Rawhide, Rawhide: A dog person cannot walk into a pet supply store (or their own grocery store, for that matter) without noticing the explosion in the number of dog chews, dental devices and edible bones that are available for sale today. Some of these are biscuit or extruded concoctions containing a mixture of ingredients, while others originate from cow skin (rawhide chews) or are the left-over body parts of a hapless food animal (pig/lamb ears, hooves, and bully sticks).

If you do not know what a bully stick is, ask your mother. Better yet, ask your father.

Even as the selection of these items has expanded, nutritional information about them is still glaringly absent. Since all of these products are intended to be chewed slowly so that pieces or the entire product will be gradually consumed by the dog, we should at least be informed as to whether these items are actually digested by dogs, should we not?

Are they digestible? Dry matter digestibility refers to the proportion of a food that a dog’s gastrointestinal tract is capable of breaking down (digesting) and absorbing into the body. When we talk about the digestibility of a dog food, we are primarily concerned with its nutrient value and ability to nourish the dog. However, when we are considering the digestibility of rawhide treats, chews and dental products, the concerns are different but equally important. Any portion of a chew that is broken off and swallowed will travel through the length of a dog’s gastrointestinal tract, just like any other food. And, if the dog is able to bite off large chunks or swallow an entire chew at once, that piece has the potential to cause digestive upset, impede normal gut motility, or in the worse case scenario, cause obstruction if it is not dissolved and digested as it moves along.

Recently, a team of researchers at the University of Illinois decided to examine exactly this question by comparing the digestibility of different types of dog chews.

The studies: Two studies were conducted, both using an in vitro (test tube) technique that has been validated as a measure of the digestive conditions that occur in a dog’s stomach (gastric digestion) and small intestine (intestinal digestion). The first study compared in vitro dry matter digestibilities of sample products from six broad categories of dog treats (1). All of the products that were tested were produced by Hartz Mountain Corporation and the study was funded by the company. The second study compared just two types of treats, pork skin versus beef rawhide chews (2). The researchers also measured digestibility of the pork skin chew using a feeding trial with dogs. (For an explanation of digestibility trials with dogs, see Scoopin’ for Science). The reason for not doing a feeding study with the beef rawhide chew was not explained in the paper.

Results: Together, the two studies reported several interesting differences between the digestibility of dog chews:

1. Chews made from pig’s ears, which are composed primarily of cartilage and the protein collagen had very low gastric (stomach) digestibilities (14 %). Although these were almost completely digested in the intestinal environment (90 %), the lack of change in gastric acid means that a pig’s ear treat, if swallowed, would potentially leave the stomach intact and enter the small intestine will little change in size and consistency.
2. Similarly, with the exception of one product, rawhide chews made from cow skin were very poorly digested in the stomach. Intestinal digestion was almost complete for one product, but others continued to have low digestibility, even in the intestinal environment. The researchers noted that feeding rawhide chews to a dog who tended to consume large pieces could increase a dog’s risk for intestinal blockage.
3. When a pork skin chew was compared directly to a beef rawhide chew, the pork skin product’s digestibility was significantly greater than that of beef rawhide chew. After six hours, which is approximately the time it takes for a meal to begin to leave a dog’s stomach and enter the small intestine, the pork chew was more than 50 percent digested, while the rawhide was only 7.6 percent digested. This low rate of gastric break down continued even when tested up to 24 hours. After simulation of digestion in the small intestine (the major site of digestive processes in dogs), the pork skin rawhide was almost 100 % digested, while the beef rawhide reached only 50 to 70 % digestion under the same conditions. Rawhide was digested up to 85 % only when exposed to the intestinal conditions for 24 hours.
4. When dogs were fed one pork skin chew per day along with their normal diet, the overall digestibility of the diet increased. This corroborates the in vitro results and supports the conclusion that the pork skin chews were highly digestible.

Take Away for Dog Folks:

One of the most interesting results of these studies was the finding of such a large difference between the digestibility of pork skin versus beef rawhide chews. Because some dogs consume these types of chews rapidly and swallow large chunks, the fact that pork chews but not beef rawhides are highly degraded in the stomach and are highly digestible overall, is of significance to dog owners. These data suggest that if an owner is going to feed some type of rawhide chew (and mind you, I am not advocating for feeding these types of treats), but if one was choosing and had a dog who might consume the treat rapidly, feeding a pork skin chew appears to be a safer bet than a beef rawhide chew.

Second, it is important to note that all types of rawhide-type chews are composed of collagen, a  structural protein that makes up most of the connective tissues in the body. This is true for ears, pig skin, rawhide, and yes, even bully sticks. As these data show, collagen can be highly digestible (or not). The difference most likely depends on the source of the product and the type of processing that is used, both of which vary a great deal among products.

Feeding  dog a chew that is composed of collagen, even when it is highly digestible collagen, does not a nutritious treat make. Although collagen is a very important and essential protein in the body, it is not a highly nutritious food protein because it is composed almost completely of non-essential amino acids and is deficient in four of the essential amino acids. What this means from a practical perspective is that even though certain types of rawhide chews are found to be highly digestible and safe (from a digestibility perspective), this does not mean that they are providing high quality nutrition to the dog. In fact, they do not. While this research is important for pushing the peanut forward regarding the safety of these products in terms of digestibility, effects on gut motility, and risk of blockage, we still need more information (and selection) of chews for dogs that are both digestible and nutritious.

Cited Studies:

1. de Godoy MRC, Vermillion R, Bauer LL, Yamka R, Frantz N, Jia T, Fahey GC Jr, Swanson KS. In vitro disappearance characteristics of selected categories of commercially available dog treats. Journal of Nutritional Science 2014; 3:e47;1-4.
2. Hooda S, Ferreira LG, Latour MA, Bauer LL, Gahey GC Jr, Swanson KS. In vitro digestibility of expanded pork skin and rawhide chews, and digestion and metabolic characteristics of expanded pork skin chews in healthy adult dogs. Journal of Animal Science 2012; 90:4355-4361.

(Note: These studies and this blog essay do not address the ongoing and well-publicized problems with chicken jerky treats and illness in dogs. That is a topic for another time, another post).

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