At K9Bioactives, we recognize that a healthy gut is the cornerstone of health for our pets. While fiber has long dominated gut health strategies, research demonstrates that animal-derived fermentable substrates (ADFS)—like hydrolyzed collagen, skin, cartilage, lung, glucosamine, etc.- can also support gut health.

Two studies— one by Butowski et al. (2024) and another by Depauw et al. (2012)—highlight the unique fermentation properties of these animal substrates, offering insights into how they can support the gut health of our dogs and cats.

Animal-Derived Substrates

Even now, pet nutrition focuses almost entirely on dietary fiber and, more recently, probiotics to support digestive health. However, both carnivores and omnivores in the wild consume bones, tendons, skin, fur, etc., which are not extensively digested.  Research does show that these animal tissues can yield beneficial gut metabolites, functioning similarly to different dietary fibers to support the microbiome and gut integrity.

In vitro fermentation using canine and feline fecal inocula

The 2024 study by Butowski et al. looked at the in vitro fermentation of several animal-derived ingredients using fecal inocula from cats and dogs.

Substrates used

• hydrolyzed collagen
• tendon
• mechanically deboned meat
• bone cake (the remnants after the MDM process),
• lung
• skin (bovine hide)
• corium (Commonly labeled as collagen chews)

Results

• Hydrolyzed Collagen resulted in the highest levels of butyrate but, at the same time, had the highest level of ammonia. On the other hand, it also had the lowest levels of indole in both dogs and cats. Butyrate functions as an energy source for colonocytes and supports the integrity of the intestinal barrier. While both ammonia and indole
• Bone cake was more fermentable in the feline sample, while MDM was more effective in the canine sample underscoring the importance of tailoring the formulation to each species.
• Significant differences were observed in the production of ammonia and indole. While butyrate is beneficial in that it is an energy source for the colonocytes, both ammonia, and indole, if accumulated excessively, can have negative impacts on gut health by promoting inflammation and disrupting the gut barrier function.
• Principal Component Analysis showed that Hydrolyzed collagen and bone cake created distinct metabolite profiles compared to the other substrates, which indicates that, similar to fiber, not all Animal Substrates offer the same benefits.

Caveat

One major caveat to interpreting the results is that the canine and feline fecal inocula used in the study came from dogs and cats already consuming these ingredients at higher levels than what is commonly found in most dog and cat foods, with the canine diet containing 43.93% protein 41.78% fat, 1.18% crude fiber 10.2% ash, while the feline diet contained 43.14% protein 41.83% fat 1.31 crude fiber and 9.69% ash. Meaning the microbiome of both the dogs and cats used in the study might contain specific bacterial strains that can ferment these substrates. In contrast, the microbiome of dogs and cats consuming less of these ingredients might not be similarly adapted to ferment these substrates.

Comparative insights from a strict carnivore (Cheetah)

The 2012 study by Depauw et al. looked at the fermentation patterns of the same and several additional animal substrates using cheetah fecal inocula. 

Substrates Used:

• Rabbit hair
• Rabbit skin
• Rabbit bone
• Chicken Cartilage
• Hydrolyzed bovine collagen
• Glucosamine
• Glucosamine-Chondroitin
• Fructooligosaccharide (Positive Control)
• Cellulose (Negative Control)
• Casein

Results

• Animal substrates like glucosamine, chondroitin, collagen, and cartilage, while not fiber, could stimulate beneficial fermentation patterns commonly associated with fiber.
• Other animal tissue, such as rabbit bone, hair, and skin, results in very little fermentation, meaning not all animal substrates offer the same benefits for gut health.
• Chicken cartilage was much more rapidly fermentable compared to other substrates such as glucosamine, chondroitin, or hydrolyzed collagen. Meaning different substrates vary not only in terms of total fermentability but also the speed at which they are fermented.
• Ammonia and phenolic compounds were higher after the fermentation of collagen and rabbit bone, underscoring the importance of balancing substrate choice to maximize gut health benefits while minimizing potential risks. 
• The study provides further evidence that we must balance animal-derived fermentable substrates with fiber based on their fermentation profiles and potential impacts on gut health.

Caveat

The caveat is that cheetahs are not dogs or cats. Additionally, the cheetahs were being fed a diet very dissimilar from what most dogs and cats are consuming, with diet one consisting of 1.5 Kg/D Beef Muscle Meat with 30g/kg vitamin-mineral mix, and diet two consisting of 2 to 2.5 kg/d whole rabbit.

Protein Fermentation: Balancing the Good and Bad

Protein fermentation can be a double-edged sword. While beneficial compounds like butyrate can be produced, the breakdown of protein can also lead to the generation of potentially harmful metabolites:

• Ammonia: Elevated levels can be toxic to gut epithelial cells which can lead to increased intestinal permeability.
• Indole: Indole can support mucosal barrier function, but excessive production is linked to inflammation and dysbiosis.
• Phenolic Compounds: Fermentation of aromatic amino acids such as tyrosine and phenylalanine can generate phenolic compounds like p-cresol, which have been implicated in disrupting mucosal barrier integrity.
• Branched-chain fatty acids (BCFAs): Derived from the fermentation of branched-chain amino acids (valine, leucine, isoleucine), BCFAs can have differing effects—while they may have some roles in signaling, excess levels can be associated with dysbiosis.
• Biogenic Amines: Compounds such as putrescine, cadaverine, and histamine can also arise during protein fermentation. While biogenic amines do play physiological roles at low concentrations (e.g., regulation of intestinal blood flow and immune responses), their excessive accumulation has been linked to gut inflammation. Managing the production is crucial for maintaining intestinal health.

Undigested Fat

While not the focus of either paper, undigested fats can also undergo fermentation in the gut. When undigested fat reaches the large intestine, it can be fermented by specific bacterial populations. Fermentation of fat is less efficient than saccharolytic or proteolytic fermentation, but it can result in the production of potentially harmful by-products, such as hydrogen sulfide and other sulfur-containing compounds. These compounds can disrupt the balance of the gut microbiota and contribute to conditions like diarrhea and gut inflammation.

Balancing Animal Substrates and Fiber Fermentation

While animal substrates like collagen can enhance butyrate production, balancing protein fermentation with dietary fiber fermentation is critical. Both Prebiotics (Inulin, FOS, GOS) and other Fermentable fibers undergo saccharolytic fermentation, producing SCFAs like acetate, propionate, and butyrate without the harmful by-products of protein breakdown.

A combination of prebiotics, fermentable fibers, and carefully selected Animal Substrates can:

• Maximize beneficial SCFA production.
• Limit the accumulation of toxic metabolites like ammonia, indole, phenols, and biogenic amines.
• Promote a more diverse and resilient gut microbiome.

Additionally, balancing protein and fiber fermentation supports the overall microbial ecosystem. Many beneficial bacteria thrive on fiber, whereas an imbalance favoring protein fermentation can encourage the growth of potentially pathogenic bacteria. This balance is crucial for sustaining a healthy gut barrier and systemic immunity.

The goal is not to replace fiber with protein fermentation substrates but to integrate them thoughtfully to support optimal gut health.

Conclusion:

At K9Bioactives, we are excited to translate this cutting-edge science into innovative, effective gut health solutions for pets.