Controls of the maintenance of eating and of the termination of eating determine meal size. Taste, especially sweet taste, is the only known unconditioned stimulus that affects eating once it has begun. Odors are crucial for flavor preferences, but their contribution is conditioned.
Feedback signals from the gastrointestinal tract provide unconditioned controls of meal termination. Most important are stomach volume, which is detected by mechano-receptors in the gastric muscles, and small intestinal nutrient content, which is detected by chemoreceptors in the intestinal mucosal. Feedback information is encoded in the periphery as neural impulses in the vagus or splanchnic nerves or in the concentration of gut peptide hormones such as cholecystokinin (CCK).
Other feedbacks unrelated to the ongoing meal also control meal size. Most interesting are two hormones whose average blood levels are correlated with body fat content, insulin and leptin. The control of meal size and food intake by body fat can be dramatic or weak, depending, for example, on genetic variation, on the availability, palatabil-ity, variety, and energy density of food, and, probably, on conditioning.
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