Dietary fiber: the clinician’s secret weapon

Introduction

The systemic health of animals and maintenance of gastrointestinal tract (GIT) homeostasis relies heavily on influences from dietary fiber, which consists of a diverse assortment of non-digestible carbohydrates 1. Primary roles of fiber within the gut include physically altering the digesta, modulating appetite and satiety, regulating digestion, and acting as a microbial energy source through fermentation, but beyond this, dietary fiber also has widespread systemic effects and myriad benefits throughout the body 1, as outlined in Figure 1. This review will cover fiber function, health benefits, reporting in pet foods, and the clinical utility of modifying fiber in dogs and cats with gastrointestinal disease, with an aim to provide relevant information for practical application in veterinary practice.

Fiber fundamentals

Physiochemical properties, including fermentability, solubility and viscosity, are useful classification schemes for different dietary fiber types, as they often provide insight into potential functional properties of the fiber source. Understanding the categorization is applicable to many clinical situations, and knowing the trends of specific fiber types can be helpful when modifying fiber in the diet for individual animals.

Fermentability is a measure of how microorganisms in the GIT can chemically breakdown fiber, and individual fiber sources may be non-fermentable, slowly fermentable, moderately fermentable, or rapidly fermentable. Short chain fatty acids (SCFA) and other metabolites produced from fermentation are important factors in the understanding of how dietary fiber translates to the myriad health benefits seen both within the GIT and systemically 1. Increased fiber fermentation rate generally corelates with decreased GIT transit time, decreased fecal bulk, and increased SCFA and other metabolite production 1,2. Cellulose and hemicellulose are examples of non-fermentable or slowly fermentable fibers which will pass through the GIT with little change to their structure, retaining more ability to bulk feces 2. Alternatively, pectins and gums are typically highly fermentable fibers, and yield a high proportion of SCFA.

Solubility of a fiber source is based upon its ability to disperse in water. Dispersion in water allows the fiber to create viscous gels in the GIT, and solubility of individual fiber sources can also vary widely from completely soluble to completely insoluble 1. There is broad overlap between solubility and fermentability, with many rapidly fermentable fibers (such as pectins and gums) being the most soluble, while poorly fermentable fibers (cellulose, hemicellulose) being some of the most insoluble.

Viscosity is a measure of the ability for fiber types to thicken and form gels in solution. Again, this property can be drastically different between fiber types and is seen most commonly among a subset of soluble fiber types. The viscosity properties are lost post-fermentation; therefore, those soluble fibers that are slowly fermentable (such as psyllium) are able to maintain the gel-like properties over longer period within the GIT*.

* https://onlinelibrary.wiley.com/doi/full/10.1111/apt.15129

Fiber and pet food labels

As stated previously, fiber consists of non-digestible carbohydrates. These carbohydrates and their metabolites (such as SCFAs) account for only up to 5% of energy extracted from the diets of dogs and cats. Given this limited contribution, dietary fiber is not considered an essential nutrient, and many institutions which set requirements for minimum and maximum nutrient concentrations do not have a minimum requirement for fiber in food for either dogs or cats. This is supported by short duration studies which reported no adverse effects in dogs fed carbohydrate-free diets 3,4.

Potential concerns related to excess dietary fiber include reduced mineral absorption, diluted caloric concentration, and gastrointestinal signs including diarrhea, cramping and bloating 1. There are no published studies that document unintended fiber-related nutritional deficiencies developing in dogs and cats fed complete and balanced, commercially available diets. Likewise, fiber deficiency may also cause gastrointestinal and systemic health concerns, but in this situation the deficiency appears to be relative to the individual animal’s needs rather than an absolute deficiency. Clinically, this is most commonly recognized in animals where increased dietary fiber content can help to ameliorate gastrointestinal clinical signs. Based on this information, when healthy dogs and cats are fed commercial pet foods, both fiber deficiency and excess are unlikely.

