The carnivore connection to nutrition in cats
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Cats also have several physiologic adaptations that reflect their expected low CHO intake. The first of these is that cats lack salivary amylase, the enzyme responsible for initiating CHO digestion.
In addition, cats also have low activities of intestinal and pancreatic amylase and reduced activities of intestinal disaccharidases that break down CHOs in the small intestines.
These specific differences do not mean cats cannot use starch. In fact, cats are extremely efficient in their use of simple sugars. However, it does underscore their development as carnivores and the expected low amounts of grain in their typical diet.
These digestive differences may mean that high amounts of CHO in diets may have untoward effects on cats. For example, high amounts of CHO in diets decrease protein digestibility in cats because of a combination of factors, including increased passage rate.
Increased amounts of CHO in diets also causes a reduction in fecal pH in cats, which is caused by incomplete CHO fermentation in the small intestines that results in increased microbial fermentation in the colon and increased production of organic acids.
In cats, the liver also has several distinct features that influence disaccharide metabolism. In most animals, hepatic hexokinase (a constitutive enzyme) and glucokinase (an inducible enzyme) are active and responsible for phosphorylation of glucose for storage or oxidation. Cats differ in that they have minimal function of hepatic glucokinase, and the activity is not adaptive (ie, activity cannot be up-regulated when the diet contains large amounts of CHO).
In addition, cats also have minimal activity of hepatic glycogen synthetase (the enzyme responsible for converting glucose to glycogen for storage in the liver). Again, the likely reason for low hepatic glucokinase and glycogen synthetase activity in cats is a metabolic program that uses gluconeogenic amino acids and fat, rather than starch, in their diet for energy.
As a result, cats have limited ability to rapidly minimize hyperglycemia from a large dietary glucose load.
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