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‘Gut feeling’ on postbiotics


What are postbiotics and how are they formed?

The microbiota within the gut of dogs and cats is capable of many roles and functions without which the animal body cannot achieve optimal performance. The highest microbial colonization and the most intense microbial activity in dogs and cats is within the large bowel. It is an anaerobic chamber with slow ingesta transit where the final phase of digestion takes place via the action of microbial enzymes. Unlike the small intestine, the colon doesn’t produce digestive enzymes, thus the digestive process in this part of the alimentary tract is fully orchestrated by commensal microbes. Gut bacteria depend on their host to provide the necessary nutrients to promote microbial growth. The conventional methods to influence the intestinal microbiota include macronutrients in food, prebiotics, probiotics or synbiotics (Figure 1).  In return, bacteria produce or make available small molecular weight metabolites during their life cycle. These metabolites are called postbiotics and provide benefits or potentially detrimental adverse effects  to microbes themselves and to the host. The term postbiotics is relatively new and refers to the metabolic product or byproduct secreted by live commensal bacteria (e.g. short chain fatty acids – SCFA, indol, indoxyl sulfate, phenolic metabolites, vitamins), or released after bacterial lysis (e.g., enzymes, peptides, polysaccharides, cell surface proteins, organic acids).

how prebiotics are formed

Figure 1. Postbiotics are metabolites secreted by live bacteria, released after bacterial lysis or they are released and further metabolized trapped compounds in fibers. They may act locally in the colon or be taken up by the host into the systemic circulation. The conventional methods used to influence postbiotic synthesis are through manipulation of gut microbes with prebiotics, probiotics, synbiotics and macronutrients in food.

Fermentation of complex carbohydrates in the intestinal tract contributes to energy production in various animal species. It is the core activity of the microbiota which allows energy to be created in the absence of oxygen from a wide variety of substrates that escape digestion in the upper gastrointestinal tract. In dogs and cats, fermentation produces rather few calories, but in herbivores, fermentation provides a majority of the daily energy requirement.  The large intestine of the dog and cat receives ileal content which has undergone digestion by the action of animal enzymes, such as proteases, lipases and amylases. However, this material still contains some undigested carbohydrates (including unabsorbed starches, fiber, oligosaccharides, and resistant starches), undigested dietary protein and fat, all of which microbes readily utilize as fermentation substrates for their own growth. In addition to dietary compounds, the colonic microflora is presented with sloughed epithelial cells, pancreatic enzymes, bile acids, and mucus. During the fermentation process, various gases are produced including hydrogen, carbon dioxide and methane. Gases readily diffuse across the intestinal mucosa and are eliminated both through the lungs and expelled as flatus. The final product of colonic microbial and host digestion is excreted from the body as feces.

Why do postbiotics matter to dogs and cats?

The large bowel contains a range of different bacteria species which have enzymes to digest and utilize various substrates.  These include saccharolytic species that break down and ferment carbohydrates, proteolytic bacteria that degrade proteins, peptides and amino acids, methanogens, and other bacteria that grow on the intermediate products of fermentation, such as hydrogen, lactate, succinate and ethanol.1 When it comes to utilization of substrates, some anaerobes are specialists focusing on a single substrate for fermentation while others are generalists, capable of fermenting a variety of substrates. For example, acetate is produced by many bacteria, but propionate and butyrate tend to be produced by specific bacteria.2


The most important determinant of intraluminal events is the amount and type of substrate available to microbes for digestion. This will determine which postbiotics are being formed.