There are a variety of different analyses used to measure fiber in veterinary diets and reported in pet food labeling. The most common include crude fiber (CF), total dietary fiber (TDF), insoluble dietary fiber, and soluble dietary fiber. The CF laboratory method is prevalent in certain countries due to requirements by governing bodies, but it does not accurately characterize the amount of fiber in a diet. This is because it mostly assesses insoluble fiber content and provides little assessment of soluble fiber sources 5. For this reason, TDF is the standard for human foods, and is increasingly popular and recommended among veterinary nutritionists 5. If TDF is not reported for a given diet, veterinarians and owners should be cognizant of the limited ability of other methods (such as CF) to determine dietary fiber content (Figure 2).

Fiber administration in nutrition plans

The amount of TDF in dog and cat food ranges greatly, from as little as 0.1 g/100 kcal to more than 11 g/100 kcal. Common types of gastrointestinal veterinary therapeutic dry diets, including hydrolyzed and easily digestible diets, often provide modest amounts of dietary fiber (1.0-2.5 g TDF per 100 kcal). Diets specifically formulated for fiber-responsive gastrointestinal disease often contain mixed types of fiber sources and provide increased fiber content (TDF: 4.5-6.0 g/100 kcal and 2.6-2.9 g/100 kcal for dogs and cats, respectively). Diets formulated for weight management typically have the highest fiber content (TDF 7.5-11.0 g/100 kcal and 4.6-7.6 g/100 kcal for dogs and cats, respectively).

Diets manufactured by reputable pet food companies are formulated to be complete and balanced as well as providing specific fiber types and concentrations in the guaranteed analysis. If feeding a fiber-enhanced diet is not possible, an alternative approach is to add fiber to a basic complete and balanced diet, but if using this approach the clinician must be aware of potential complications, including administration issues, palatability concerns, and the potential to unbalance the base diet if done incorrectly. Psyllium husk (Figure 3) is one of the more common fiber types used for this purpose, and is to be preferred over food additives (e.g., canned pumpkin (Figure 4)) due to the relative fiber concentrations between these sources. Comparatively, fiber in foods like canned pumpkin are less concentrated and require significantly more volume to be administered compared to sources like psyllium husk 1. Specific dosing of fiber sources for a desired clinical effect is also unknown, with limited data on this in dogs and cats; there is a single published study in dogs with fiber-responsive large bowel disease that reportedly responded to a median daily dosage of 30 mL (2 tablespoons) psyllium fiber (range 3.75-90 mL (0.25 to 6 tablespoons)) 6. As such, fiber supplementation in this manner is largely empirical and may be judged by clinical signs (Figure 5). Clinicians should also remember that when using single source fibers for supplementation, any given individual source may not yield the same clinical benefits as diets with mixed fiber sources. Lastly, it is also recommended to gradually increase the quantity of fiber supplemented to avoid adverse events which may be more common with rapid changes in administration (Figure 6).

GIT effects and health benefits

Fiber affects gastric emptying rate, stool transit time, fecal bulking effects, fecal water concentration, bacterial metabolism, and stimulates mucus secretion (Figure 1) 1. Most commonly, soluble and viscous fiber types delay gastric emptying and increase small intestinal transit time, while insoluble fibers often increase gastric emptying time and decrease transit time in both the small intestine and colon 1. Ease of defecation can also be altered by dietary fiber and its ability to increases fecal volume and weight (Figure 1). Soluble, fermentable fibers increase stool weight, moisture concentration, and softness by increasing water retention, fecal dry matter percentage, and bacterial concentration. Insoluble, minimally fermentable fibers increase fecal mass through water retention and its dry matter bulking capacity 1. However, these expected effects often vary between studies on different fiber types in dogs and cats 1, and when using dietary fiber the clinician should be aware that atypical responses are possible.

Fiber may also decrease the digestibility of a diet. This effect is observed with slowly fermentable fibers which take the GIT longer to digest 2. Alternatively, fermentable, soluble fibers may have a higher chance of affecting nutrient availability. This is based on the ability of such fibers to form gels; these then create a physical barrier to nutrient digestion.

Fiber as a prebiotic and the gut microbiome

Dietary fiber can also function as a prebiotic. Prebiotic fibers are able to be used by host microorganisms and the associated metabolites can provide health benefits 7, and in particular prebiotic fiber markedly impacts the gut microbiome in dogs and cats. In order to be a prebiotic, the fiber must be resistant to digestion by host enzymes, promote the growth of symbiotic gut microbes, and fail to promote the growth of pathogenic bacteria 7. Oligosaccharides and inulin (mainly from chicory) are common prebiotic fibers that often appear in dog and cat diets. However, studies regarding the impact, dosage ranges for supplementation, and the synergistic effects of prebiotics and other dietary fibers on dog and cat health are lacking, and further research is warranted.

