The Psychobiological System Of Appetite Control

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It is now accepted that the control of appetite is based on a network of interactions forming part of a psychobiological system. The system can be conceptualized on three levels (Figure 8.1). These are the levels of psychological events (hunger perception, cravings, hedonic sensations) and behavioural operations (meals, snacks, energy and macronut-rient intakes); the level of peripheral physiology and metabolic events; and the level of neurotransmitter and metabolic interactions in the brain (2). Appetite reflects the synchronous operation of events and processes in the three levels. When appetite is disrupted as in certain eating disorders, these three levels become desynchronised. Neural events trigger and guide behaviour, but each act of behaviour involves a response in the peripheral physiological system; in turn, these physiological events are translated into brain neurochemical activity. This brain activity represents the strength of motivation to eat and the willingness to refrain from feeding.

The lower part of the psychobiological system (Figure 8.1) illustrates the appetite cascade which prompts us to consider the events which stimulate eating and which motivate organisms to seek food. It also includes those behavioural actions which actually form the structure of eating, and those processes which follow the termination of eating and which are referred to as post-ingestive or postprandial events.

Even before food touches the mouth, physiological signals are generated by the sight and smell of food. These events constitute the cephalic phase of appetite. Cephalic-phase responses are generated in many parts of the gastrointestinal tract; their function is to anticipate the ingestion of food. During and immediately after eating, afferent information provides the major control over appetite. It has been noted that 'afferent information from ingested food acting in the mouth provides primarily positive feedback for eating; that from the stomach and small intestine is primarily negative feedback' (3).

Appetite Diagram

Figure 8.1 Diagram showing the expression of appetite as the relationship between three levels of operations: the behavioural pattern, peripheral physiology and metabolism, and brain activity. PVN, paraventricular nucleus; NST, nucleus of the tractus solitarius; CCK, cholecystokinin; FFA, free fatty acids; T:LNAA, tryptophan: large neutral amino acids; GLP-1, glucagon-like peptide 1. (See Blundell (2) for detailed diagram)

Figure 8.1 Diagram showing the expression of appetite as the relationship between three levels of operations: the behavioural pattern, peripheral physiology and metabolism, and brain activity. PVN, paraventricular nucleus; NST, nucleus of the tractus solitarius; CCK, cholecystokinin; FFA, free fatty acids; T:LNAA, tryptophan: large neutral amino acids; GLP-1, glucagon-like peptide 1. (See Blundell (2) for detailed diagram)

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