János Tossenberger (2011)
Carbohydrates are inexpensive feed components providing energy, assisting protein digestion and the utilization of minerals. The minimum carbohydrate requirement of the dog is not known. Various studies suggest that it is possible to maintain the dog on a carbohydrate free diet, provided it contains sufficient quantities of glucose precursors (amino acids, glycerol) to produce glucose, which is essential for the metabolism. Glucose in the blood is an immediately available and metabolisable energy source for every cell of the body and particularly for the brain, the red blood cells and the kidneys. Gestating and lactating bitches, however, need carbohydrate for whelping and for raising healthy puppies.
The dog digests different carbohydrates in a different way. Puppies utilize lactose efficiently and cover 8 to 10 % of their energy requirement from it. Lactose from cow milk may cover up to 20 – 30 % of the energy requirement of an adult dog, but as its absorption is slow, these levels may lead to osmotic diarrhea (Bokori, 1993). While adult dogs readily utilize saccharose, it is not recommended to add it to the milk replacer of young puppies, because nursing pups are unable to digest saccharose due to the low saccharose digesting enzyme activity in the small intestine.
Crude starch is not readily digestible for the dog because of the low alpha-amylase activity of the pancreatic fluid. The low amylase activity of the pancreas is characteristic of carnivores, thus it is not recommended to feed ingredients high in crude starch, because it may cause fermentative dyspepsia in the colon. Such feed ingredients should be processed by some form of heat treatment (e.g. extrusion). Autoclaving, micronization and flaking are also suitable procedures for this purpose (Hegedűs, 1995). Flake ingredients are increasingly used by the feed industry today.
Non-starch carbohydrates (e.g. fiber) and non-digestible plant materials delay gastric emptying and are necessary for maintaining normal intestinal peristalsis. The energy content of dog food for obese dogs can be reduced by increasing its crude fiber content. The optimum crude fiber content of the diet is usually recommended at 2 – 3 % of the dry matter content. Based on the findings of domestic and international research aimed at determining the nutrient requirements of dogs it can be concluded that individual researchers differ in their opinion concerning the role of carbohydrate in dog nutrition. While there are no questions raised concerning the essential nature of protein and fat as energy supplying nutrients, the results of feeding trials conducted with the purpose of determining the carbohydrate requirement are extremely varied. As far as protein and fat requirements are concerned, the differences in opinions only pertain to the optimum levels for specific physiological stages.
In summary, it can be concluded, that carbohydrate – although physiologically an essential nutrient – can not be considered an indispensable ingredient of foods and diets for dogs. It is possible, that contrary to several other mammalian species, the canine liver is capable of producing sufficient quantities of glucose through gluconeogenesis, using amino acids and glycerol in the process. When feeding carbohydrate free diets, the animals should be given substantially more protein and fat in order to provide sufficient quantities of glucose precursors for their body. Particular care should be taken in meeting the nutrient requirements when the body is under increased stress (growth, gestation, lactation).
When the nutrient supply of bitches in late gestation is inadequate it may lead to the synthesizing of less (intermediate) glucose by the metabolism than what is needed to cover the requirement. In consequence, dams in advance gestation and kept on carbohydrate free diets may develop hypoglycemia on the last week and ketosis (pregnancy toxemia) during whelping.
It should also be noted, that proteins and fats are considerably more expensive nutrients than carbohydrates. If the ration contains so much protein that it exceeds the transforming potential of the animal, it will on the one hand increase feeding costs, and on the other hand not only a part of protein will be lost, but it will also be a burden for the dog with the resulting harmful metabolites. Carbohydrates facilitate protein digestion, utilization of minerals, and in the case of high energy requirement reduce the utilization of fats and proteins for energy. When the energy requirement is covered from increased lipolysis in the liver, the number of ketone bodies may escalate in the blood, leading to health impairment. Bearing all this in mind, it can be concluded, that although dogs can be sustained on a carbohydrate free diet, the manufacturing of carbohydrate free dog foods, and the carbohydrate free feeding of these animals is not a practical option due to the relatively low price of starch. Financial considerations therefore justify the production of dog foods containing carbohydrates.