Metabolic factors governing the control of eating are generally separated into those associated with turning on eating (hunger) and those associated with turning off eating (satiety). Hunger (usually accompanied by eating) develops during the fasting phase of metabolism, after the ingestion and absorption of nutrients has been completed. Satiety (usually accompanied by cessation of eating) develops during the absorptive phase of metabolism, when a meal is being ingested and while nutrients are being absorbed from the intestine.
The principal sources of energy for all tissues are glucose and fatty acids, which may be drawn directly from the bloodstream. Energy reserves are stored in three different forms: protein, fat, and glycogen, which must be converted to glucose and fatty acids to be used. During the absorptive phase, metabolism is directed at the accumulation of these reserves; glucose and amino acids are converted to glycogen and fat, and amino acids are also stored as protein.
In the fasting phase, metabolic processes are aimed at utilizing energy. After a meal, blood glucose is used for energy needs, but as the supply of available glucose diminishes, energy reserves must be converted to usable forms. Glycogen is converted back to glucose and fat to fatty acid. These processes are associated with hunger. In the event of long-term deprivation or starvation, there is an increasing demand for energy derived from the conversion of amino acids to glucose, and fatty acids to ketones. These conversion processes provide the substrate for internal influences on eating.
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