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 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 thelowest hanging fruit, energy-wise through glucose burning. in your normal well-fedstate your body breaks down glycogen molecules to produce glucose: thefriendly carbohydrate that keeps your cells well-fed and functioning. the average person can typically goabout six hours after feeding in the glucose burning phase before they startfeeling hungry, and probably grumpy. at that point yourbody has burned through all that lunchtime glucose and is turning towardfatty acids: the building blocks of fat moleculesstored in your tissues to fuel itself.
this, which kicks off the first bigmetabolic shift as your body enters phase two of starvation: fat burning. the fat-burning phase canlast from days to weeks. during this stage called ketosis, ourlivers metabolize fatty acids and the smaller fat-chain derivatives called ketone bodies; they replaceglucose as the main energy source. these compounds come in three differentwater soluble configurations that move from the liver to the heart, brain, andother 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 carbhigh protein diet are maybe in ketosis is pretty often. itdoesn't mean they're starving, it just means thattheir bodies are running differently because either they're not eating enoughcarbs from which to get glucose, or they'requickly burning through them. but if you think that eating butter covered baconto burn body fat sounds like a good deal, you should know your brain might be less than amused. your braincan't directly use those fatty acids as
fuel because they're too big to squeezethrough 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 fourcans of coke if one of them's diet. so in your first day or two without food whilethe rest of your body starts fueling itself on fatty acids, the brain drains the last bit is stored glucoseuntil it really runs out.
but you probably noticed that no one dies after just twodays without food: that wouldn't make very muchevolutionary sense. luckily, your brain is smart and it has a backupplan. within a few days have no food, your brain recalibrates its glucoserequirement from 120 grams to about 30 grams, and it changes the menu. the brain nowstarts eating those processed ketone bodies, which, because they're smallerthan 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 areto continue to look for food. but even if you are semi alert you're definitely not in agood mood.
as a hungered is continually deprived of vital nutrient, itgets to feeling depressed, anxious, lethargic, socially withdrawn, and startsto have trouble concentrating and comprehending. the third, and final phaseof starvation begins when all your fat stores are burned up, and the bodyswitches to using its final reserves proteins. this phase is marked by a rapidmuscle 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, sothings go downhill pretty quickly as
your body starts eating itself, basically, in aprocess of internal self cannibalism called catabolysis. still, your everoptimistic body tries to run damage control by eating up your least vitalcells for as long as possible as it holds out hope for more food. but, after weeks of no food -- once yourbody has burned through all available glucose, fatty acids, and protein, musclemass, it's got nowhere left to look, and theend is near. even mind that by this point your body is sogrossly deficient in vitamins and
minerals and has such a weakened immunesystem not just about anything could kill you. in the end, the ultimate cause of death duringstarvation is often cardiac arrest, due to excessive tissue degradation. the heart simply can no longer supportitself with such limited resources. it's enough to make us all, i hope, reallyappreciate everything we eat, at least for this one day. thank you forwatching this episode of scishow, if you have any questions, comments, or ideas for us
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