A complete review of prebiotic fibers is beyond the scope of this paper, but fermentation of prebiotic fibers and the associated changes in microbial populations and microbial metabolites are an important outcome of prebiotics. SCFAs produced by bacterial fermentation of fibers act as an energy source for both the microbes in the colon and the host via the portal circulation 1. Locally in the gut, these assist in maintenance of a healthy epithelium by regulating bacterial proliferation, improving colonic blood flow, reducing reliance on gluconeogenesis, and preventing colonization by pathogenic bacteria (Figure 1). Systemically this translates to improved glycemic control, modulation of gene expression, immunoregulatory effects, and neuronal regeneration 1. The local and systemic benefits of prebiotics are an important area for further investigation in dogs and cats.

Systemic effects and health benefits

Dietary fiber decreases the caloric density of foods and increases satiety, emphasizing its role in weight management 1. The addition of insoluble fiber to the diet allows an animal to be fed a calorically neutral, larger volume of food with improve satiation. Outcome results of fiber-based weight management studies can differ, and the studies often include multiple variables, but the available evidence supports increased dietary fiber intake in modulating weight management and satiation.

Fiber and macronutrient composition in the diet also impacts postprandial glycemia and insulin responses (Figure 1) 1. Highly viscous, soluble fibers appear to be most effective due to their ability to impair glucose uptake from fiber-induced gel formed in the intestines. There are studies showing improved glycemic control and decreased post-prandial hyperglycemia linked to increasing dietary fiber concentration, soluble or insoluble, which are reviewed elsewhere 1. Less research has been performed in cats to understand fibers effect on glycemic control in this species, and the two currently available studies are contradictory 1. Despite the improvements noted in some studies, none have been able to cause an effect size large enough to alter overall administered insulin dose or improved clinical signs.

There are many changes to the GIT as animals age. These include reduced digestibility of crude protein and fat, increased intestinal transit time, and alterations in basal metabolism, post-prandial responses, intestinal and pancreatic secretions, and physical activity. Fiber supplementation may be able to maintain intestinal transit time, gastrointestinal function, and prevent constipation and diarrhea in elderly populations, but this has not been formally studied, and fiber recommendations in geriatric dogs and cats are presumptive. Importantly for geriatric populations, if the animal does not have a relative fiber deficiency, high fiber intake may not be recommended (especially in lean or underweight geriatric animals) due to the impact on digestibility and GIT function in those animals.

Additional benefits of dietary fiber may include improvements in immune, cardiovascular, and reproductive health (Figure 1) 1. In human medicine, fiber supplementations may reduce the risk of neoplasia, cardiovascular disease and coronary heart disease, and improve reproductive health status, but there is limited exploration of these fiber effects, as well as other systemic outcomes, in dogs and cats, and further investigation is needed on these potential benefits of dietary fiber.

Fiber and management of GIT disease

Acute diarrhea

Fiber-enhanced diets are applicable to the management of dogs with acute diarrhea 8,9. This was first documented in a study on twenty-two dogs randomized to receive one of two diet options co-administered with metronidazole. When comparing groups, the dogs fed the high fiber diet exhibited greater improvement in fecal scores compared to the control diet 8. The diets in this study differed in concentrations of soluble and insoluble fiber, with the higher fiber diet containing a TDF of 6.1 g/100 kcal compared to the second diet with lower fiber content at a TDF of 1.5 g/100 kcal.

Another study evaluating conventional treatment options for acute colitis in 59 client-owned dogs also demonstrated a clinical benefit with fiber-related dietary management 9. The cases were randomized into three placebo-controlled groups, with one group that received an easily digestible diet in combination with placebo, a second group which received an easily digestible diet in combination with metronidazole, and a third group which received a psyllium-enhanced easily digestible diet in combination with placebo. The dietary fiber content in the easily digestible diet was reported as a TDF of 1.5 g/100 kcal and the fiber-enhanced easily digestible diet contained a TDF of 2.8 g/100 kcal. Nutritional management alone was found to be superior to metronidazole in combination with nutritional management based on time to resolution of clinical signs. This study was notable as it was the first study to compare two common empirical management strategies, and demonstrated a benefit to dietary management for acute colitis in dogs 9.

