Pet food protein sources: animals, plants and by-products

Dogs and cats don’t really require protein in their foods; they require proper levels and bioavailability of amino acids, which multiple sources can supply.

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Andrea Gantz
Andrea Gantz

Before discussing the pros and cons of various protein sources, I thought we should revisit the importance of dietary protein for dogs and cats. Dietary protein is required for many reasons. First and foremost, dietary protein provides essential amino acids that are required for the synthesis of many different proteins in the body. Secondly, dietary protein also provides non-essential amino acids that are used for gluconeogenesis and/or energy for maintenance, growth, gestation and lactation in dogs and cats (NRC, 2006).

It’s about amino acids, not just crude protein

For those who formulate foods based on amino acid content, one could make the argument that dogs and cats don’t really require protein. They require amino acids in the proper levels and proportions (e.g., ratios) and these must have a high bioavailability – i.e., they must be available for absorption and utilization. Therefore, crude protein levels of food alone do not tell the consumer, retailer or veterinarian if the dog or cat’s amino acid requirements will be met from a food.

If you recall my previous blog post “Pet food nutrition: it’s not just about high protein,” high crude protein does not equal high quality. Providing formulated levels of amino acids is a good start in supplying information about your foods; however, knowing the apparent digestibility or availability of each amino acid is ideal.

Determining apparent digestibility or availability of amino acids

Estimations of amino acid availability of protein sources can be determined by measuring the digestibility of each amino acid at the end of the small intestine (Gross et al., 2010). Notice I said the small intestine. Any amino acids or protein that are not digested or absorbed prior to leaving the small intestine will be fermented by microbes in the hindgut and produce fecal odors compounds. These amino acids reaching the hindgut are not available nor usable to the animal to meet amino acid requirements. Therefore, utilizing total tract digestibility results in the overestimation of protein and amino acid digestibility.

Small intestinal digestibility of various protein sources in dogs

Gross et al. (2010) summarized all the peer-reviewed studies from 1999 to 2005 for small intestinal digestibility of different protein sources in dogs. For purposes of this discussion, I selected common protein ingredients utilized in pet food today and summarized them in Table 1. Additionally, I selected foods with a rice as the common carbohydrate source and a 20 percent crude protein level to make the proper comparisons amongst the ingredients.


Common protein ingredients used in pet foods.


Unfortunately, no one to date has been able to successfully measure amino acid or crude protein small intestinal digestibility in cats. As a result, there is no peer-reviewed data for cats. One could review the total tract data; however, as mentioned previously, it would not provide accurate information. As a result, I will not be showing any cat data in this discussion.

Your dog is still not a wolf!

As you can clearly see in Table 1, the amino acid digestibility is similar across all protein sources except for soybean meal. The lower soybean meal digestibility is not surprising because it often has oligosaccharides and fiber (think flatulence) that can alter and reduce digestibility. As a result, most nutritionists limit its use in formulas.

If you were to compare protein sources on digestibility alone, one could make the argument that they are equal. Since the body does not care about the source of the amino acids, these protein sources can be used interchangeably or to complement each other. Consequently, many pet food companies utilize multiple protein sources in their formulations (animal and plant combined) to meet an animal’s requirements.

Ever check the ingredient statement on a bag of food?

At this point, you are likely thinking “not my brand”; however, have you reviewed the entire ingredient statement? Most products in the marketplace will start with meat, followed by meals (fish, chicken, beef, etc.) and plant proteins (potato protein, chickpea flour, pea protein, etc.). Again, the utilization of complementary proteins ensures optimal nutrition and supports the concept I mentioned above, while attempting to target a high protein content or guaranteed analysis.

For those who are still debating my first blog post, “Your dog is not a wolf!”: If a dog can digest the starch and protein components of plants, do you still think you should feed them like a wolf? If so, wait until I cover the benefits of fiber for dogs and their digestive tracts in a future blog post.

It’s still not about high protein

As I mentioned in my “Pet food nutrition: it’s not just about high protein” blog, it is about protein quality. Protein quality is impacted by the concentration and composition of essential amino acids, non-amino acid nitrogen (nucleic acids, amines, amides), digestibility and the bioavailability of each essential amino acid.

Above we focused on the digestibility of amino acids in various protein sources. So, I would be remiss if I did not show the typical moisture, protein and amino acids levels of the protein sources I discussed (Table 2). Showing the two tables together should help give you a better picture of whether or not the protein is high quality.


Protein quality is impacted by the concentration and composition of essential amino acids, non-amino acid nitrogen, digestibility and the bioavailability of each essential amino acid.

What do I do with this knowledge?

So now, you are probably wondering what to do with this knowledge. First and foremost, it should be evident that it is important to recognize the importance of optimal levels of essential amino acids and their availability regardless of protein source.

Also, it doesn’t take a calculator nor mathematician to see that the probability of obtaining a high-quality-protein pet food (amino acid level and availability) with meat alone is not likely (look at moisture, as indicated in Table 2 by percentage of dry matter, and amino acid levels). Especially if you are formulating for proper amino acid levels and ratios to prevent any imbalances or antagonisms (e.g., lysine-arginine) in a kibble format.

As a result, most foods in the marketplace will utilize complimentary protein sources along with crystalline amino acids to properly formulate and balance their foods. This combination ensures the optimal nutrition for your dog or cat. It is very common to see foods with an animal protein, a plant protein and one or more of the following amino acids: L-Lysine, DL-methionine, L-threonine, L-tryptophan and taurine.

Animal by-products are making a resurgence

As you can clearly see from the tables, animal by-product meals in the table are high in essential amino acids and are highly available. Unfortunately, over recent years these ingredients have been demonized by marketing of “not included” claims.

However, new companies have found novel ways to market and glamorize these ingredients by calling them out individually. While at recent trade show, I saw companies calling out green tripe (stomach lining), livers, gizzards, lung and blood as ingredients in their foods or treats. I even saw dehydrated rabbit feet, duck heads, duck feet, turkey tails and turkey fries. I guess it all comes down to marketing sometimes!

Next topic: Navigating through pet food product claims: Part 1

Next time we will discuss this topic: “Navigating through product claims: Part 1.” If there are topics you would like to have discussed, feel free to comment below or reach out via LinkedIn:



Gross et al., 2010. Macronutrients: In Small Animal Clinical Nutrition, 5th Edition. Topeka, Kansas: Mark Morris Institute, p. 49-105.

Hendricks et al., 2002. Nutritional Quality and Variation of Meat and Bone Meal. Asian-Aust. J. Anim. Sci. 10: 1507-1516.

Murray et al., 1998. Raw and Rendered Animal By-Products as Ingredients in Dog Diets. J. Nutr. 12: 2812S-2815S.

NRC, 2006. Proteins and Amino Acids in Nutrient Requirements of Dogs and Cats. Natl. Acad. Press, Washington, DC. p. 6–138.

Yamka et al., 2003. Evaluation of low-ash poultry meal as a protein source in canine foods. J. Anim. Sci. 2003. 81:2279–2284. 

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