First, let’s break down what a carbohydrate is. Carbohydrates can be classified as monosaccharides, disaccharides and polysaccharides. For this discussion, we will be focusing on the polysaccharides, starch and fiber. Starches are digestible in the small intestine while fiber is only digested in the large intestine. Some examples that are commonly found in canine diets would be oat, barley, rice and various fruits and vegetables though there are many other possibilities as well.

Can Dogs Digest Carbohydrates?

One of the main arguments against feeding carbohydrates in canine diets is that wolves don’t eat carbohydrates, so our companion animals shouldn’t either. As dogs have been domesticated, they have developed the ability to efficiently digest carbohydrates which researchers have linked to 3 main genes, though the research for the third is still lacking. 

In human digestion, the breakdown of carbohydrates begins in the mouth with salivary amylase that initiates chemical digestion along with mechanical digestion from chewing. Since dogs don’t produce salivary amylase, only mechanical digestion occurs in the mouth. Once the food is chewed and swallowed, carbohydrates remain relatively unchanged until they reach the small intestine where pancreatic amylase begins the breakdown of starch into glucose and maltose [7]. There have been many studies done on the amylase activity in relation to starch digestion in both wolves and domesticated dogs by studying the gene AMY2B- the gene responsible for the production of pancreatic amylase. It has been shown that wolves carry only 2 copies of AMY2B, but dogs were found to carry up to 30 copies depending on breed [1, 3, 4]. With each additional copy of AMY2B, there is an estimated 5% increase in serum amylase activity which means that dogs with higher copies of AMY2B may have an increased ability to digest starch [2, 3]. In addition to breed variation, it has been shown that the spread of prehistoric agriculture where the dogs originate may have an effect on their ability to digest starch. Dogs that originate from non- agrarian regions such as Siberian Huskies, Greenland Sled Dogs and other arctic breeds, typically have lower copy numbers of AMY2B while dogs that originate from agrarian areas have higher copy numbers. This demonstrates that those breeds that were domesticated alongside the spread of agriculture may have allowed them to adapt in order to digest carbohydrates [2,10,11]. Through personal experiences or discussions with other dog owners, I have noticed that Huskies (3 copies of AMY2B) tend to do better with diets that are lower in carbs while breeds like the Saluki (24 copies of AMY2B) show better results with diets containing higher levels of carbohydrates. It is also important to mention that the preferred diets of these dogs may be related to the sports that they were originally bred for (endurance vs. sprinting). 

The second important gene related to starch digestion in dogs is maltase-glucoamylase (MGAM) which is responsible for catalyzing the hydrolysis of maltose to glucose. The enzyme maltase is necessary to break down the disaccharide maltose into the monosaccharide glucose which is the main sugar found in the body and acts as an energy source to the body’s cells. While dogs and wolves have the same number of copies of MGAM, there are some differences that show that dogs have a greater ability to digest starch. One of the main differences is that dogs are able to produce longer versions of maltase which is typically only found in herbivores and omnivores. This suggests that an increased length may lead to better digestion of starch [3]. More research needs to be done on this particular gene to know for sure what its role in starch digestion is. 

As with humans, dogs are not able to digest uncooked carbohydrates well but cooking them increases the starch digestibility close to 100%. When starches are cooked, the small granules that make up carbohydrates expand which allows pancreatic amylase to have more ability to digest. There have been various studies done to show digestibility of different forms of carbohydrates such as rice, corn, oat, potato, tapioca, quinoa, millet and legumes. A study conducted by Carciofi et al. examined 6 extruded diets with different starch sources and found starch digestibility for all of the diets was over 98%. The carbohydrates included in this study were cassava flour, brewer’s rice, corn, sorghum, peas or lentils [5]. In another study, rice, corn and oats were examined where cooked oats were found to have a starch digestibility of 95.5% [9]. 

Should a Dog’s Diet Contain Carbohydrates?

The short answer is maybe. I do not believe in a one size fits all approach in canine diets because different dogs are going to do better with different macronutrient profiles. This could be due to many reasons such as breed, activity levels and medical conditions. We’ll discuss some of the reasons owners may choose to include carbohydrates in their dogs diet. 

Carbohydrates are protein sparing

As stated above, dogs don’t have a nutritional requirement for carbohydrates but they do have a requirement for glucose. Glucose is used as an energy source for many body cells as well as in the synthesis of other nutrients such as nonessential amino acids, DNA and RNA. Glucose can either come from one of two sources- starch or proteins. If the diet is low in starch, the body will use proteins to provide necessary glucose to the body tissues via a process called gluconeogenesis. Gluconeogenesis uses the amino acids found in muscle proteins to make glucose. This will in turn decrease the amount of protein available for processes such as tissue repair and growth. Providing small amounts of carbohydrates in the diet will ensure that protein is used for its main functions [7]. 

