientists are discovering
that it's not so simple.

So, what is astonishing
is the fact

that we ingest more than
a million calories a year.

And yet we don't oscillate
between supermodel

and an opera diva over
the whole period of a year.

We stay pretty stable.

So I think the best way to think
about how weight is regulated

is to think of it
as a set point, right?

That is, that our body
is defending

a very particular weight under
a certain set of circumstances.

And it's obviously not set

because the body weight
can change.

What is set is what is

the minimum body weight
for that individual.

If you drop your body weight
below this threshold,

the body will begin to do
whatever it has to do

to prevent your dying
of starvation.

♪♪

But few Americans grasped

just how hard the body fought
against weight loss

until a reality TV show
called "The Biggest Loser"

captivated audiences.

Yeah, I made it!

One of the heaviest contestants
on season eight

was Danny Cahill.

Yeah!

For Danny,

competing on the show was
a shot at salvation,

something he had longed
for since childhood.

♪♪

When I was a kid I was like,

if you could have one wish,
what would you wish for?

And you know, most people say,
"A million dollars,"

you know, or-or something.

I'd say, "I wish
I could be like that person

that eats the same as me and
seems to never gain weight."

As a child,
Danny tended to be heavy,

but by the time he was a father,
he struggled with obesity.

His doctor warned that
unless he slimmed down,

he might not live
to see his kids grow up.

I was mortified
at how big I'd gotten.

And it seemed like
it happened overnight.

It just sneaks up on you.

You don't feel the pain.

So, when you gain a half a pound

and a half a pound and
a half a pound every week,

well, in a year
that's 30, 40 pounds.

And in five years,
it's a 150 pounds.

Danny saw "The Biggest Loser"
as a second chance.

Look at that word,
"Believe," okay?

Cutting calories and exercising
around 45 hours each week,

he hoped to shed a pound a day.

Let's go.

Top it.

I went there with a purpose.

I went there to lose the weight.

I went there to break a record.

I went there to-to right my life

and to get the weight that I've
been carrying around for years

off of me.

♪♪

As the season progressed,
Danny pushed harder,

cutting back to
800 calories a day.

Seven months later,
when he stepped on the scale

for the final weigh-in,

he had lost a
staggering 239 pounds.

I was mentally exhausted,
I was physically exhausted.

In fact, looking back at it,
I go, "How did I do that?

"How did we do that?

That is crazy."

♪♪

Danny was determined
to keep the weight off at home.

He delayed returning to work

so he could exercise
several hours a day.

When his book tour ended,

he went back to his job
surveying land.

Moving less and
feeling constantly hungry,

the weight came back.

♪♪

Danny regained over 100 pounds,
and he was not alone.

Six years later, N.I.H.
scientist Kevin Hall found

that 13 of the 14
contestants he examined

had regained much
of their lost weight

or were even heavier.

♪♪

When you make extreme changes

to your diet or
physical activity patterns,

your body responds
very strongly,

both in terms of slowing down
the number of calories

that you're burning, as well as

increasing your appetite
and hunger.

And those types of processes
that are biological,

are well beyond willpower.

Close the door.

Research has revealed
that as dieters lose fat,

leptin levels fall,
triggering hunger.

In.

Out.

As Danny regained weight,

his leptin levels
should have rebounded.

But did they?

You started out kind of

more or less where
you would expect

for your amount of body fat.

But six years after
"The Biggest Loser,"

here's where you ended up.

And I was told that
that would tell your body

that you're starving
and that you needed to eat.

So, how am I going
to handle this,

being hungry all the time with
my hormone levels out of whack,

and is that ever going
to correct itself?

Thyroid hormones also fell.

And that's one reason

as contestants shed pounds,
their metabolic rate...

The number of calories they burn
while resting... dropped.

What was the surprise

was that six years later,

their metabolic rate was still
at the same level

despite regaining
all that body weight.

You started off
at zero, basically...

And compared to
other contestants,

after Danny's weight rebounded,

his metabolic rate
had fallen the furthest.

