Introduction

Definition

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

Obesity

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.

Diabetes mellitus

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].

Hypercalcemia

Hairballs

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.

Conclusion