Medically speaking, starvation occurs when a body doesn't get enough calories and nutrients.  Whether brought on by poverty, famine, being lost at sea, voluntary hungry 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 5 or 10 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 2 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 1 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 6 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 switch kicks off the first big metabolic shift, as your body enters phase 2 of starvation:  Fat burning.  The fat burning phase can last from days to weeks.  During this stage, called ketosis, our livers metabolize fatty acids into 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 and brain and other tissues during periods of fasting to try to keep things going.  Some people, like hard-training triathletes or body builders or those on a super low-carb, high-protein diet, may be in ketosis 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're thinking 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% of our stored energy to function properly.  They're kind of greedy like that, needing about 120 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 the first day or two without food, while the rest of your body starts fueling itself on fatty acids, the brain drains the last bit of 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 back-up plan.

Within a few days of 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 hungry brain is continually deprived of vital nutrients, it gets to feeling depressed, anxious, lethargic and 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: protein.  This phase is marked by 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.  Keep in mind that by the end of this point your body is so grossly deficient in nutrients and minerals and has such a weak immune system that 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 or comments or ideas for us, we're in 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.