Your metabolism was about
800 calories a day lower

than what we would expect.

I was shocked.

My body burned 800 calories
less than a normal man

that was the same
height and weight.

And I just want
people to realize

that obese people
aren't just lazy people.

There are a whole lot
of things at play here,

and one of them is our biology.

So the vast majority
of humans fail on a diet.

And, from an evolutionary
perspective,

that makes sense because we
never evolved to lose weight.

As soon as you go on a diet,

that activates what's called
a starvation response

to help us hold onto energy.

The brain remembers
that set point.

The brain is a powerful organ.

And the hypothalamus is powerful
in knowing what our weight was.

And so the brain wins.

♪♪

There's another reason

why it's hard to voluntarily
control your weight.

It's obvious when looking
at identical twins,

that the shape and size
of their bodies...

And even their gestures...
Are remarkably similar.

That's because they share
100% of their genes.

♪♪

But that's not true
for fraternal twins.

They can differ
in gender and size,

because they only share
half of their genes.

So if you systematically compare

identical to
non-identical twins,

what you conclude is that

obesity is as or more genetic
than any trait

that's been studied
with the exception of height.

These twin studies have shown

that the heritability of obesity
is around 40% to 70%.

So, obesity is not just
about the environment,

not just about your lifestyle.

Your genetics determine

why some people gain weight
more easily than others.

Jump!

♪♪

To find genes that
influence obesity,

a global consortium
is analyzing genomes

at centers like
the Broad Institute.

♪♪

Over 1,000 genes
have been identified

that may play a role
in determining weight.

Most are for common obesity
and have small effects.

But not all.

The data, mapped
to 23 chromosomes,

is publicly available to
scientists like Ruth Loos.

As she homes in
on a specific location,

she can search
for individual genes.

To date, researchers have
identified eight different genes

which can cause severe
early-onset obesity.

And most of these genes,
they act in the brain.

They control food intake,
they control hunger,

satiety, reward...

Basically components
that we can think of

as controlling willpower.

And there are
other gene variants

that don't cause obesity
but may make carriers heavier.

One impacts leptin signaling.

If you carry that mutation,

then you would weigh
about 15 pounds more

than someone who does not.

And we see that
one in 5,000 individuals

in the general population
carries that mutation.

♪♪

But while genes can increase
your risk for gaining weight,

they can also protect you.

One gene variant, found in about
six percent of the population,

makes carriers always feel full
instead of hungry.

Now this variant
doesn't make you fat.

It predisposes
to making you leaner.

So, we need to understand
that there are people

who are very susceptible and
people who are very resistant,

that obese people are
not morally inferior,

they're biologically different.

Obesity is defined by
your body mass index, or BMI,

a calculation that divides your
weight by your height squared.

A BMI above 25 is classified
as overweight,

and higher than 30 as obesity.

In the 1980s, obesity rates in
the U.S. began rising sharply,

reaching higher than 39%
in many states.

Obesity by far

is the greatest public health
challenge of our time.

It is the number-one
chronic disease

that we're dealing with
here in the United States.

And it's leading to at least

a hundred different disease
entities that we know of.

So we will never

get ahead of diabetes,

we'll never get ahead of
cardiovascular disease,

we'll never get ahead of cancer

unless we address
the root cause.

And the root cause is,
in many cases, obesity.

♪♪

And it's not just
a U.S. epidemic.

In just three decades, obesity
nearly tripled worldwide.

According to the
World Health Organization,

one problem might be a decrease
in physical activity,

causing us to burn
fewer calories.

So is one solution
to simply be more active?

♪♪

Back in Tanzania,
Herman Pontzer and Brian Wood,

working with a team of
international scientists,

hope to find out.

They're giving members of the
Hadza community GPS devices

to measure how far
they travel each day.

GPS.

The answer might shed light

on why the Hadza don't suffer

from the chronic diseases
seen in the West.

Without antibiotics
and vaccines, sadly

a lot of kids, you know, don't
make it to their 15th birthday.

