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Integrated responses to feeding, comparative aspects
The optimal utilisation of nutrients requires an integrated response of the gastrointestinal tract to ingested food. Broad mechanisms are similar in all mammals, and involve sensing food components through olfaction, taste and specialised receptors within the stomach and intestines. The sensing of food components leads to release of gut hormones and activation of nerves, which in turn modify digestive functions. Bacteria, viruses, fungi and potentially injurious substances in foods activate tissue defence mechanisms. While the responses to nutrients lead to broadly similar changes in appetite, satiety and food-seeking behaviour, gastrointestinal motility, release of digestive enzymes and induction of nutrient transporters, the requirements in different animals differ. To simplify discussion, we can divide species into ruminant foregut fermenters (such as cattle and sheep), non-ruminant foregut fermenters (e.g., kangaroo, colobus monkey), hind-gut fermenters (such as horse), and auto-enzyme dependent digesters (pig, human) that also gain nutrition from hind-gut fermentation. Ruminants are efficient digesters because the rumenal movements are able to stratify food into gas, fluid and particle components, retaining food to be digested in the forestomach and passing more fully digested material into the abomasum and duodenum, and also being able to return food from the forestomach to the mouth for mastication and limited enzyme exposure. Poultry have multiple stomachs that allow for storage, digestion and tituration, but not fermentation. Ruminants lose efficiency in that most carbohydrate is utilised by gastric bacteria and very little glucose reaches the small intestine. Thus glucose must be synthesised from short chain fatty acids produced by bacteria, whereas species such as pig and human convert carbohydrate to glucose enzymatically. Thus ruminants are more prone than other groups to enter into negative glucose balance, for example during post-partum lactation. Obligatory by-products of fermentation are carbon dioxide and methane. Foregut fermenters are also advantaged by being able to readily utilise vitamins produced by fermentation. It is thought that coprophagy by hind-gut fermenters, such as rabbits, provides access to such vitamins. Foregut fermentation also contributes to detoxification, for which hindgut fermenters and autoenzymic digesters rely primarily on the liver. With these differences in mind, it is necessary to closely consider what information can be readily extrapolated between species. Conversely, we can point to similarities in neural and hormonal signalling systems and products of digestion, achieved in different ways, that are available for energy utilisation and incorporation into tissues.
Keywords: digestive physiology; fermentation; glucose; comparative