Postbiotics derived from carbohydrate and fiber fermentation

Providing primarily carbohydrates to the gut microbiota results in a saccharolytic type of fermentation. The amount of dietary carbohydrate that comes into the colon depends on many factors, including meal size, food processing (cooking, extrusion), food form (e.g. whole grain vs refined starch), and rate of enzymatic digestion by the host and gut transit. The digestive function of the colon primarily involves the bacterial breakdown of carbohydrates not digested in the small intestine, such as resistant starch, oligosaccharides or fiber. The major postbiotics generated by canine and feline intestinal microbiota are straight short chain fatty acids (SCFA) acetate, propionate, and butyrate (Brosey BP Gastrointestinal volatile…. 2000). They are mainly products of microbes with saccharolytic function and they have important physiological roles (Figure 2). SCFA provide a powerful driving force for movement of sodium and water out of the large bowel lumen, which allows protection from diarrhea.3 Acetate, propionate, and butyrate reduce inflammation via receptors on immune cells and alteration of gene expression.4 A number of animal studies show that SCFA maintain mucosal integrity and growth.5 Acetate and propionate are energy substrates for microbial growth but they are also absorbed from the colon and provide a source of energy for the body. Interestingly, up to 7% of the metabolic energy of dogs, and to a lesser extent in cats, is produced by microbial fermentation in the colon. Without the presence of gut microbes, these energy substrates would never form and the energy would remain trapped within the fiber and lost in the feces. Butyrate is an important energy fuel for colonocytes and has the capacity to modify cell growth in the colonic epithelium via modulation of nucleic acid metabolism.1  It is also well recognized that SCFA stimulate canine and feline colonic smooth muscle contraction.6,7 The prokinetic effect of SCFA provides another mechanism by which dietary fiber is beneficial in the management of colonic motility disorders.

Physiological functions of short chain fatty acids (SCFA)

Figure 2. Physiological functions of short chain fatty acids (SCFA)

Postbiotics derived from plant polyphenols

Polyphenols are diverse class of plant metabolites, associated with the color, taste, and defense mechanisms of fruits and vegetables. Plants synthesize them as a protection against UV light and pathogens. They can be found in skin, flesh, and fiber of many fruits and vegetables. Polyphenols are molecules with reported antioxidant, anti-inflammatory, anti-microbial and disease modulating properties, and they possess their bioactive roles after ingestion, either in their native form or as metabolites.8


Not all polyphenols are absorbed with equal efficacy and they are often transformed before absorption.


The absorption of polyphenols from citrus fruit was evaluated in dogs.9 Ten healthy beagles were administered 70 mg citrus flavonoids as a grapefruit extract contained in capsules, while two additional dogs were used as controls and given an excipient. The grapefruit flavanone naringin, along with its bacterially transformed metabolites naringenin and naringenin glucuronide, was detected in dog plasma. This study demonstrates that oral administration of polyphenols has potential to reach measurable concentrations in the circulation. Their native chemical structures as well as metabolites have potential to provide biological activity not only within the gastrointestinal tract but beyond the gut as well. 


Many polyphenols are extensively metabolized by intestinal and hepatic enzymes as well as by the intestinal microflora. This transformation modulates their biological activity. Some polyphenol metabolites produced by gut microbiota are even more biologically active than their precursors. The gut microbiota plays a unique role in liberating fiber-bound polyphenols from fruit and vegetable fiber sources and transforming dietary polyphenols into biologically active species that can act locally within the colon or systemically when absorbed. Many colonic metabolites of plant polyphenols have been found in the urine, confirming that some colonic absorption occurs in animals.10


An interesting mechanism of action of polyphenols is their antimicrobial effect. Phenolic compounds have demonstrated inhibitory effects on both the growth and adhesion of pathogenic bacteria, such as Clostridium spp., E. coli, and Salmonella typhimurium, while promoting proliferation and adhesion of beneficial bacteria, such as Lactobacillus or Bifidobacterium.11,12 This makes selective antimicrobial effect of polyphenols superior to that of antibiotics, which reduce beneficial, as well as harmful bacteria. As such, plant polyphenols have clinical implications in veterinary gastroenterology and nutrition.


