Cats and dietary fiber
Nutritionists and veterinarians have been interested in dietary fiber as a component of pet foods or as a supplement for many years. Fiber has traditionally been used to modify stool quality and as an aid in weight management, but more recently fiber has been shown to have effects on the gastrointestinal microbiome and may play a role in the management of various disorders.
Fiber is surprisingly difficult to define but is often classified by its properties, such as solubility in water, viscosity, and fermentability in the gastrointestinal tract. Many commercial cat foods contain two or more fiber-containing ingredients.
Dietary fiber, and in particular slowly fermentable fibers such as cellulose and peanut hulls, have been shown to be an effective means of increasing dietary bulk in the gastrointestinal tract without supplying additional calories.
Recent reviews suggest that the effect of fiber in cats with diabetes mellitus is unknown but that low-carbohydrate, low-fiber diets may be indicated.
Recommendations for nutritional therapy of constipation vary widely; some authors suggest highly digestible, low-fiber diets while others prefer high-fiber diets or fiber supplementation.
IntroductionNutritionists and veterinarians have been interested in dietary fiber as a component of pet foods or as a supplement for many years. Fiber has traditionally been used to modify stool quality and as an aid in weight management, but more recently fiber has been shown to have effects on the gastrointestinal microbiome and may play a role in the management of various disorders. This article offers a brief overview of dietary fiber and a discussion of the potential benefits for several common feline medical conditions.
Dietary fiber is often classified by its properties, such as solubility in water, viscosity, and fermentability in the gastrointestinal tract. Table 1 summarizes the characteristics of common fiber sources used in pet food. It can be difficult to predict the health effects of any one fiber source, and many commercial cat foods include two or more fiber-containing ingredients. For example, fermentable fibers can act as energy substrates for bacteria in the lower intestinal tract which results in the production of short-chain fatty acids 3. These in turn are absorbed and can modify the structure and function of intestinal cells. Nonfermentable fibers tend to increase fecal weight and volume and may decrease intestinal transit time 3. Fiber sources with high viscosity often have an increased water-binding capacity that can result in softer, moister feces. It is likely that varying the amounts and types of fiber will affect a cat’s gastrointestinal microbial population, although research into the microbiome is in its early stages 4. Figure 1 illustrates the solubility and viscosity of different fiber sources.
Fiber and its role in disease
In North America, obesity is the most common form of malnutrition in veterinary patients; it is estimated that 35.1% of adult cats are either overweight or obese 5 (Figure 2). Obesity predisposes cats to a variety of diseases such as diabetes mellitus, hepatic lipidosis, urinary tract disease, orthopedic disease, and dermopathies.
Dietary fiber has been used to help manage obesity in both dogs and cats. In particular, slowly fermentable fibers such as cellulose and peanut hulls have been shown to be an effective means of increasing dietary bulk in the gastrointestinal (GI) tract without supplying additional calories. Sources of dietary fiber in commercial pet foods can thus be beneficial, as they are likely to result in the consumption of less food energy. Mixed dietary fibers are also thought to promote the attenuation of glucose absorption from the GI tract, induce gastric distention which stimulates the cholecystokinin-satiation signaling pathway, delay gastric emptying, and stimulate longer ileal transit time 6. While the effects of fiber on food consumption in cats are not well described in the literature, there is speculation that fiber-enhanced diets might reduce overeating and thus play a protective role against the development of feline obesity 7.
Hunger-driven begging behaviors often compromise client compliance and can result in failure to achieve weight loss in obese-prone individuals. The inclusion of fiber in commercial diets is thought to induce satiety to the extent that it decreases these unwanted behaviors. A study evaluating weight-loss strategies in overweight cats found reduced begging scores (less vocalization and owner-seeking behavior) with a diet containing high-water-binding capacity fiber compared with a diet containing primarily insoluble fiber 8. Therefore, not only the amount but the type of fiber may have an effect on satiety.
Despite the potential therapeutic properties of fiber on obesity, it should be noted that its inclusion can also result in decreased protein digestibility; weight loss diets must compensate for this by increasing dietary protein concentrations. Moreover, the ratio of slowly to rapidly fermentable fibers is important because research has shown that when rapidly fermentable fibers are included in high concentrations sufficient to promote satiety, GI side effects such as flatulence and diarrhea may be seen 9. Overall, despite the paucity of information available on fiber in feline patients and conflicting results on its efficacy, its inclusion in commercial diets may prove useful for weight loss in obese-prone patients.
In one randomized controlled crossover study 12, 16 diabetic cats were fed a high-fiber (HF) diet containing 12% cellulose (dry matter basis) or a low-fiber (LF) diet (added cornstarch). Pre- and post-prandial blood glucose measurements were lower when the cats ate the HF diet. Insulin doses and glycated hemoglobin concentrations were not significantly different and 4 cats did not show improvement in blood glucose on the HF diet. The authors concluded that the results supported feeding a diet with added cellulose to cats with DM. These data were consistent with an unpublished study conducted by the same authors, in which 9 of 13 diabetic cats had improved glycemic control on a HF diet. However, there were other differences between the two study diets that may have influenced the results; the LF diet was lower in protein and higher in carbohydrates than the HF diet, and caloric intake was reduced on the HF diet. The four cats that did not respond to the HF diet had a lower average body weight (4.7 kg vs. 5.5 kg), so the amount of body fat may be a significant variable 12.
