From an artistic point of view, forced starvation and the emotions that a person experiences are very powerfully described in Knut Hamun's novel "Hunger".
0-6 hours after your last meal
Soon after you eat, your body starts breaking down glycogens - polysaccharides containing carbohydrates - to produce glucose. When you eat well, glucose is the main energy supplier. Glycogen, on the other hand, plays the role of a "reserve" while the supply of glucose from the outside stops.
It should be noted that not all people get energy from the "burning" of glucose. If you are on a low-carb diet and / or do intense strength training, your body is in a state of ketosis, meaning it has increased levels of ketone bodies - compounds that are formed when fat is broken down instead of glucose. Technically speaking, in ketosis, you are not hungry, despite the lack of glucose, you are simply in a different metabolic regime.
6-72 hours after the last meal
And now you you are already starving and therefore enter a state of ketosis in any case: your glycogen stores are depleted and the body has no choice but to extract energy from fat stores. This is the first significant metabolic phase shift since you stopped eating.
In ketosis, you live as you did except the brain cannot use fatty acids as a fuel source. Therefore, during the first 24 to 48 hours of fasting, the brain will consume the remaining glucose stores while the rest of the body has already entered ketosis. The problem is that the brain needs 120 grams of glucose every day, which is equivalent to the amount of sugar contained in three cans of cola. This means that you will be sorely lacking glucose. In theory, the brain will have to die. But this will not happen, because your body will do something like a backup.
First, ketone bodies are transformed in such a way as to still overcome the blood-brain barrier, which prevents them from entering the brain. The brain, in turn, will adapt to consume ketones, extracting up to 30% of the required energy from them within 3 days. Starting from the 4th day, the amount of this energy will increase to 70%. In addition, the brain will learn to get by with 30 grams of glucose instead of 120.
Surprisingly, humans are the only species among animals with a brain that can do without glucose. Apparently, it was a gift of evolution, which allowed us to survive more than one period of prolonged fasting.
More than 72 hours after the last meal
Nevertheless, your brain still lacks about 10 g glucose per day. And there is nowhere else to take it, except from the proteins of your own body. At this stage of the fast, the protein in the cells of the body will begin to break down, releasing amino acids into the bloodstream. These amino acids are converted into glucose in the liver, and the brain is happy again.
How your body will work if you starve
But not your body. You have entered a phase of autophagy, literally self-devouring. This does not mean that you are tempted to gnaw off a limb for food. This is knowncheat that your muscle mass is rapidly deteriorating. Fortunately, the body is able to balance this process, selectively destroying less important cells for vital functions and supporting more important ones.
It is clear that, despite all these maneuvers, your body is far from in order. Hunger negates the functioning of the immune system, mainly due to the lack of vitamins and minerals. In theory, you can die at any time from immune-related diseases.
But it is more likely that you will continue to live in the above-described mode as long as the body's resources - glucose, fats, muscle cells - will not be completely exhausted. The final stage of fasting will be accompanied by two diseases: marasmus and kwashiorkor (severe degeneration due to lack of protein).
Insanity will begin as a result of extreme energy deficiency due to insufficient amount of protein and calories. Kwashiorkor will lead to swelling and enlargement of the liver, which makes your belly swell like starving children in creepy photos from disadvantaged areas. An enlarged abdomen will create a feeling of distension, and you will, contrary to logic, begin to feel full.
The final stage of prolonged fasting - death - will be caused by a heart attack or arrhythmia due to extreme tissue degradation caused by autophagy, as well as due to severe electrolyte imbalance.