Cats: obligate carnivore (Proceedings)
Cats are not able to adapt to varying levels of carbohydrates in their diets due to various changes in the digestive and absorptive functions of intestine. Salivary amylase, the enzyme used to initiate digestion of dietary starches is absent in cats, and intestinal amylase appears to be exclusively derived from the pancreas. These enzymes were not necessary in a prey based diet with minimal starch content. The level of pancreatic amylase is only 5% that of dogs. The sugar transporter in the intestine is nonadaptive to changes in dietary carbohydrate levels. Disaccharide activity (i.e. the brush border enzymes responsible for sugar digestion) is also non-adaptive and only ~40% that found in dogs. These changes evolved because cats had little natural carbohydrate intake and it was not necessary to have systems intact that were not of little use to the animal.
Despite these adaptive changes, cats are able to still use carbohydrates in their diets, with a sugar digestibility of ~94% with a few exceptions. Lactose digestion declines sharply in kittens after 7 weeks of age. This is due to a decrease in intestinal lactase activity that is typical in mammals. Most adult cats can consume small amounts of milk without problems, but larger amounts (>1.3 gram/kilogram of body weight) can lead to signs of bloating, diarrhea and gas.High amounts of dietary carbohydrate levels can negatively impact on diet digestibility. With high levels of dietary carbohydrates, decreases in protein digestibility are seen due to a combination of factors including reduced fecal pH caused by incomplete carbohydrate digestion and increased microbial fermentation in the colon with increased production of organic acids. Cats have a vestigial cecum and short colon, which limit their ability to use poorly digestible starches and fiber for energy through bacterial fermentation in the large bowel.
The liver of most animals has two active enzyme systems for converting glucose to glucose-6-phosphate (the first step to forming glycogen-the storage form of glucose within the cells); hexokinase and glucokinase. The glucokinase system is used primarily when a large load of glucose is received by the liver as would be seen with a high carbohydrate meal. Cats have very low liver glucokinase activity and therefore limited ability to metabolize large amounts of simple carbohydrates by this route. Blood glucose levels in carnivores are more consistent with less postprandial fluctuations because glucose is released in small continuous boluses over a longer period of time as a result of gluconeogenic catabolism of proteins. If the protein is not included in the diet, body muscle and organ tissue will be used. Cats did not need to handle large carbohydrate loads and therefore only use the hexokinase system for glucose metabolism.
Domestic cats are thought to have descended from the small African wildcat (Felis silvestris libyca), a cat naturally found in the deserts of Africa. Because of this ancient relationship, cats today maintain this adaptation to a dryer environment. Cats seem to be less sensitive to the stimulus of thirst, and are able to survive on less water than can dogs.
With this decreased response to thirst, cats may ignore minor levels of dehydration (up to 4% body weight). They are able to compensate for this reduced water intake by forming highly concentrated urine. 4 Cats adjust their water intake based on the dry matter content of their diet rather than the moisture content. They consume 1.5-2 ml of water/g of dry matter. This 2:1 ratio of water to dry matter is similar to that of their typical prey. Practically, this means that cats consuming a dry food diet will consume about half the amount of water through their diet and drinking, compared to cats eating a canned food diet.