Finally, a study published in abstract form demonstrated the benefits of fiber-enhanced diets in shelter puppies with acute diarrhea 10. Due to the abstract format, limited information about the study diets was available, but they included multiple sources of soluble and insoluble fiber. The higher fiber diet was 4.3% CF and 2.0% soluble fiber versus the lower fiber diet which contained 3.3% CF dry matter basis and 0.3% soluble fiber (g/100 kcal not reported for either diet). The fiber-enhanced diet resulted in faster resolution of clinical signs and improved fecal scores.

Although there is less supportive evidence, fiber-enhanced diets also appear efficacious in cats with acute diarrhea. Similar to the study above 10, a study published in abstract form demonstrated the benefits of fiber-enhanced diets in shelter kittens with acute diarrhea. Once again limited information about the study diets was provided, but they included multiple sources of soluble and insoluble fiber. The higher fiber diet was 3.0% CF and 1.6% soluble fiber, whilst the lower fiber diet was 1.1% CF dry matter basis and 0.2% soluble fiber (g/100 kcal was not reported for either diet). Despite similar final fecal scores between diets, the fiber-enhanced diet resulted in fewer days to resolution of clinical signs 11.

In summary, no adverse effects or complications associated with dietary therapy for acute diarrhea were noted in these studies, and when studied in comparison to antibiotics, there was less injury to the gut microbiome 8,10,11. Based on this information, there is evidence for the use of fiber-enhanced diets for the management of acute diarrhea, particularly colitis, in both dogs and cats, although the ideal fiber types, fiber dosage and administration duration is currently unknown.

Chronic diarrhea

Similar to acute diarrhea, multiple studies support the use of fiber-enhanced diets in chronic diarrhea management. The most common indication for dietary fiber is in the presence of colitis (large bowel) signs, but the following studies also demonstrate preliminary evidence in some enteritis (small bowel) or enterocolitis (mixed bowel) clinical scenarios 12,13,14.

The first study described the outcome of dogs treated with 3 different diets (low fat, high fiber, or restricted antigen) while concurrently administered anti-inflammatory corticosteroids 12. The fiber-enhanced diet was the second most effective, being reported to control clinical signs in approximately 75% of cases; the dogs on the restricted antigen diet exhibited the highest response rate at 85%, and the low-fat diet exhibited the lowest response rate at 18%.

Further support for fiber-enhanced diets was demonstrated in a retrospective study in 25 dogs with chronic colitis, which documented a switch to dietary fiber enhancement as the most common approach that resulted in resolution of clinical signs 13. Once again, highly digestible diets and hypoallergenic diets were documented to be effective in smaller percentages of the cases. Since this was not a randomized controlled study, it is impossible to determine the relative effectiveness between these dietary approaches. Furthermore, another uncontrolled study using a commercially available diet in dogs described fiber-enhanced diets resulting in colitis resolution 14. All three of these studies support the concept for fiber-enhanced diets in chronic diarrhea dogs, as evidenced by improved fecal scores.

There are also studies supporting the use of fiber-enhanced diets in both non-inflammatory and idiopathic chronic colitis 6,15. One study reported the response of 27 dogs with chronic idiopathic large-bowel diarrhea to predominantly soluble fiber supplementation with various base diets 6; the majority of dogs (96.3%) exhibited either an excellent or good clinical response to fiber supplementation. In another study using a commercially available diet enriched with insoluble fiber in combination with other therapies, the response in 12/19 dogs was attributed to the dietary modification 15. In both studies, administration of fiber allowed for adjunctive medical treatments to be either tapered or discontinued completely in some dogs, and therefore support the use of fiber-enhanced diets with soluble, insoluble, and mixed fiber sources for canine idiopathic large bowel diarrhea.

