Controls of feeding in pigs

The physiological controls of feeding behavior in pigs are reviewed. Feeding patterns, central nervous system integration of the control systems and the influence of taste and olfaction are briefly considered, but the emphasis of the review is on the specific control systems that determine meal size. The glucostatic mechanism of stimulation of eating operates in the pig, but is probably an emergency control in response to a severe deficiency of glucose available to the central nervous system. The effective controls determining meal size are predominantly inhibitory signals initiated within or near the gastrointestinal tract by the presence of food. There is evidence that a rise in osmoconcentration caused by the arrival of foodstuffs in the duodenum during a meal can inhibit feeding behavior to a degree proportional to the hypertonicity of the duodenal content. Arrival of chyme in the duodenum will also trigger the release of cholecystokinin (CCK). Exogenous CCK injected by various routes inhibits feeding, suggesting that endogenous CCK acts as a satiety or inhibitory signal during meals. Gastrointestinal distention during meals probably also acts as an inhibitory signal, but only preliminary studies have been made to delineate the role of this factor in feeding behavior. It is concluded that although considerable research has been conducted to reveal what control mechanisms operate in pigs, even the mechanisms already investigated are not firmly established as operating in normal meals. The osmoreceptive, CCK and gastrointestinal distention control systems are promising hypotheses worthy of further study, and the search for other control systems that may also participate continues.