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Carbohydrates in the Dog's Diet

By Lew Olson • December 2005 Newsletter
In this unit, I am going to describe what carbohydrates are, their values as food for dogs, and whether they are necessary in a dog’s diet.

Carbohydrates are categorized as monosaccharides (glucose, galactose and fructose), disaccharides (two monosaccharides held together by a glycosidic bond) and polysaccharides, which includes starch, cellulose and glycogen. Disaccharides and polysaccharides must be broken down into monosaccharides for digestion. One form of monosaccharide would be honey. Examples of a disaccharide would be sucrose, maltose and lactose (found in milk). Polysaccharides would include cellulose (the cell walls of plants), starches (such as grains, corn and potatoes) and glycogen. (1)

The pancreas releases amylase, a digestive enzyme to break down starch to maltose. Dogs, unlike omnivores, do not have amylase in their saliva to help break down starches in the mouth. Consequently, they are not as efficient at digesting starches, and have a difficult time with a diet high in most complex carbohydrates, which will stay in the dogs’ digestive tract longer, causing more energy to be used to try and absorb these foods. The consequence of eating a diet high in starches is seen in bulky, thick stools. (2)
The colon is not built to absorb nutrients, except in the releasing of water and electrolytes (notably sodium and chloride) to the body. This helps in preventing dehydration. Diarrhea in dogs can cause a problem with dehydration, as the colon does not have the time needed to complete its work.
Colitis and irritable bowel syndrome can also cause problems with absorption of water. While a high fiber diet is often recommended for dogs with this condition such a diet may be contraindicated due to the dog’s short digestive tract.
The starches in grains and carbohydrates is useful in herbivores and humans for enhanced digestion, but the starches in the grains slow down the digestion process for dogs and can cause irritation and spasms in the large intestine. The dogs’ digestive tract must labor to digest starches, which can add more time in the digestive tract.
Highly digestible diets are easier for a dog to digest and cause less irritation to the intestines. High fiber diets have not proven to be effective in dogs with gastrointestinal problems and W. B. Armand suggests diets instead should be bland, low in fiber and highly nutritious. (3)
Feces are most often 25% solid material, and 75% water. This can vary with the type of diet fed. A diet based more on carbohydrates, such as grains, tends to produce larger stools and contain more water. A diet of raw meat and bones produces stools that are very small, and contain less moisture. Odor is dependent on the amount of bacteria available for fermentation, and so a diet of meat (protein) and bones (calcium) produces stool that is far less odoriferous than one of grains.
Grains need a longer digestion period, and spend more time in the digestive system. The carbohydrates that were not entirely digested in the small intestine continue to ferment in the colon. The bacteria in the large intestine also contribute to the development of Vitamin K, necessary for blood clotting. These same bacteria produce the B vitamins, which are passed in the feces. Animals that eat a diet primarily of processed dog food may become attracted to eating feces in order to gain this valuable nutrient.
Dogs may also become stool eaters for the bacteria and enzyme content. By ingesting the stool, they are returning valuable ingredients to the digestive tract to aid in assimilating nutrients. Adding digestive enzymes or beneficial bacteria to the dog’s diet, along with a B complex vitamin may help eliminate this distasteful habit.
An important point is that dry dog foods are composed mainly of cereal, consisting largely of corn, wheat, rice and/or soy. While dog food companies would have you believe that grains are a good source of protein, the fact is that dogs have a very difficult time digesting and utilizing protein from carbohydrates. Studies show dogs do best on animal protein, which is much more easily digested and assimilated. (See last’s month newsletter on protein.)
Carbohydrates are often listed in canine nutrition books as an essential part of the diet, yet no percentages or ratios are designated for this nutrient. In one text, it says “Carbohydrate is physiologically essential to the dog and cat; however it is not essential in the diet.” (4) Another text says, “The fact that dogs and cats do not require carbohydrates in the diets is usually immaterial because the nutrient content of most commercial foods includes at least a moderate level of this nutrient.” It goes on to state that most commercial dog foods contain up to 60% carbohydrates. (1)