But if you make it to 15
in the Hadza population,

there's a great chance you're
going to live to be in your 60s,

even 70s and 80s,

and with a much healthier body

than we would often have
in the West.

GPS data reveals why.

Hunters, like Dofu,
walk about nine miles a day,

while women foragers
walk about five.

That's more exercise than the
average American gets in a week.

But do the Hadza people also
burn more calories each day?

I had spent many months
living with the Hadza.

I had gone with them
during the day of foraging

and come back to camp, and
I felt so tired and exhausted.

I felt of course the energy use
that we would be detecting

would be much higher
than in Western populations.

To find out, volunteers like
Dofu drink a special water

with hydrogen and oxygen
molecules that can be traced.

Uh-huh.

Over the next two weeks,

by tracking the depletion
of these molecules,

the scientists can measure
the number of calories

burned each day.

So we've come out here,
we collect all this data,

you know, putting urine samples
on liquid nitrogen,

shipping them to one of
the best labs in the country.

And as the results came back,

I wasn't sure what
I was seeing at first.

It's, uh, 77.

Despite being more active,

Hadza men only burn, on average,
2,500 calories per day,

and Hadza women about 1,900,

the same amount as an average
man and woman in the U.S.

It's a really
counterintuitive result,

and it really surprised us.

Somebody who is sedentary,
working a desk job in the U.S.,

is burning the same
number of calories

as a Hadza man or woman

who's so much
more physically active,

and that's even after
you account for things

like body size,
body composition, age,

gender, all of those factors.

The research suggests that no
matter what our lifestyle is,

our body protects us by keeping
the total calories burned

each day within a narrow range.

And that's why, in the long run,

exercise alone won't
make you thin.

I think what
the Hadza data tell us

is that we can change our
lifestyles however we want,

and more activity is
always better,

but it's not going
to burn more calories,

'cause our bodies adjust to
these more-active lifestyles.

The implications are huge.

If obesity is not due
to sedentary lifestyles,

it must be caused by
eating too many calories.

And according
to Daniel Lieberman,

that became possible as humans
changed their environment.

So in the last 10,000 years
we've transformed

both how we get energy,
actually, and how we use energy.

And the first big shift occurred
with the origins of agriculture,

when we shifted from simply
going out and getting food

out there in nature
to growing the food ourselves.

Farming allowed humans to grow
carbohydrate-rich foods

like corn, wheat, and rice.

And then we invented machines

and industrialized
how we grow food.

We've increased the scale
of calories that we produce

by orders of magnitude.

And the result is, for the first
time in millions of years,

we have more energy
than we know what to do with.

♪♪

And it's not just
an excess of calories.

Scientists suspect
that obesity may be caused

by the changing nature
of the foods we eat.

♪♪

More than 50% of calories
in the U.S.

are consumed in the form
of ultra-processed foods,

low in fiber, but full
of fat, sugar, and salt.

Packed with calories,
they've been labeled obesogenic.

But do they cause us to overeat?

To find out, Nora Volkow has
been scanning the brains

of patients with
and without obesity.

To me the most important aspect
about obesity

is understanding that

the food itself has made
changes in your brain

that are driving your
inability to stop eating.

♪♪

These changes begin

as food activates
our brain's reward system,

releasing a feel-good chemical
called dopamine.

Through pleasure,
dopamine motivates us

to find and eat rewarding foods.

Imaging reveals that
high-fat, sugary foods

can overwhelm the
brain's reward system,

flooding it with
too much dopamine.

Our bodies actually have evolved

to try to maintain
a homeostatic state.

And that means that
if there's too much stimulation

with dopamine, you start
to see that receptors

that are sensitive to dopamine

are down regulated,
they decrease.

♪♪

Volkow found that
dopamine receptors,

seen as red in the brains
of control subjects,

are reduced in people
with obesity.

As dopamine signaling goes down,

our capacity to be able to
inhibit desires goes down.

So, I say, "I'm not going
to eat the chocolate,

"or I'm not going
to eat the donut.

I don't want to eat it."

Can I stop it?

