The First 24 Hours
Glycogen, the storage form of carbohydrate in the liver, is broken down into glucose for usage. The central nervous system uses glucose as energy source. Also the body breaks down triglycerides (neutral fats) stored in the fat cells into free fatty acids to be used as energy for performing the functions of muscle, kidney and heart. The insulin concentration is low to facilitate the release of free fatty acids and to prevent glucose from being taken up by peripheral tissues (glucose is reserved for the brain and nervous tissue during fasting).
Day Two to Day Four
The glycogen store in the liver is depleted after 24 hours. The body breaks down muscle into amino acids (the building block of protein) to be used in the liver and kidney. Elevated levels of triglycerides are broken down into free fatty acids and glycerols. Both glycerols and amino acids are converted into glucose for the brain and nervous system. The free fatty acids continue supplying energy to kidney, muscle and heart. Ketone is synthesized in the liver by breaking down fat and used by muscle mostly as energy source. During this time, the brain still uses glucose converted from amino acids primarily but it has started to also utilize ketones.
Over Two Weeks
The need for glucose is less at this time. Muscle protein is broken down in the liver and kidney. Kidney is working harder to neutralize the increased amount of acidic ketones. Triglycerides are still being broken down to provide energy. Meanwhile, the brain mainly uses ketones. People may experience headaches because of ketosis.
If continue fasting after fat storage is depleted, the body will again break down muscle proteins to provide energy. This weakens the respiratory muscles, heart muscles and immune system and death may result from respiratory infections.