Whether brought on by poverty, famine, being lost at sea, voluntary hunger strikes or medical conditions,

the physiology of starvation follows the same process no matter the cause.

And it really is a process: it takes time to starve to death.

Without oxygen, our bodies shut down in five or ten minutes.

Without water, we can last maybe a couple days -- a week at most.

But depending on the circumstances, a person might go as long as two months without any food at all.

During this time the body passes through three distinct metabolic phases as

it desperately tries to stay alive until it finds food again.

phase one is where your body picks the lowest hanging fruit, energy-wise

through glucose burning. In your normal well-fed state your body breaks down glycogen

molecules to produce glucose: the friendly carbohydrate that keeps

your cells well-fed and functioning.

The average person can typically go about six hours after feeding in the

glucose burning phase before they start feeling hungry, and probably grumpy.

At that point your body has burned through all that

lunchtime glucose and is turning toward fatty acids:

the building blocks of fat molecules stored in your tissues to fuel itself.

This, which kicks off the first big metabolic shift as your body

enters phase two of starvation: fat burning.

The fat-burning phase can last from days to weeks.

During this stage called ketosis, our livers metabolize fatty acids and the

smaller fat-chain derivatives called ketone bodies;

they replace glucose as the main energy source.

These compounds come in three different water soluble configurations that move

from the liver to the heart, brain, and other tissues during periods of fasting

to try to keep things going.

Some people, like hard-training triathletes or body

builders are those on a super low carb high protein diet

are maybe in ketosis is pretty often. It doesn't mean

they're starving, it just means that their bodies are running differently

because either they're not eating enough carbs from which to get glucose,

or they're quickly burning through them.

But if you think that eating butter covered bacon to burn body fat sounds like a good deal,

you should know your brain might be less than amused.

Your brain can't directly use those fatty acids as

fuel because they're too big to squeeze through the blood-brain barrier.

And our brains are a big energy suck -- they demand about 25 percent of our

stored energy to function properly.

The kind of greedy like that needing about a

hundred and twenty grams of glucose a day to stay happy.

That's like three cans of coke: it's like four cans of coke if one of them's diet.

So in your first day or two without food while the rest of your body starts fueling

itself on fatty acids, the brain

drains the last bit is stored glucose until it really runs out.

But you probably noticed that no one dies after just two days without food:

that wouldn't make very much evolutionary sense.

Luckily, your brain is smart and it has a backup plan. Within a few days have no food, your

brain recalibrates its glucose requirement from 120 grams to about 30 grams,

and it changes the menu. The brain now starts eating those processed ketone

bodies, which, because they're smaller than fatty acids, can get through the blood-brain barrier.

This is a great evolutionary trick to keep us

alert enough no matter how hungry we are to continue to look for food.

But even if you are semi alert you're definitely not in a good mood.

As a hungered is continually deprived of vital nutrient, it gets to feeling

depressed, anxious, lethargic, socially withdrawn,

and starts to have trouble concentrating and comprehending.

The third, and final phase of starvation begins when all your fat

stores are burned up, and the body switches to using its final reserves proteins.

This phase is marked by a rapid muscle depletion as your body cells

start to break down their own proteins into amino acids which your

brain now gobbles up.

Proteins are essential for proper cell functioning, so things go downhill pretty quickly as

your body starts eating itself, basically,

in a process of internal self cannibalism called catabolysis.

Still, your ever optimistic body tries to run damage

control by eating up your least vital cells for as long as possible as it

holds out hope for more food.

But, after weeks of no food -- once your body has burned through all available

glucose, fatty acids, and protein, muscle mass,

it's got nowhere left to look, and the end is near.

even mind that by this point your body is so grossly deficient in vitamins and

minerals and has such a weakened immune system not just about anything could

kill you.

In the end, the ultimate cause of death during starvation is often cardiac arrest,

due to excessive tissue degradation.

The heart simply can no longer support itself with such limited resources.

It's enough to make us all, I hope, really appreciate everything we eat,

at least for this one day.

Thank you for watching this episode of SciShow,

if you have any questions, comments, or ideas for us

where the comments, and on Facebook and Twitter. And if you want to keep getting

smarter with us here at SciShow, you can go to

YouTube.com/scishow and subscribe.