Domestication & Diet: Dog Genes & Cat Gut Bacteria

Swedish researchers in their comparative studies of dog and wolf DNA report that dogs show changes in genes governing three critical steps in the digestion of starch, first breaking down large carbohydrate molecules into smaller ones, then breaking these up into smaller sugars and then finally facilitating their absorption in the digestive system. Significantly there was so called gene duplication, multiple copies of a gene for amylase, produced by the pancreas that is involved in the first step of starch digestion.

Wolves had two copies while dogs had four to 30.( See Erik Axelsson et al 2013 The genomic signature of dog domestication reveals adaptation to a starch-rich diet. Nature 495:360-364. doi:10.1038/nature11837 ).

While some dog food companies may be quick to jump on this research as scientific evidence supporting their continued and widely questioned practice of manufacturing high grain/starch diets, they do need to pause and consider what these findings really mean. They do not mean that it is OK to make starches a major dietary ingredient for dogs. But they do mean that many dog breeds are more omnivorous than wolves and can digest some starches as a consequence of co-evolved selection living in close association with humans for many generations. Humans also went through similar genetic-dietary changes with the shift from being gatherer-hunters (the Paleolithic diet) to grain-eating (and dairy consuming) agro-pastoralists. But just like with we humans, dogs show nutrigenomic differences, some developing exocrine pancreatic insufficiency when after being raised on a high grain diet, or diabetes, obesity, inflammatory bowel disease, skin disorders and even epilepsy. When these symptoms disappear when they are taken off high grain/starch diets, we have medical based evidence of the probable cause. The recent inclusion of genetically engineered food ingredients in dog (and cat) foods, such as corn, rice, canola and sugar beet, may aggravate these conditions and cause other health problems as documented in Not Fit for a Dog: The Truth About Manufactured Cat & Dog Foods by veterinarians Fox, Hodgkins & Smart. This interesting genetic research revealing some of the differences between the wolf and domesticated dog genome shows how processes of adaptation operate through gene-environment interactions.

These processes, in the realm of dietary choices and what kinds of foods are available, are epigenetic, but there are additional considerations. Dietary ingredients can alter the ‘microbiome’ of the digestive system—the numbers and varieties of bacteria and other microorganisms which play an essential role in the digestion of various foods and assimilation of nutrients, coupled with critcial immune defense, metabolic and other regulatory functions.

According to a recent study in kittens fed either a high protein low carbohydrate (HPLC) or moderate protein and moderate carbohydrate (MPMC) diet, levels of proteolytic bacteria (which break down protein) were higher for kittens on the HPLC diet and levels of saccharolytic bacteria (which break down carbohydrates) were higher for kittens on the MPMC diet. This illustrates how adaptive processes can operate it the level of the digestive system’s bacterial microbiome. (see Kelly Swanson et al, British Journal of Nutrition, Aug 31, 2012). They also looked at relationships between the diets and physiology. The kittens fed the MPMC diet had high levels of bifidobacteria, which was linked to higher blood ghrelin levels. Ghrelin is a hormone that stimulates appetite and thus may be linked to weight gain. At the same time, the bifidobacteria may promote better gastrointestinal health. Low levels in humans have been linked to inflammatory bowel disease. While pet food manufacturers still marketing both moderate and high carbohydrate diets for cats may applaud this study of a small number of cats, it should be noted that kittens on a high protein diet had higher levels of bacteria that break down protein. This study does nothing more than to demonstrate how diet affects gut flora/bacterial populations, and in no way justifies feeding cats and kittens a ‘balanced’ diet of moderate amounts of protein and carbohydrate. This study simply demonstrates how commensal and symbiotic intestinal bacteria are affected by different diets, just as the Swedish study comparing dog and wolf DNA reveals what one would anticipate in terms of genetic adaptations over generations in dogs to dietary changes associated with a domesticated existence. The cat study does not look at the long-term differences between obligate carnivore felines being fed biologically appropriate and inappropriate diets in terms of health and longevity, even though their changed gut flora may help cats adapt to a high carbohydrate kibble diet. The wolf-dog comparisons do not consider the long-term consequences of raising dogs on a more ancestral, wolf-like diet with little or no carbohydrates. This is a popular trend today, and has some merits for those dog breeds and individuals (yet to be systematically identified) with low genetic duplication for pancreatic amylase production and therefore limited ability to process starches. The notion that too much protein in the diet can cause kidney disease is erroneous, lacking biological and clinical evidence to my knowledge, but that is not to dismiss the legitimacy of concern regarding protein quality and quantity when the kidneys are actually diseased. Furthermore, recent studies of canine nutrition in both pups and old dogs have shown that various amino acids and essential fatty acids, which are either not available or of limited availability in vegetarian diets, play a vital role in cognitive development and immune system function, and may help prevent declines in both functions with age.