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I recently came across a UN study which advocated that controlled fasting can be beneficial for the the human body and I am fascinated by stories of wilderness survival against the odds.

It piqued my interest to ask; what exactly happens to a human body during fasting and ultimately starvation?

I understand that there is a period of glycogen use/exhaustion and functioning becomes impaired but what medically happens to the human body (organs, muscles, protein, body fat, cognitive processes etc) and over what timeline (24 hours to death). I read that halitosis is an interesting side-effect etc.

Assume the human body has ample access to water and can avoid dehydration.

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closed as too broad by rg255, AliceD, March Ho, kmm, James May 15 '16 at 23:57

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Have you read this? $\endgroup$ – MattDMo May 12 '16 at 11:58
  • $\begingroup$ I have now - it raises a few more questions though; how long after running out of glycogen does the body wait before using adipose tissue? Does it burn adipose as long as it exists or does it dual-use muscular fuel as well? $\endgroup$ – Venture2099 May 12 '16 at 12:08
  • $\begingroup$ Please edit your post and update it with your current question. $\endgroup$ – MattDMo May 12 '16 at 13:16
  • $\begingroup$ This is a very broad question, there are tons of literature on how the human body adapts to fasting in various ways. Please be more specific. $\endgroup$ – Roland May 12 '16 at 16:25
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Actually, the body goes into the fasting state about eight hours or so after the last meal, usually when the guy finishes absorbing nutrients from the food.
Normally, body glucose, which is stored in your liver and muscles, is your main source of energy. When in a fasting state, your body uses up the glucose that you have stored to provide energy. If you are still fasting after this store is used up, your body will start to burn fat to create energy, therefore leading to weight loss. After fasting for a few days, there will be a higher level of endorphin in your blood which will cause you to be more alert and, weirdly, giving you a feeling of mental well being. Eventually, if you are starved for too long, your body will start to break down muscle protein in order to create energy (that is why, during WWII, most POW's held by the Japanese were very thin - they were starved and in order to survive, their bodies had to 'eat' their own muscle) Technically, starvation is when your body starts to eat your protein, i.e. Muscle.
During starvation, your body will thin out, because your fat has been burned and your muscle is being 'eaten'. This obviously leads to weakness. When your fat is being burned, you go through a detoxification process which is caused because any toxins stored in your body's fat are dissolved and removed from the body. Your kidneys are very efficient in being able to maintain the body's water and salts, like potassium and sodium. But these can be lost through perspiration. This eventually leads to dehydration which does many things to your body, like: not allowing for your body to regulate body temperature; your heart works harder (most of your water is in your blood (about %$60$) and when you lose the water, your heart has to 'catch' up and pump more to keep the blood flowing and cooling you down - now, your heart is working harder which could give you a greater risk of heat exhaustion); you can feel cranky, drowsy, confused, forgetful. These are a lot of things that happen when your body is deprived of what it needs in terms of food and liquid.

This is what happens, and this is very general, when your body is in its starvation mode. I am sure that there are many more things, but I could only give a brief answer.

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