Formulating pet food with functional fibers

Despite not being an essential nutrient, fiber is present in almost every commercial pet food. It dilutes the energy content in a given diet and lacks nutritional value. As a result, fiber has developed a reputation as a ‘filler’ used to reduce production costs.

Unfortunately, this limited view distracts pet owners from the importance of fiber in pet food, particularly functional fibers.

Fiber properties

Dietary fibers are indigestible carbohydrates or lignin molecules that resist degradation by digestive enzymes. Primarily, different fibers are classified based on their solubility in water:

• Soluble fibers dissolve when they come into contact with digesta, influencing stool viscosity.
• Insoluble fibers provide mechanical support to produce easy-to-pass stools. They do this without dissolving in water, without affecting viscosity and without undergoing fermentation.

Functional fibers are a specific subset of dietary fibers that have been isolated, tested and proven to demonstrate physiological benefits. But not all dietary fibers are functional! Examples of functional fibers include psyllium (isolated from the husks of psyllium seeds) and beta-glucans (isolated from oats, fungi and yeast), which are viscous and fermentable. Fructooligosaccharides (FOS) – which are polysaccharides synthesized from sucrose – are fermentable, while cellulose is an insoluble fiber that potentially aids defecation.

State of play

There’s more to nutrition than picking the right combination of nutrients. Without a supportive gut microenvironment, this becomes impossible. Current approaches focus on prebiotics: fermentable fibers that are digested by gut microbes to produce short-chain fatty acids (SCFAs).

This is particularly important considering the popularity of high-protein ‘ancestral diets’ recently adopted by many pet owners. These diets usually have upwards of 50% protein content, which can exceed the digestive capacity of the intestinal tract. In this case, proteins reach the colon where proteolytic fermentation can generate toxic levels of ammonia.

A recent study by the University of Bologna suggests that supplementing high-protein diets with fructans, like FOS, can alter the gut microenvironment to favor the growth of saccharolytic bacteria that produce acetate, propionate and butyrate. These SCFAs can provide as much as 7% of the metabolic energy required by dogs (and to a lesser extent by cats). Among these, butyrate is the preferred substrate and is utilized as an energy source for more than 90% of colonocytes to maintain colon integrity.

In cases of inflammatory bowel disease and colitis, the loss of epithelial adhesions significantly impairs the absorptive capacity of the colon. Many studies have shown that increasing butyrate levels in the colon can improve colonocyte permeability, restore tight junctions (adhesion proteins) and decrease inflammatory mediators within the mucosa.

The role of butyrate has been instrumental to the point that animal farming operations use encapsulated butyric acid as an oral supplement. While this has been effective, pet food manufacturers can naturally increase the production of butyrate (and other SCFAs) by including functional fibers like FOS into their diet formulations.

The challenges

Guava fiber sounds like it could have an appealing taste, but this isn’t the case. In fact, the addition of fiber is notorious for decreasing kibble taste, aroma and texture. A recent study investigating the effect of 8 different fibers in kibble formulations suggests that increased fiber content and reduced particle size negatively affect kibble palatability. Based on comparisons of 3-12% fiber formulations, high-fiber diets were more bitter, stale, and associated with an iron aftertaste.

An exciting approach to incorporating more functional fiber into the diet involves electrospinning fiber to create microcapsules capable of holding even more fiber. Using a double-layer co-encapsulation system, functional fibers like inulin and resistant starches were packaged into fiber-based microcapsules with probiotics. Not only does this provide a method of delivering added fiber, but it can also improve the bioavailability of supplements and medications.

Future of functional fibers

Up to 80% of cats over the age of 15 are affected by chronic kidney disease (CKD), making it a significant concern in the context of pet welfare. Recent studies suggest that fermentable fibers have an amino acid-sparing effect that can reduce nitrogenous waste, ultimately reducing kidney stress.

In one study, adding functional fibers (guar gum and beet pulp) to a low-protein diet increased fermentation-derived propionic acid and related metabolites in dogs. These SCFAs generated energy through the Krebs cycle, avoiding the further breakdown of protein sources that can lead to the accumulation of nitrogenous waste products.

Severe CKD can also lead to the buildup of toxins in the blood due to poor kidney filtration, referred to as uremia. A research group from Oregon State University noted a significant reduction in uremic toxins associated with indoles after adding FOS to the diet of cats diagnosed with CKD (IRIS Stage 1 and 2).

This amino acid-sparing effect has also been noted in liver disease, emphasizing the importance of understanding the interplay between functional fibers, the gut microbiome and disease prevention.

Common misconceptions

1) All fibers provide regular bowel movements

Regular bowel movements require functional fibers present throughout the colon for stool output and water content to increase. However, randomized clinical trials demonstrate that fermentable fibers like inulin and resistant starches do not affect either stool output or water content, and thus are not directly involved in the regularity of bowel movements. This is likely due to fiber fermentation occurring too early along the digestive tract, a common challenge in diet formulation. For functional fibers to positively affect stool quality, they should reach the distal colon and be detectable in the feces.

2) Crude fiber is what matters

Most pet food companies list crude fiber content on the label. Unfortunately, this only accounts for a fraction of the insoluble fibers, underestimating the true fiber content by as much as 95%. Total dietary fiber (TDF) is more accurate and provides information regarding soluble and insoluble fibers. Usually, veterinary diets will present TDF or have this analysis readily available.

3) Carnivores don’t need fiber

Cats are obligate carnivores, but even they benefit from functional fibers. Major veterinary diets include functional fibers like psyllium, beet pulp and FOS in their formulations to help support digestive health. A recent review by Hill’s Nutrition highlights the importance of oligosaccharide prebiotics like FOS to increase SCFA production and promote the growth of Bifidobacterium populations when incorporated into diets.