Beyond traditional gastrointestinal outcome measures, there are also reports that dietary fiber can improve stooling behaviors and quality of life as detailed by dog owners. The study documenting these effects suggested that the downstream effect of the fibers in producing increased antioxidant and anti-inflammatory metabolites may contribute to these overall health outcomes 16. Further research should focus on the long-term benefits of fiber supplementation in chronic gastrointestinal disease dogs.

The prognosis for dogs with fiber-responsive chronic gastrointestinal disease is also reported to be good. Long-term control in studies looking at the effect of soluble fiber treatment was said to be at least good in 96% of dogs 6; when using predominantly insoluble fiber diets, 63% of dogs responded to diet. Long-term follow-up indicated that after initial response, long-term supplementation and dietary management was associated with the best clinical signs control 6. In all the studies mentioned, fiber-enhanced diets and fiber supplementation was safe and well tolerated by the dogs.

Evidence for nutritional management of chronic diarrhea in cats with fiber-enhanced diets is limited and focuses on the results of only a few studies 17,18,19. In a small study of 12 cats with chronic diarrhea, where the majority responded to either diet alone or diet plus ancillary medications, the most common diets utilized were either high in fiber or supplemented with fiber 17. In an additional crossover study, a mixed-source soluble fiber diet in 19 cats with chronic diarrhea resulted in improved fecal scores and lowered defecation frequency compared to a control diet 18. The third study evaluated fiber sources which produce anti-inflammatory antioxidant metabolites and documented better outcomes compared to other fiber sources in diarrhea cats 19. These studies support the concept of fiber-responsive chronic diarrhea in cats. As such, there is strong evidence for utilizing fiber-enhanced diets or fiber supplementation in the management of chronic diarrhea in both dogs and cats, with the majority of evidence supporting its use in colitis cases.

Constipation

Based on data extrapolated from other species, expert option, and data in healthy dogs and cats, dietary fiber appears to be beneficial in management of canine and feline constipation cases 20. There are potential reasons why both soluble and insoluble fiber properties may be effective in constipation. As stated previously, insoluble or non-fermentable fibers increase fecal bulk which stimulates peristalsis in the GIT 21. Soluble fiber forms gels within the gastrointestinal tract, which assists in movement of feces 21. Highly soluble and fermentable fibers may further benefit constipated animals from the products of fermentation; specifically, these fibers have been linked to the production of SCFA, which has a prokinetic effect and increases fecal water content 22.

Cats are commonly affected by constipation 23, and fiber sources that promote antioxidant and anti-inflammatory compounds have been shown to result in improved clinical signs 19. Another study looked at cats which were treated with either diet alone or diet in combination with adjunctive medications, and improvement in fecal scores improved in both groups 24; the diets in this study utilized a mix of fiber sources, including psyllium. Although limited in number, these two studies support the use of fiber-enhanced diets in feline constipation (Figure 7). However, when recommending fiber for feline constipation management, the clinician should first evaluate for evidence of severe colonic dysmotility (e.g., megacolon) where fiber would be contraindicated.

Clinical studies on the effects of fiber in naturally affected constipated dogs are lacking, although experimentally, administration of fig paste fiber improved fecal passage in dogs without complications 25. Furthermore, a symbiotic using the prebiotic inulin was shown to have a laxative effect in healthy dogs, suggesting that this may be effective in constipation management 26. Additional research is needed to document the effects of fiber in naturally occurring canine constipation management, but preliminary research is promising, and as with diarrheal disease, further trials are needed in both dogs and cats to examine whether a specific type or amount of fiber is optimal for constipation management.

Conclusion

Fiber is a component of nearly all diets fed to dogs and cats and represents an untapped area which can be manipulated to benefit animals and ameliorate clinical signs. For clinicians to competently and confidently implement fiber in practice, a basic understanding of the health benefits, pet food labeling, fiber physiochemical properties, and the clinical indications associated with fiber, is necessary. One of the most prevalent opportunities for dietary fiber management of disease is in canine and feline gastrointestinal disease. Pet food companies offer a variety of fiber-enhanced diets, and research shows that this offers many advantages over alternative conventional treatment options (such as long-term antibiotic or immunomodulatory medications) in selected cases. As such, when managing gastrointestinal disease, dietary fiber should remain a viable treatment option and modality for both dogs and cats.