New emerging controlled studies have shown that dietary intervention with a prebiotic blend which included fiber-bound polyphenols modulate canine and feline gut microbiota towards a more ‘health-promoting profile’ by increasing the relative abundance of beneficial bacteria.13 In one controlled study, a food containing pea powder, tomato pomace, and broccoli powder for felines led to a significant increase in beneficial Bifidobacterium and reduction in the potentially detrimental genera Clostridium and Eubacterium (Figure 3).14 Similar results were observed in dogs

Adult cats fed a test food containing pea powder, tomato pomace, and broccoli powder had significantly increased fecal proportion of Bifidobacterium as compared to cats fed the control food.

Figure 3. Adult cats fed a test food containing pea powder, tomato pomace, and broccoli powder had significantly increased fecal proportion of Bifidobacterium as compared to cats fed the control food.14

Postbiotics derived from protein putrefaction

A large variety of protein substrates are available for microbial metabolism in the colon, including dietary proteins that by-pass small intestinal digestion, sloughed epithelial cells,  hydrolytic enzymes secreted by the pancreas, or mucin glycoproteins. Anaerobic degradation of undigested protein in the colon is a process called putrefaction. Bacterial proteases and peptidases break the protein down to peptides and amino acids, and release of ammonia (NH3) through deamination. Other metabolites of proteolysis include branched chain fatty acids (BCFA) from fermentation of branched amino acids, phenolic, and indolic compounds from aromatic amino acids, and hydrogen sulfide (H2S) from sulfur containing amino acids. Finally, decarboxylation of amino acids results in the appearance of amines in the gut.


In humans, it is accepted that carbohydrate fermentation results in beneficial effects for the host because of the generation of short chain fatty acids, whereas excessive protein fermentation is considered detrimental for the host’s health. The recent research advances show that similar trends can be applied to dogs and cats.


While some products of protein fermentation have benefits to the host, studies in animal models and in vitro show that some postbiotics of excessive protein putrefaction like ammonia, phenols, p-cresol, certain amines, and hydrogen sulfide, play important roles in the initiation and  progression of a leaky gut, inflammation, DNA damage, and cancer progression.15 Conversely, dietary fiber or intake of plant-based foods appear to inhibit this, highlighting the importance of maintaining gut microbiome carbohydrate fermentation.16 One mechanism of action by which carbohydrates inhibit putrefaction is that active carbohydrate fermentation stimulates the bacterial requirement for nitrogen to support increased growth of bacterial biomass, lowering nitrogen availability for putrefaction. The positive effects of carbohydrate fermentation also relates to the fact that SCFA lower the pH in the colon, which inhibits bacterial proteases and peptidases which are more active at neutral to alkaline pH.  Thus, promoting the production of SCFA from carbohydrates and maintenance of an acidic colonic pH will help to lower putrefaction.13,15


The effect of a high protein raw meat-based food (DMB: 76% crude protein, 0% carbohydrate, 0% crude fiber) and a commercial moderate protein dry food (DMB: 30% crude protein, 35% carbohydrate, 1.8% crude fiber) on gut microbiota was evaluated in healthy dogs.17 Dogs fed the high protein food had decreased gut microbial diversity, which is an attribute of dysbiosis, and this was accompanied by decreased production of SCFA. Authors concluded that these changes were similar to reduced diversity found in GI diseases like inflammatory bowel disease or acute diarrhea. This is indicative of an unfavorable and unhealthy microbial diversity.


Researchers at Hill’s Pet Nutrition have shown that feeding dogs food enriched with fibers containing fiber-bound polyphenols improved fecal stool quality, increased saccharolytic postbiotic concentrations, decreased stool pH, decreased microbial putrefaction, and decreased polyamine concentration, when compared to dogs fed a food without fiber-bound polyphenols.13 The stool quality was significantly improved in both healthy dogs as well as dogs with chronic enteritis . These findings show that fiber containing bound polyphenols can positively regulate the gut ecosystem and optimize fecal score.


