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Dogs efficiently digest starch through pancreatic enzymes (amylase) and intestinal disaccharidases (maltase and isomaltase). The ability to absorb nutrients in dogs is approximately 10% higher than in cats, whose intestinal sugar transport system is not adapted to a diet containing high levels of carbohydrates, as these are naturally present in limited quantities in their diet. Their liver lacks an enzyme called glucokinase, which is why cats cannot rapidly metabolise large amounts of carbohydrates or sugars.
The absence of glucokinase does not represent a problem in healthy cats, because their natural feeding behaviour consists of consuming several small meals throughout the day, which helps prevent excessive glucose load.
Sugars should nevertheless be considered a valuable source of energy and appear to be necessary to support optimal lactation.
In the absence of high levels of meat in their diet, specific nutritional supplementation is required. A cat’s energy metabolism is closely linked to protein metabolism. The latter is characterised by a higher protein requirement. Cats’ greater need for protein is related to the high enzymatic activity of the liver (transaminases and deaminases), which removes amino groups from amino acids, allowing the resulting keto acid to be used for energy production or glucose synthesis.
Unlike other species (including dogs), cats fed a low-protein diet cannot reduce the activity of these enzymes. As a result, these enzymes remain constantly active, meaning that a certain amount of dietary protein is always broken down for energy purposes.
Carnivores such as cats are limited by what the tissues of their prey provide in terms of essential nutrients and energy sources, including proteins, taurine, arginine, arachidonic acid, and niacin. Since cats cannot synthesise these nutrients from their precursors, they have maintained a high protein requirement through a unique protein metabolism.
Cats are not able to directly use dietary fibre; however, in the large intestine they possess a microflora that acts on this component through fermentation, producing short-chain fatty acids (acetic, propionic, and butyric acids). These are used by intestinal epithelial cells as an energy source. These fatty acids also provide beneficial trophic effects on the cells of the caecum and colon (increasing cell proliferation, crypt height, thickness, and depth).
Cats are capable of digesting high levels of dietary fats, as these are naturally abundant in animal tissues.
They have a specific requirement for arachidonic acid (a polyunsaturated omega-6 fatty acid), because they cannot synthesise it from linoleic acid, unlike dogs. Arachidonic acid is important for reproductive activity and blood clotting function, while linoleic acid plays an important role in growth and in maintaining healthy skin and coat.
Cats also have specific nutritional requirements for four amino acids: arginine, taurine, methionine, and cystine.