And if those areas of the brain
are not functioning properly,

no matter how much you want
not to do something,

it's very difficult
to carry through.

And this explains
why people will tell you,

"I did not want to eat the food.

"I knew I was going
to gain the weight.

Yet I could not stop myself."

♪♪

The truth about fat
is complicated.

Evolution has designed powerful
hormonal and neural signals

to ensure that we eat
and defend our weight.

I think what people really need
to understand about obesity

is that it's not your fault.

This is not a matter
of your willpower.

This is a disease.

It's a chronic disease.

It's a highly complex disease,

and there are many causes
of obesity.

So, can obesity be
prevented or treated?

The experts say maybe,

using a long-term approach,
focused on health.

I don't want to put you
on the next diet.

Diet implies
that it's short term.

I want to put you
on a lifestyle plan

that you can sustain for years.

In place.

While "The Biggest Loser"
reveals why diets fail,

contestants like Danny...
Who continue to exercise...

Kept off ten to 13%
of their weight.

And medically, that's a success,

because modest weight loss
has huge health benefits,

from lowering blood pressure
to preventing diabetes.

There're also medications that
signal the brain to feel full,

and help people lose
ten percent of their weight.

And if you want to lose
100 pounds or more,

there's another option,

one that Muriel Mena chose to
avoid the fate of her mother.

I was three, and I remember
she was talking on the phone,

and she collapsed because
she had a heart attack.

And then I just remember
E.M.T.s rushing in.

At age 44, Muriel's mom
weighed 300 pounds

and had died of heart disease.

As an adolescent, Muriel began
struggling with her weight,

just as her mother had.

I was going to the doctor's and
having to get my heart checked,

and I was like,
"Wait, this is weird.

Like, I'm 13 at a cardiologist."

It was always in
the back of my mind,

"Oh, this is what
she passed from."

By age 16, Muriel, like nearly
five million American teenagers,

had obesity, and qualified
for bariatric surgery,

which bypasses or removes
part of the stomach.

Bariatric surgery
is the only way

that most people
with severe obesity

can not just lose weight but
most importantly keep it off.

So you've been doing great.

The first thing
I want to show you is

275 was your highest weight.

Your body mass index was 43.

After surgery you went all
the way down to 172 pounds

and a BMI of about 28.

And that's really fantastic.

Originally we thought that
bariatric surgery worked

by making the stomach
much smaller,

so that you couldn't
eat a lot of food.

We now know that the hormones
that come from the G.I. tract

and go to the brain
change in a way

that cause satiety earlier,
sooner, and with much less food.

These signal changes
between the gut and brain

help the body reset
its set point for years.

But hunger can return,

because our weight is
as regulated

as our blood pressure
and heartbeat.

The ability
to control our weight

is distinctly outside
our conscious control.

People don't like to hear that.

We all want to think
that we're in control

of when we put the fork down.

But there are lots
of biological forces

that are controlling what
weight it is that you end up at.

Fat behaves differently
on all of us.

It has to do with our genetics.

It has to do with our age,
our gender,

a whole host of factors.

Everybody is very different.

And what matters is
to be healthy,

not to be perfect.

Today, Hartje Andresen
is 20 pounds heavier,

a mother, and modeling
on her own terms.

I had to overcome this fear
of gaining weight,

and that really took
some courage for me.

♪♪

Modeling has the possibility

to show people how a
healthy body should look like.

The focus needs to be more
on strength and health

rather than being
a certain size.

♪♪

And there's another insight
provided by the Hadza,

and hunters like Dofu.

What the Hadza tell us is that

diet and exercise are
two different tools

with two different jobs.

You need to exercise
to stay healthy

and to age well,

but you need to watch your diet

if you want to
watch your weight.

It's not a coincidence that

the obesity epidemic is
spreading around the world.

Wherever modern, Western,
post-industrial lifestyles

show up, obesity follows
with them.

But I don't think the solution
is to go back to the Stone Age.

I think the solution is to learn
from our evolutionary history

and get the best of both worlds.

♪♪