Tryptophan metabolites of colonic bacteria, such as phenols and indoles, are absorbed into the circulation and are cleared by normal‐functioning kidneys. Indoles have several beneficial functions such as upregulation of tight junction proteins and modulation of inflammatory genes in intestinal epithelial cells18; however one of their metabolites, indoxyl sulfate, is a potential uremic toxin that accumulates in patients with chronic kidney disease (CKD).19 In one study, cats with CKD IRIS Stage 1-4 had significantly increased indoxyl sulfate in serum when compared to healthy cats. P-cresol was also increased but it did not reach significance.20 Furthermore, cats with CKD also had lower bacterial species richness, an attribute of a healthy microbiome, when compared to healthy cats in this study.


Despite these important findings, the relationship between gut health and protein putrefaction in dogs and cats will require more research to derive specific recommendations for protein intake or carbohydrate : protein ratio to optimize gut microbial function in health and disease. Nevertheless, maintaining carbohydrate fermentation over protein putrefaction appears to provide clinically relevant benefits to both dogs and cats.

Microbial gas postbiotics

Gas produced in the colon consists of hydrogen, carbon dioxide, methane, and nitrogen. While gas is an inevitable product of microbial fermentation, not all bacterial species have the capability to generate gas, such as lactobacilli and bifidobacteria.21


Gas from the colon may be partially excreted via the lungs or as flatus. Gas production by the colonic microbiota can exert clinical consequences for the host. For example, methane formation by the colonic microflora has been a great interest to human gastroenterologists. There is strong evidence that methane is positively associated with intestinal motility disorders like constipation.22 While a causative relationship is not known, it has been shown in dogs that methane slows small intestinal transit time.23 These findings indicate that methane is not just an inert gas but it can modulate neuromuscular function of intestinal tract. There is a need for high quality clinical trials to investigate the actions of methane on intestinal motility in dogs and cats with intestinal disease, especially constipation. 

Vitamin postbiotics

Gut microbiota can synthesize certain vitamins, notably vitamin K, and B group vitamins including biotin, cobalamin, folates, nicotinic acid, panthotenic acid, pyridoxine, riboflavin, and thiamin.24


Vitamin K is a fat-soluble vitamin with antihemorrhagic activity. Because microbially synthesized vitamin K is readily absorbed by passive diffusion in the colon in most mammalian species, dietary supplementation is unnecessary for most cats and dogs. Nutritional strategies that maintain healthy gut microflora will support adequate vitamin K microbial synthesis.


The intestine of dogs and cats is exposed to two sources of water-soluble B vitamins: a dietary source (which is mainly absorbed in the small intestine) and a bacterial source (generated by the gut microbiota). The amount of B vitamins generated by microbiota varies depending on the type of dietary substrates consumed with diets rich in fiber producing more B vitamins than diets rich in simple sugars or meat.25,26 Traditionally, it has been thought that B vitamins generated in the host colon are not absorbed and subsequently lost in the feces. However, there is evidence from studies using various human and animal colonic preparations that the colonic epithelium can absorb a range of B vitamins via specific carrier-mediated mechanisms.27 The extent to which gut microbial B vitamin production contributes to canine and feline metabolism has not been documented.


Recent studies suggest that gut microbes produce many postbiotics. In general, postbiotics from carbohydrate fermentation result in beneficial effects, whereas postbiotics from excessive protein putrefaction may be detrimental. The major determinant of postbiotic production is food composition which provides the substrates to the intestinal microbes. Research shows that fruit and vegetable fibers containing polyphenols have clinical benefits to dogs and cats, including healthy gut flora composition and function, and improved fecal quality. The current research target is geared towards finding the best nutritional strategies to nourish and activate gut microbiota of dogs and cats for long term health benefits and to modify underlying gastrointestinal disease. 

Toronto 2019 Hill's Global Symposium banner

Iveta Becvarova, DVM, MS, DACVN Director, Global Academic & Professional Affairs, Hill’s Pet Nutrition Inc.

Dr Delisa Appleton


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