There have been a couple studies conducted with pregnant dogs to determine reproductive success when fed carbohydrate free diets. The first study used a diet that contained only 26% protein and resulted in only 63% of puppies being alive at birth with a high puppy mortality rate shortly after birth. The dogs became hypoglycemic and ketotic as they were nearing the end of the pregnancies. They also had reduced blood concentrations of the amino acid alanine. This shows that the diet was not providing enough protein to successfully maintain a stable blood glucose level through gluconeogenesis. In contrast, the second study used diets containing 51% and 45% protein and there were no effects on the duration of gestation, litter size or puppy viability. A third study was done to determine the protein level required to alleviate the effects of a carbohydrate free diet fed during gestation and lactation. When carbohydrate is included in the diet, pregnant dogs require ~7 grams of protein per unit of metabolic body weight but if they were fed a carbohydrate free diet, that amount increases to ~12 grams [6] . 

Carbohydrates provide dietary fiber 

Carbohydrates consist of starch, sugars and fiber. Since there is no requirement for carbohydrates, there is also no requirement for fiber but it does aid in the proper function of the GI tract. It also acts as an energy source for the microbes found in the large intestine through bacterial fermentation to produce short chain fatty acids. Fiber has been found to be helpful in a variety of health conditions including obesity and GI disease.

Obesity

One thing that’s commonly said when owners are trying to get their dog to lose weight is that they are always begging for food. Fiber can play a huge role in managing this behavior. Providing a high protein, high fiber diet has been shown to be beneficial in overweight dogs due to the increased satiety it provides [12]. Insoluble fiber such as cellulose will increase the bulk in the stomach to promote satiety without increasing caloric intake. Soluble fibers slow gut transit time, so the addition of these fibers will also promote satiety due to the slowed digestion time. However, because soluble fiber is fermented in the large intestine, too much can cause increased gas so there needs to be a balance when formulating the diet [7]. 

GI disease

This term encompasses several disorders including gastric motility disorders, IBD, IBS, colitis, SIBO and gastroenteritis just to name a few. All of these disorders respond heavily to fiber manipulation but the amounts and balance of soluble and insoluble fiber vary between disease and often even the patient. In dogs with colitis for example, diets that are low, moderate and high in fiber have all been used successfully to manage the symptoms. Dogs with IBD are often recommended diets with low soluble fiber but moderate to high insoluble fiber to normalize transit time [7]. 

Carbohydrates are useful for (some) Performance Dogs

Working dogs can be divided into 3 types of activities depending on the intensity of the physical activity. The 2 ends of the spectrum are endurance sports, where the physical activity will last many hours but at lower levels of exertion, and sprint activity, which requires high intensity physical activity that typically lasts less than 2 minutes. Activities that require moderate to low levels of intensity include sled pulling, police/ service work and frisbee trials. These dogs have been shown to derive most of their energy through aerobic pathways, which use fat. These dogs show decreased performance with the addition of significant amounts of carbohydrates in their diet, and typically require high fat diets. 

Alternatively, dogs participating in sprint activities such as racing, agility and weight pulling, use anaerobic pathways to derive energy. Anaerobic pathways use glucose rather than fat. As discussed above, protein can be used to supply the body with glucose, but supplying carbohydrates can be beneficial. One study found that increasing protein and lowering carbohydrate in a racing greyhounds diet actually decreased performance [8]. While this study used only a very small sample size and the increased time it took to complete the race was minimal, it can be argued that carbohydrates do improve performance for sprint type activities. 

Carbohydrates can be beneficial for therapeutic conditions

There are many health conditions that benefit greatly from the addition of carbohydrates in the diet. We already discussed the addition of fiber for health conditions above, so now we will discuss how starch can be an important factor in these dogs’ diets. Because this is so dependent on each dog and their condition(s), I will just touch briefly on this topic. 

