A post-absorptive state is a metabolic period that occurs when the stomach and intestines are empty. During a post-absorptive state, the body's energy needs are fulfilled from energy previously stored in the body. This state is typically reached four or more hours after food has been consumed, usually overnight and in the morning before breakfast.
A post-absorptive state differs from an absorptive state. An absorptive state occurs during the period within four hours of food consumption. During the absorptive state, the stomach and intestines contain nutrients that are being absorbed by the body. These nutrients are used to meet the immediate energy needs of the body. When the body is in an absorptive state, glucose is readily available to be synthesized into energy.
Once the nutrients in the stomach and intestines are exhausted, the body returns to a post-absorptive state during which energy needs are met by stored fuels contained in body tissue. Once a post-absorptive state is reached, the body begins converting fat to glucose. Once fat reserves are depleted, the body begins to synthesize proteins from muscle tissue. As blood glucose drop, the rate at which the body secretes insulin slows and the pancreas begins to secrete glucagon.