Carbohydrates in Pet Foods
Another commonly voiced concern about grains is that the carbohydrates they provide are not needed by dogs or, especially, cats. Some pet owners perceive that carbohydrates from grains and other sources are not digestible by dogs or cats. However, some pet owners will seek out foods with low or no grains, assuming this equates to low or no carbohydrate. In this section, the authors address the nutritional requirement, digestion, and metabolic use of dietary carbohydrates in cats and dogs and also discuss other nongrain sources of dietary carbohydrates used in pet foods.
There is no evidence that adult, nonreproducing cats or dogs have a dietary requirement for carbohydrates, but this is true for most mammalian species, including humans.
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10 Like other mammals, cats and dogs have a metabolic need for carbohydrates in the form of glucose, which is required by the brain and nervous tissues, red blood cells, renal medulla, and testes; the mammary gland during lactation; and the pregnant uterus.
11 When dietary carbohydrates are not provided,
de novo gluconeogenesis, primarily from amino acids but also from the glycerol backbone of triglycerides, provides the required glucose. Because the physiologic requirement for carbohydrates can be met by dietary carbohydrates or via gluconeogenesis, dietary carbohydrate is considered a dispensable, or nonessential, nutrient for adults. An analogy to this would be the nonessential amino acids. These amino acids are just as important to the body for normal endogenous protein synthesis as the 10 essential amino acids but, unlike those, the body is able to make the nonessential amino acids via transamination providing that sufficient substrates are included in the diet.
Cats are carnivores and evolved consuming low carbohydrate diets, so some question if they are able to digest or metabolize dietary carbohydrates. Cats and dogs both lack salivary amylase, the enzyme that can begin the process of digesting carbohydrates. However, both species have sufficient pancreatic amylase as well as intestinal disaccharidases, which allow them to efficiently digest properly processed carbohydrates.
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There are different forms of dietary carbohydrates, including simple sugars, rapidly digested and slowly digested starches (complex carbohydrates), dietary fibers, and others. Proper processing or cooking is necessary to make starches digestible to mammals, including cats and dogs. In many species, poorly digestible carbohydrates or an overload of simple sugars may induce adverse changes in intestinal metabolism.
14 This also is true for cats given high quantities (25%–40% of the diet) of sugars or raw (indigestible) starch.
15 However, when properly processed complex carbohydrates are provided as a major component of balanced diets (eg, 25%–50% of the diet being dry matter), cats are easily able to digest and use the carbohydrates. In fact, both cats and dogs can digest properly cooked carbohydrates, such as those from grains, with greater than 90% efficiency.
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Post-absorption, both cats and dogs will use the glucose from dietary carbohydrates to help meet their physiologic demand. Studies have shown that cats will increase carbohydrate metabolism, or oxidation, when carbohydrate intake increases and will likewise increase or decrease protein oxidation when intake of that nutrient changes.
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18 However, having more or less carbohydrate in the diet does not significantly affect gluconeogenesis or blood glucose concentrations in normally fed cats.
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Grain-free diets are not necessarily low in dietary carbohydrates. Although grains are a common source of dietary carbohydrates, other sources of carbohydrates can include potatoes, beans, tapioca, peas, and other vegetables and fruits. Many grain-free pet foods contain these alternate sources of dietary carbohydrates and may contain at least as much carbohydrate as traditional grain-containing diets. For pet foods that truly are low in carbohydrate, that component of the diet is often replaced with increased dietary fat, which can increase the risk for undesirable weight gain.
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