There are 3 macronutrients that make up the diet- protein, fat and carbohydrates. Some conditions, such as GI disorders recommend low fat diets while others, such as kidney diseases are typically lower in protein (in terms of keeping minerals close to the recommended allowance). When diets are formulated for these dogs, one must make a decision on how to distribute the macronutrients in the best way for that dog. A dog with colitis for example would likely need a low fat diet- so most of the macronutrient profile would need to be carbohydrates and protein. In order to keep fat and carbohydrate low, large amounts of lean protein would be needed. Some potential problems with this would be cost to feed (lean meat is much more expensive) and balancing mineral levels in the diet. Most (but not all) dogs with colitis also tend to do better with higher levels of soluble fiber, which would mean increasing the amount of carbohydrates [7]. Now, what if that dog also has a kidney condition such as a history of struvite crystals. Lowering protein would not be the main goal, but keeping an eye on certain mineral levels is important and can be hard to achieve with lean meat. Lean meat is going to be more concentrated in nutrients than fattier cuts would be. 

Another example would be a dog with a history of pancreatitis. Low fat, low to moderate protein diets are often recommended for these patients. High fat diets can be a factor in the onset of pancreatitis especially if the dog has a history. In addition, protein also promotes a strong pancreatic response so it is recommended to keep this low as well at least while the dog is recovering. This all means that the diets of dogs with a history of pancreatitis, or those recovering from an episode are likely going to need diets fairly high in carbohydrates- even as high as 80%. It is possible to slowly lower the carbohydrate content down after healing but they may always need a high carbohydrate diet [7].

Conclusion 

While a dog does not have a dietary requirement for carbohydrates, it can be argued that the addition can be a very beneficial addition to their diet. They can play an important role in the health of the dogs digestive tract as well as the management of several different health conditions. I do not prescribe to a one size fits all approach to canine diet formulation, I do encourage at least a small amount of carbohydrates in the diet if the dog tolerates it. 

Sources

1. Arendt, Maja et al. “Amylase activity is associated with AMY2B copy numbers in dog: implications for dog domestication, diet and diabetes.” Animal genetics vol. 45,5 (2014): 716-22. doi:10.1111/age.12179
2. Arendt, M, et al. “Diet Adaptation in Dog Reflects Spread of Prehistoric Agriculture.” Heredity, vol. 117, no. 5, 2016, pp. 301–306., https://doi.org/10.1038/hdy.2016.48. 
3. Axelsson, Erik et al. “The genomic signature of dog domestication reveals adaptation to a starch-rich diet.” Nature vol. 495,7441 (2013): 360-4. doi:10.1038/nature11837
4. Botigue´, L. R. et al. Ancient European dog genomes reveal continuity since the Early Neolithic. Nat. Commun. 8, 16082 doi: 10.1038/ncomms16082 (2017).
5. Carciofi, A C et al. “Effects of six carbohydrate sources on dog diet digestibility and post-prandial glucose and insulin response.” Journal of animal physiology and animal nutrition vol. 92,3 (2008): 326-36. doi:10.1111/j.1439-0396.2007.00794.x
6. Case, Linda P., et al. “Carbohydrate Metabolism.” Canine and Feline Nutrition: A Resource for Companion Animal Professionals, 3rd ed., Mosby, Maryland Heights, MO, 2011, pp. 75–79. 
7. Hand, Michael S., et al. Small Animal Clinical Nutrition. 5th ed., Mark Morris Institute, 2010. 
8. Hill, R C et al. “Effect of increased dietary protein and decreased dietary carbohydrate on performance and body composition in racing Greyhounds.” American journal of veterinary research vol. 62,3 (2001): 440-7. doi:10.2460/ajvr.2001.62.440
9. Moore, M. L., Fottler, H. J., Fahey, G. C., & Corbin, J. E. (1980). Utilization of Corn-Soybean Meal-Substituted Diets by Dogs. Journal of Animal Science, 50(5), 892–896. https://doi.org/10.2527/jas1980.505892x 
10. Ollivier, Morgane, et al. “Amy2b Copy Number Variation Reveals Starch Diet Adaptations in Ancient European Dogs.” Royal Society Open Science, vol. 3, no. 11, 1 Nov. 2016, https://doi.org/10.1098/rsos.160449. 
11. Reiter, Taylor, et al. “Dietary Variation and Evolution of Gene Copy Number among Dog Breeds.” PLOS ONE, vol. 11, no. 2, 2016, https://doi.org/10.1371/journal.pone.0148899.
12. Weber, Mickaël, et al. “A High-Protein, High-Fiber Diet Designed for Weight Loss Improves Satiety in Dogs.” Journal of Veterinary Internal Medicine, vol. 21, no. 6, 28 June 2007, pp. 1203–1208., https://doi.org/10.1111/j.1939-1676.2007.tb01939.x.