Physiology of Nutrient Digestion

2. Physiology of Nutrient Digestion

Protein Digestion (Figure 2)

Protein digestion is located in the upper gastrointestinal tract. Cats are normally very efficient in protein digestion and the apparent digestibility of proteins is similar to dogs (Zentek et al., 1998; Funaba et al., 2005). The digestive capacity of the younger cat may be lower than that of adult animals, due either to the physiological development of the gut or diet-induced enzyme modulation (Harper & Turner, 2000).

Figure 2. Digestion and absorption of proteins.

Protein digestion is initiated in the stomach. A sequence of proteolytic enzymes is required to split the dietary proteins. Most important are endopeptidases such as pepsin or trypsin. Proteins are initially digested by pepsins (Shaw & Wright, 1976). Pepsins require an acidic environment for their activation: cats produce a highly acidic gastric secretion, the pH in the feline stomach varies from 2-3 (Hall, 2000). Pepsin is deactivated as soon as it enters the alkaline milieu of the duodenum and jejunum.

The small intestine has a slightly alkaline pH due to the secretions of the epithelial glands and the bicarbonate-rich pancreatic juice (Williams, 1996). This is necessary for the continuation of protein digestion by the proteolytic enzymes of the pancreas and the small intestinal mucosa. Feline trypsin seems to occur in one isoform only and trypsinogen, which is activated to trypsin by the activity of intestinal enterokinase, is closely related to the trypsinogen in other mammalian species (Steiner et al., 1997). Luminal protein digestion releases small peptides and amino acids that are transported through the brush border and absorbed by specific active carrier-mediated transport mechanisms through the gut wall.

Uptakes of arginine and lysine were high throughout the suckling period and the perinatal intestinal hyperplasia observed in many other mammalian species seems to be absent in cats (Buddington & Diamond, 1992).

Carbohydrate Digestion (Figure 3)

The cat’s ability to digest and tolerate such complex carbohydrates as starch is very high, although amylase activity in pancreatic tissue and small intestinal content is low compared to most other species (Kienzle, 1993). It decreases in the lower gut, probably due to intensive microbial degradation. Dietary carbohydrate levels had no obvious inductive effect on disaccharidase activities.

Figure 3. Digestion and absorption of carbohydrates.  

Maltase, isomaltase and sucrase activity did not depend on age. In contrast, lactase activity decreased from newborn kittens to adult cats and only few adult cats can have significant lactase activity in the jejunum (Kienzle, 1993). The tolerance for simple sugars is much more limited due to a limited intermediary capacity for sugar metabolism compared to most other species (Morris et al., 1977; Kienzle, 1994; Appleton et al., 2004). Apparent total digestibility of sugars was determined in adult cats and reached almost 100%. However, the prececal digestibility may be considerably lower depending on the sugar source and the degree that the starch is cooked (Kienzle, 1993).

Fat Digestion (Figure 4)

Cats are well adapted to fat digestion. Fats are not only important energy sources but also have additional functional properties (Bauer, 2006). Obviously, healthy cats can tolerate high dietary fat levels without a negative impact on digestive function. An age-related reduction in apparent fat digestibility was observed in cats fed on different fat sources with different degrees of saturation. Saturated fatty acids had a slightly lower apparent digestibility in young and senior cats (Peachey et al., 1999).

Figure 4. Digestion and absorption of fats.  

Fat digestion may be severely impaired in cats with exocrine pancreatic insufficiency (Nicholson et al., 1989) or in animals with impaired bile secretion. Bile acids are not only important for the solubilization of fatty acids but also for the activation of pancreatic lipase (Strombeck, 1996b). Bile acids are reabsorbed in the ileum and re-circulated to the liver. The absorbed long chain fatty acids are re-esterified in the intestinal epithelium and incorporated into chylomicrons before the release into lymphatics. Medium chain fatty acids can be absorbed directly into the blood, but palatability of medium chain fatty acids is usually low in this species (MacDonald et al., 1985).