Cats Have Increased Protein Digestibility as Compared to Dogs and Improve Their Ability to Absorb Protein as Dietary Protein Intake Shifts from Animal to Plant Sources
Authors: Golder Christina M. , Weemhoff James , Jewell Dennis E. 
Published: April 2020

This retrospective study used 226 dogs and 296 cats to evaluate whether protein absorption was influenced by species and within species and the influence increasing the percentage of total dietary protein as plant protein had on protein absorption. All protocols were reviewed and approved by the Institutional Animal Care and Use Committee, of Hill’s Pet Nutrition, Inc. A minimum of six dogs or cats were assigned to each study and the Association of American Feed Control Officials protocol for digestibility was used to evaluate protein digestibility. In short dogs and cats were fed for a minimum of 6 days for dogs and 10 days for cats. Following this there were 5 days of fecal collections. Dogs and cats varied from 1.1 to 12.8 years of age (dogs 6.4 ± 3.1 cats 7.3 ± 2.7). There were 459 dog studies and 427 cat studies completed. Dietary inclusion of animal based and plant protein was calculated and the change in protein digestibility evaluated for any response to the changing makeup. The percent of protein from plants was used as well as the percentage of food as fiber as the independent variables with protein digestibility the dependent variable. Increasing fiber was associated with decreasing protein digestibility in dry and canned cat food and dry dog food. This analysis showed that both dogs and cats were able to digest dietary plant protein with protein digestibility in dogs decreasing as plant protein increased from 92% to 90.8% at 50% plant protein in dry dog food and 89.1 to 83.7% in canned dog food. In cats, as plant protein increased, protein digestibility increased from 93.5 to 96.8% and in canned from 95.5% to 97.4% at 50% of the protein from plants. Both of these calculations are with foods with 0% fiber in order to exclude the negative effect of fiber. When individual plant high concentration protein sources (excluding the protein from whole grains) were evaluated (i.e., soybean meal, soybean protein isolate, corn gluten meal and rice protein concentrate) it was observed in the dog that there was no slope in response to increasing protein from these sources. The intercepts were 87.5, 91.1, 89.5 and 92.3 percent for soybean meal, soybean protein isolate corn gluten meal and rice protein concentrate, respectively. In the cat there was a significant positive effect on protein digestibility by increasing concentration of corn gluten meal (protein digestibility equals 92.9 + 0.093 * percent protein from corn gluten meal). There was no response to the other plant protein sources on digestibility with the intercepts of 93.7, 95.5, and 96.4 percent for soybean meal, soybean protein isolate, and rice protein concentrate. In summary, as the dietary protein shifted from striated muscle and other animal proteins to plant based proteins, there was a decline in the protein digestibility in the dog (1.15% at 50% protein in dry dog food) while in cats increasing dietary plant protein was associated with increasing protein digestibility (3.35% increase at 50% protein in dry cat food). Protein digestibility of food is similar if not enhanced when the plant protein sources are concentrated from soybeans (soybean isolate, soybean meal), corn (corn gluten meal) or rice (rice protein concentrate).Support or Funding InformationFunding was from Hill’s Pet Nutrition, Inc.