L16 Intro GI Physiol 2011 - COPYRIGHT Mammalian Physiology...

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-1/8- COPYRIGHT Prof. Beyenbach Mammalian Physiology BIOAP 4580 2011 GASTROINTESTINAL PHYSIOLOGY The broadest view of animal nutrition includes feeding strategy, diet and gastro-intestinal physiology. The details of GI-physiology are profoundly influenced by the diet and feeding behavior and consideration of one without understanding of the other is fraught with peril. For example, animal diets can be broadly divided into those based primarily on plant matter versus those based on animal matter. For the overwhelming majority of animals, diet is a matter of evolutionary development, not choice. Since the nutritional content of different foods varies considerably, the effective utilization, ingestion, digestion, and assimilation of nutrients require behavioral, physiological, and biochemical adaptations. By the standards of what animals require for development and maintenance, grass – even good grass – is relatively nutrient poor. Those animals that survive on a diet consisting largely of grasses have evolved mechanisms for utilizing plant material that are absent in carnivores. For example, to realize the caloric value of indigestible cellulose – what we consider fiber – cattle have a 4 part stomach. One part is the rumen, a fermentation vat where symbiotic micro-fauna break the cellulose down into various nutrients that can be absorbed by the intestine. In addition, the fermentation vat really acts like a bio-reactor, where the growing microfauna become an important source of protein. Ruminants are considered fore-gut fermenters and the potential for them to harvest the micro-fauna is a significant advantage to their feeding strategy. Horses and rabbits also eat a diet high in grasses and other leafy matter but they are monogastric, not ruminant. However, they also use symbiotic micro-fauna to aid in the digestion of cellulose. Their fermentation vat is in the colon and/or caecum. Alas, “hind-gut” fermenters do not have the ability to digest the luminal micro-fauna. But nutritionally challenged horses and rabbits can be observed to feed on excrement, known as “copraphagy”, where first a soft stool is passed and then re-ingested to claim its nutrient value (and proteins) and later a hard stool is passed for its excretory value. Well nourished, i.e., grain fed, horses may not exhibit copraphagy. All of the nutrient requirements for the development and maintenance of the animal are obtained through the diet, either directly or as precursors. Dietary requirements are those nutrients that must be obtained in the diet because the animal lacks the ability to synthesize them de novo from precursors. Humans have a dietary requirement for 8 (or 9) amino acids from which all other nutritionally required amino acids can be derived by available metabolic pathways. Members of the Felidae (cats) have a dietary requirement for taurine. Taurine is not available from any plant source, making the Felidae obligate carnivores. Recall that taurine is an intracellular osmolyte that participates in the regulation of cell volume. Fish are
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L16 Intro GI Physiol 2011 - COPYRIGHT Mammalian Physiology...

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