High fiber carbohydrates are often found in dog foods. They can include bran, beet pulp, rice hulls, peanut hulls, cellulose and plant gums. Fiber is not digestible by dogs in these forms. (1) Dog food companies use these to help dogs with digestion, but there is fiber available that can give dogs more nutrients, such as pulped vegetables. However, these would not maintain their integrity in a processed food.
Regarding fiber, Dr. Kronfeld states, “Fiber does not have a minimal requirement in dogs. On the other hand, fiber from cereal seems the most important factor contributing to the large bowel bleeding in racing sled dogs which are consuming three to five times more food than the average resting adult.
Too much fiber appears harmful when the colon and rectum are severely challenged.” He also goes on to point out that dogs were eating meat and fat for ten thousand years, and while it has only been about 100 years that they have been eating fiber, they appeared to do fine without it before. (5) Fiber also decreases the absorption of calcium, magnesium, zinc and iron and depresses energy and protein digestion. Part of the reason for this is the high cellulose content of fiber, which is indigestible by dogs. (4)
Starches pose another problem in canine digestion. Most grains contain phytin, which inhibits the absorption of calcium, magnesium, iron, zinc and also iodine. Dr. Kronfeld reports the NRC recommendations for minimal nutrients are based on a diet of 50% meat and meat by products. He suggests that diets that are predominantly grain, (such as commercial, processed feeds) need to increase the mineral amount by twice the amount recommended by the NRC. (4)
Dr. David Kronfeld reports that carbohydrates are most important for dogs in two situations: puppies just coming off the mother’s milk (which is 12% carbohydrates) and the lactating bitch, which needs three times the usual turnover of blood glucose for production of milk.
He goes on to state “no carbohydrates need be provided in the diet for pups after weaning or adult dogs, not even for those subjected to hard work. The liver is easily able to synthesize sufficient glucose (from amino acids derived from protein and glycerol derived from fats) for transport in the blood and utilization in other tissues”. (4) He also goes on to state that he feels the high carbohydrate content in dog foods is what contributes to coprophagy (stool eating), and hypoglycemia.
The last argument for using grains in dog foods is for the carbohydrates as an energy source. Many texts state that the glucose found in grains is necessary for stamina, endurance and performance. However, fat can convert to glucose in the liver, if given in high enough doses. This process, which is called gluconeogenesis, is easily achieved in dogs fed a high ratio of protein (50%) and a higher ratio of fat than recommended by the NRC.
Studies completed in the 1980’s of pregnant and lactating bitches that when bitches were fed a carbohydrate free diet, their protein and fat levels were increased, they whelped well, had a comparable amount of survival of pups to bitches fed carbohydrates, and produced a good supply of milk. It is important to increase the protein and fat for dogs fed a carbohydrate free diet so that gluconeogenesis (conversion of fat to glucose) can occur. (1) (4) (6)
Even though carbohydrates may not offer much value to a dog, adding pulped vegetables to the diet in small quantities is not harmful, and may have some nutritional value. I would probably recommend feeding vegetables and/or grains at a ratio of 1/6 or less of the diet. The fiber in these foods offers more bulk to the stool, and more energy trying to digest them. For those who want more information on feeding vegetables in the diet, I have an article with more information here in the July 2005 Newsletter.here:
Next month we will discuss fats and fatty acids in the diet, see you then and Happy Holidays!
1. Case, Linda P MS, Carey, Daniel PD, DVM and Hirakawa, Diane A, PhD, Canine and Feline Nutrition, ( Mosby Press, 1995) 17-18
2. Kronfeld, DS Phd DSc MVSc, Some Nutritional Problems in Dogs, (Canine Nutrition, University of Pennsylvania, School of Veterinary Medicine, 1972) 32-33
3. Armand, WB VMD, Diet and Gastrointestinal Problems, (Canine Nutrition, University of Pennsylvania, School of Veterinary Medicine, 1972), 49
4. Kronfeld, DS PhD DSC MVSC, Home Cooking for Dogs, The Staples, Meat, Meat by-Products and Cereal, (American Kennel Club Gazette, July, 1978) 55
5. Kronfeld, DS PhD DSc MVSc, Home Cooking for Dogs, Food Energy-Carbohydrates, Fats and Proteins, (American Kennel Club Gazette, June, 1978) 62
6. Kienzle, E and Meyer, H. The Effects of Carbohydrate-Free Diets Containing Different Levels of Protein on Reproduction in the Bitch, (Nutrition of the Dog and Cat, Cambridge University Press, 1989) 254-25
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