WEBVTT

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NARRATOR: What is the difference
between these athletes' muscles,

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these aging muscles,
and even these dancers' muscles?

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All are covered in skin,
cushioned by fat,

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supported by bone,
fed by blood vessels.

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But deep within our muscles'
cells lies an organelle,

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which produces most of
our body's energy,

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the mitochondria,
the powerhouse of the cell.

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Some cells have
many mitochondria;

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others fewer.

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Some mitochondria are intact;

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others damaged.

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And the number and condition
of mitochondria

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makes a huge difference
in all of our cells.

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Dr. Vamsi Mootha
does groundbreaking research

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on mitochondria.

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He explains how mitochondria
affect cells

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that make up muscle fibers.

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VAMSI MOOTHA, MD:
Our skeletal muscle

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actually has two different
types of fibers -

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dark meat and white meat -

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and sometimes called slow twitch
and fast twitch fibers.

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And depending on
what activity we're doing,

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we'll either use more
of the fast twitch fibers

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or more of the slow twitch
fibers.

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So during a sprint,

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we'd wanna use
the fast twitch fibers.

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Fast twitch fibers
need energy very rapidly.

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Slow twitch fibers,
on the other hand,

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have to be able to perform
for a long period of time.

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These slow twitch fibers
are dark

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because they're actually
packed with mitochondria.

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That's actually
what makes them dark.

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NARRATOR: Slow twitch fibers
have more mitochondria

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for more sustained energy.

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MOOTHA, MD:
As we do exercise training,

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we're actually telling
our muscle cells

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to produce more mitochondria
within our skeletal muscle.

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NARRATOR: How do these reserves
of mitochondria

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provide energy for endurance?

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- Each of the cells in
our body use something called

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ATP or
adenosine triphosphate

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as the high energy currency

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and this is the energy source
that's going to be fueling

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the various motors,

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the various pumps
in each of our cells.

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So when we eat food,
for example, glucose,

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that's going to enter
into our cells

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and through three main
metabolic pathways

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that glucose is gonna
get converted into ATP.

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And the three main pathways
are known as glycolysis,

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the Krebs cycle and
the electron transport chain.

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NARRATOR: These three pathways
are linked in our cells.

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Glycolysis occurs
outside the mitochondria,

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uses no oxygen and quickly
produces only two ATP.

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It also produces a chemical
that triggers the Krebs cycle.

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Both the Krebs cycle and
the electron transport chain

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happen
inside the mitochondria,

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use oxygen and together
produce as many as 36 ATP,

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fuel essential
for endurance performance.

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MOOTHA, MD:
After about 30 seconds

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or after a minute or two,

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depending on how much
glycogen reserves we have,

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we need to switch over
into other fuels,

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fuels that get metabolized
through the mitochondrion.

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We actually have a fixed number
of muscle cells.

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So when we're doing
exercise training,

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one of the things we're doing

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is we're increasing
mitochondria number.

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NARRATOR: But not all exercise
training is equal

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as Arianna Greene saw when
she danced The Nutcracker.

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ARIANNA GREENE:
I remember being dumbfounded

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when the prima ballerina
came backstage

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and I thought
she was gonna die.

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She was panting.

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She said, "Please somebody,
stagehand, undo my costume.

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I can't breathe.
I can't do anything."

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And later, I realized
that this happens all the time.

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These beautiful dancers
don't have stamina,

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don't have endurance.

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NARRATOR: Dancers train for
precision, strength and control,

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but they may not build up
stores of mitochondria

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in their muscles,

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so they rely heavily on
fuel produced in glycolysis.

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But no matter how much
mitochondria we have,

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at some point they perform
less and less efficiently.

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Part of the reason
has to do with a unique feature

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of mitochondria.

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They have their own
second set of circular DNA

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and that DNA
may have everything to do

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with the difference
between these muscles and these.

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MOOTHA, MD:
There's a mitochondrial

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DNA theory of aging.

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Mitochondrial DNA have poor
DNA repair mechanisms.

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And on top of that,

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mitochondria DNA tend to
accumulate more mutations

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at a higher rate
than the nucleus

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and this is because
the mitochondrion

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is exposed to a lot of oxygen

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and sometimes the oxygen
can have toxic metabolites

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that can lead to mutations
in the DNA.

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So we believe
that one of the things

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that's happening
is that as mitochondrial DNA

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accumulates mutations,

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our ability to produce energy
in an efficient manner decreases

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and that this is what
might be contributing

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to the aging process.

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NARRATOR: Oxygen invigorates
the work of mitochondria,

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but may also lead
to their destruction.

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Our cells are the stage

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for everything that happens
in our bodies -

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life, energy production,
aging -

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and mitochondria
play center stage.