Digestion and Metabolism

In sheep and other ruminants, the exposure of ingested food to the metabolic activities of ruminal bacteria, protozoa and fungi (see Mackie et al., Chapter 4, this volume) has profound implications for the digestion and metabolism of food.Plant carbohydrates, usually the major sources of energy in ruminant diets, are largelyfermented to short-chain fatty acids (SCFA). These consist mainly of acetate, propionate andbutyrate and are readily absorbed from the rumen and metabolised in tissues to supportmaintenance and production. The degradation of ß-linked glucose polymers (cellulose andhemicellulose) from plant cell walls is of obvious nutritional benefit, since these materials arenot hydrolysed by the endogenous enzymes of the host animal. In contrast, the readyfermentation of starch and other α-linked glucose polymers, which are potential sources ofglucose if they reach the small intestine (SI), implies that only small amounts of glucose areabsorbed from the SI (see p.106).