3.4 Digestion and Absorption
Digestion is the process by which ingested materials are reduced to molecules of small enough size or other appropriate characteristics for absorption, i.e., passage through the gut wall into the blood stream. This generally means that proteins are hydrolyzed to amino acids or to polypeptide chains of a few amino acids, digestible carbohydrates to simple sugars, and lipids to fatty acids and glycerol. Materials not absorbed are by definition indigestible and are eventually voided as faeces. Digestibility ranges from 100 percent for glucose to as little as 5 percent for raw starch or 5-15 percent for plant material containing mostly cellulose (plant fibre). Digestibility of most natural proteins and lipids ranges from 80 to 90 percent.
Digestion is a progressive process, beginning in the stomach and possibly not ending until food leaves the rectum as faeces. Most studies of digestion simply compare the protein, lipid and carbohydrate content of the faeces with that of the feed. A study on digestion in channel catfish by Smith and Lovell (1973) showed continuing digestion (and absorption) of protein during passage through each part of the gut (Table 1). The methods employed in this study are discussed in Section 4 below. The comparison of faeces collected from the rectum and from the water also points out the hazard of incomplete recovery of faecal matter being likely when collection is done from outside the gut. Most of the protein digestion occurred in the stomach, but also continued in the intestine.
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