Protein Quality in Pet Foods

This article goes beyond the “numbers” on pet food labels to take a closer look at determining protein quality in pet foods.

My recent article, Comparing Protein in Commercial Pet Foods, covered making sense of the Guaranteed Analysis on pet food labels. In that article, I showed how to convert the “as fed” listings to “dry matter” (DM) in order to accurately compare amounts of protein (and fat). I did not discuss protein quality in pet foods, however, as protein quality is not a variable in the DM equation. The amount of protein on a pet food label is strictly about “numbers”, which is why many mass market commercial pet food companies can produce products that look good on paper while really containing inferior quality protein ingredients that are not beneficial — and may even be harmful — to our pets’ health. This is where scrutinizing the label’s ingredients list becomes critical. In this article, I will discuss protein quality in pet foods so that you can look beyond the “numbers” to further determine the quality of your dog or cat’s food.

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The Right Type of Protein

You have probably read statements on the Internet such as, “High quality animal protein sources are necessary for your dog or cat.” But what is “high quality animal protein”? After all, most foods contain protein in varying amounts, including plant-based foods such as soy, wheat and corn. And, according to mass-market commercial pet food companies, the protein in these ingredients is “high quality.” Let’s take a closer look.

Here is a comparison from the USDA’s Nutrient Database of the protein content of various foods per 100 grams (3.5 ounces) of the food:

• Raw apple with skin slices – .26 grams
• Wheat flour, whole-grain – 13.21 grams
• Chicken, raw – 21.39 grams
• Beef, round, top round, steak, separable lean and fat, trimmed to 1/8″ fat, all grades, raw – 22.06 grams
• Peas, raw green – 5.42 grams
• Rice, white, medium-grain, raw, unenriched – 6.61 grams
• Soybeans, mature seeds, raw – 36.49 grams

Soy – a plant derived protein – beats out beef and chicken in the amount of protein it contains. But, is it high quality protein?

Protein quality is based on two major factors: the amount of essential amino acids it contains and how easily the animal’s body can digest and assimilate those amino acids (known as bioavailability). A quick reminder: Amino acids are the building blocks of protein and are therefore essential to life. Essential amino acids are amino acids that an animal’s body cannot synthesize in large enough amounts to maintain an adequate protein turnover and so they must be provided by the diet. Non-essential amino acids are still extremely important, but they need not be provided by diet because the animal can synthesize them endogenously (internally).

Let’s take dogs as an example. Dogs have a requirement for 22 different amino acids. Their bodies’ manufacture 12 of them, so those are the non-essential amino acids. The other 10 are essential because their bodies are unable to produce them in sufficient quantity and so they must come from dietary sources.

Let’s compare the content of the 10 essential amino acids for dogs between beef, wheat flour and soy.

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Now, this is very interesting! Soy is the winner in terms of both total amino acids and essential amino acids. So, why not use it? This is where bioavailability comes in.

Protein Bioavailability

When determining protein quality in pet foods, looking at bioavailability is essential. We know that high quality proteins must contain the correct amount of essential amino acids. However, that is only part of the story. Certain proteins may contain an ideal essential amino acid profile, but they are not provided in a form that is highly bioavailable, meaning that they are not readily digested, absorbed and utilized by the animal’s body. This greatly reduces the overall quality of the protein. After all, it doesn’t matter how great the amino acid profile is if the animal can’t benefit from it.

And that leads us back to soy. It may look great on the outside, but is it so great on the inside? Let’s see….

Protein Inhibitors

Certain proteins contain compounds that inhibit amino acid digestibility. These compounds are known as “anti-nutritional factors”. Trypsin inhibitors found in legumes are major anti-nutritional factors. Trypsin inhibitors reduce the biological activity of trypsin, an enzyme in the digestive system that breaks down protein. Interestingly, soybeans aren the most concentrated source of trypsin inhibitors among common food and feed protein sources.^[1]

Studies in rats and pigs have found that trypsin inhibitors from soybeans, kidney beans and other grain legumes cause reductions of up to 50% in protein and amino acid digestibility and reductions of up to 100% (yes, that’s a total reduction) in protein quality. Ironically, subjecting soy to sufficient heat treatment can inactivate its trypsin inhibitor activity, but application of prolonged heating to destroy all inhibitor activity also adversely affects its protein bioavailability,^[1] which is a major problem in dry extruded (kibble) foods.

Tannins are anti-nutritional factors found in sorghum and other cereals as well as fava beans and grain legumes. Studies in rats, poultry and pigs have found that high levels of tannins in these ingredients can significantly reduce protein and amino acid digestibility by up to 23%. In addition, phytates, another anti-nutritional factor found in cereals and legumes, has been shown to reduce protein and amino acid digestibility by up to 10%.^[1]

Interestingly, the negative health effects of anti-nutritional factors in rats are shown to be more pronounced in the elderly as compared to the young, indicating that older animals are more susceptible to adverse effects of these compounds.^[1]

Upon detailed inspection, we see that what appear on the surface to be “high quality” proteins, such as soy, really are not high quality at all due to low bioavailability. Obviously, this is not information that we hear about from mass-market pet food manufacturers touting grain-based foods for dogs and cats as “high quality”!

(Note: Soy, corn and wheat also have many other negative health effects associated with them. For example, soy is goitrogenic and disrupts estrogen function, while corn contains lectins and wheat contains gluten. We will discuss these issues in upcoming articles, but please note that they greatly reduce the quality of the protein beyond the issue of bioavailability. After all, anything that is harmful at the cellular level does not make for a high-quality protein.)

The Bottom Line

Protein quality in pet foods is every bit as important as the quantity of protein in the food. For the health of our companion animals, let’s choose the freshest, least processed, highest-quality proteins possible and avoid products that rely on inferior quality soy, wheat and corn-based proteins.

In my next article, I will discuss how you can evaluate pet foods to determine the quality of their proteins. It might seem simple, but ingredient lists can be deceiving. Sometimes, a food that lists meat as the first ingredient really has less meat-based protein than a food that lists corn as the first ingredient. Pet food companies may also use a tactic called “ingredient fractioning” or “ingredient splitting” to make it look like a product contains a lesser amount of grains than it really does.

As always, I hope that you enjoyed this article and that it helps you and your four-legged companion to enjoy many healthy, happy years together.

1. ^[1] Sarwar Gilani G,C W Xiao & Cockell KA. (2011). ‘Impact of Antinutritional Factors in Food Proteins on the Digestibility of Protein and the Bioavailability of Amino Acids and on Protein Quality’, British Journal of Nutrition, vol. 108, pp. S315–S332. ↩

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