Another randomized study 13 evaluated two wet (canned) diets in cats with DM that were also being treated with insulin. The moderate-carbohydrate, high-fiber (MC-HF) diet contained approximately 11% crude fiber and 26% carbohydrate (on a dry matter basis) while the low-carbohydrate, low-fiber diet (LC-LF) contained approximately 1% crude fiber and 15% carbohydrate. Most, but not all, cats showed improvement in blood glucose and fructosamine in both diet groups. By week 16, insulin was able to be discontinued in 68% of cats on the LC-LF diet and 41% of cats on the MC-HF fiber. The authors concluded that cats were more likely to be well-regulated or revert to a non-insulin-dependent state when fed a LC-LF diet. However, the two diets differed in ingredients (ground corn in MC-HF vs. soybean meal and corn gluten meal in LC-LF) and in fat content (41% in MC-HF vs. 51% in LC-LF, metabolizable energy basis). Therefore it is difficult to determine if the fiber, carbohydrate, or fat content or particular combination of ingredients in the two diets were responsible for the results 13.
Current guidelines for humans with diabetes include medical nutrition therapy 14. While dietary fiber consumption is associated with lower all-cause mortality in people with DM, there is little evidence that glycemic control is improved with increased intake of fiber or whole grains. Several dietary fiber studies in dogs with DM have yielded mixed results, and decreased caloric intake with high-fiber diets may be responsible for improvement in glycemic control 11. At present, there is little evidence to support a direct effect of dietary fiber on DM in cats, but weight loss strategies that may include higher fiber can play a role in the management of these cases.
While randomized, controlled studies are lacking, some veterinarians advocate the use of fiber supplementation for dogs and cats with IBD if diarrhea is a clinical feature. When food hypersensitivities are suspected as the underlying etiology, novel or hydrolyzed protein diets are the typical recommendation. Adjunctive fiber supplementation may be administered to these therapeutic diets to improve intestinal motility and water balance and normalize microflora 6; psyllium husks and wheat bran have been used effectively in such cases. In other instances, patients with IBD respond best to highly digestible, low fiber diets, therefore fiber supplementation may be contraindicated for some individuals. Assessing each pet on an individual basis is critical in optimizing the clinical outcome.
Recommendations for nutritional therapy of constipation vary widely. Some authors suggest highly digestible, low-fiber diets, while others prefer high-fiber diets or fiber supplementation 17. Different types and amounts of dietary fiber have different effects on large intestinal function. Certain poorly fermentable fibers such as cellulose act as bulk-forming laxatives and can help distend the lumen of the colon and increase the rate of fecal passage 17. However, cellulose has different effects on fecal dry matter and quality in dogs depending on type and fiber length 18. Other fibers such as psyllium can form a viscous gel (because of its high water-binding capacity) which helps ease the passage of feces. Diets that are low in fiber and highly digestible can reduce the amount of feces produced but do not stimulate motility or fecal passage 17. As dehydration is one of the underlying causes of constipation, wet (canned) diets are often suggested to increase water intake along with parenteral fluid therapy when necessary. However, wet diets vary in fiber types and amounts and may not be appropriate for all cats with constipation.
Only one study has been published looking at the effects of a commercial diet in constipated cats 19. In this uncontrolled clinical trial, a moderate-fiber dry commercial diet containing psyllium as the predominant fiber (along with other fiber sources including chicory, fructo-oligosaccharides, mannan-oligosaccharides, rice and corn) was fed to 66 constipated cats. Assessments of fecal consistency and subjective improvement in clinical signs were evaluated by both veterinarians and owners. The results indicated that 56 cats completed the trial and all ate the diet and had improvements in fecal scores. Most cats that were on current medications for constipation were able to decrease or stop the drugs completely. While there was no control group or diet in this study, the positive results support the use of this psyllium-enriched diet as a primary or adjunctive therapy in managing cats with constipation and obstipation 19.
Dietary interventions are often ineffective because hypercalcemia tends to develop from increased bone resorption and renal tubular reabsorption of calcium. A dietary change to a product low in calcium will only benefit the subset of patients with increased intestinal absorption of calcium such as with hypervitaminosis D. High-fiber diets have been reported to decrease the risk of hypercalcemia and calcium oxalate urolithiasis in humans by increased binding of intestinal calcium, prevention of calcium absorption from the GI tract, and a decrease in GI transit time 20. There are very few studies in the veterinary literature to support this finding. Fiber-enhanced diets were associated with resolution of hypercalcemia in 5 cats with idiopathic hypercalcemia and calcium oxalate urolithiasis 21 but no effect was observed in another study 22. Further evaluation of the role of nutritional therapy in cats with hypercalcemia is required before conclusive recommendations can be made.
Commercial hairball control diets are available that include various types and amounts of fiber. A survey of dry and wet feline diets with hairball claims available in the U.S. found that the fiber sources listed among the ingredients varied widely, and included powdered cellulose, dried beet pulp, soybean hulls, dried chicory root, rice hulls, rice bran, pea bran meal, pea fiber, oat fiber, inulin, and psyllium. Published studies are lacking. A crossover trial comparing a maintenance diet with one with added fiber in 102 cats over 2 months showed an average of 21.5% fewer hairballs and 21.8% reduction in vomiting frequency 24. Another research study in 16 healthy cats compared the effect of two dry diets, one moderate fiber (6.9% as fed) and one high fiber (14.2%), on fecal hair excretion 25; after 3 weeks, cats on the high fiber diet excreted an average of twice as much hair as cats on the moderate fiber diet. The results indicated either the amount or the type (psyllium and cellulose) of fibers in the high-fiber diet increased hair passage through the intestinal tract and may reduce the incidence of hairball regurgitation or vomiting.
ConclusionBoth the amount and type of dietary fiber in the diet can impact on intestinal health and function, and have been shown to play a beneficial role in the treatment of various clinical conditions. Further studies are needed to determine the effects of specific diets and fiber supplementation in cats.
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