When humans eat, their bodies take the calories, metabolize them and use them to provide the body with energy. The body needs the energy for everything including, but not limited to, brain function, the function of vital organs and the maintenance of a heartbeat.
After not eating for as little as 6 hours, the body starts to enter ketosis, which helps the body dull the pains for appetite and uses fat for fuel. The brain cannot use fatty acids as a source of energy. The brain continues using the glucose in the blood for the next 24 to 48 hours. After this time, the brain starts to use ketone bodies as fuel and reduces the amount of glucose energy used from 120 grams a day to 30 grams.
After 72 hours, the body enters autophagy in which it starts to consume protein in the cells. The protein turns into amino acids that can be converted to glucose. This is the period in which muscle mass starts to waste away, balancing metabolic needs along with the needs to stay active and look for food. Once the body's energy supplies have been exhausted, there is the threat of death, usually due to a heart attack or complications in the heart's rhythm. This is caused by the tissue degradation brought on by autophagy or the imbalance of the body's electrolytes.