Physiological and Metabolic Regulation of Feed Intake

摘要

Evidence is presented showing there is a close relationship between voluntary energy intake and energy expenditure in animals. The reduction in feed intake in modern pig genotypes has been argued by some people to be limiting the rate of protein deposition, whereas others believe the fall in intake is a consequence of a reduction in energy deposition as pigs were bred to be leaner. Until recently, it has not been possible to distinguish cause from effect in the association between energy intake and energy use. However, increased knowledge over the past decade shows that animals closely monitor their short term energy status and the degree of adiposity. Two systems (adenosine monophosphate-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR)) continuously monitor the immediate energy status of an animal through the ratio of adenosine monophosphate (AMP):adenosine triphosphate (ATP). In addition, insulin and leptin monitor the lipid content of the body and ghrelin the leanness ofthe animal. These hormones interact with AMPK and mTOR, which stimulate responses in peripheral tissues and in the melanocortin system of the hypothalamus. Within the melanocortin system, AMPK controls malonyl-CoA concentrations, which regulates the expression of orexigenic or anorexigenic peptides that act through higher centres of the brain to stimulate a sensation of hunger or satiety. In many situations experienced by pigs raised commercially, the capacity of the individual to consume sufficient energy for optimal energy metabolism is limited by various dietary, climatic, disease and social constraints. The impact of these constraints and possible mechanisms through which they operate are described. One important constraint requiring greater understanding is how variation in the physical nature of indigestible fibre and the source of other nutrients interact to alter rate of passage of digesta to increase intake with the negative effects of the intestinal brake that reduces intake.