The Role of Dietary Fiber in Pet Nutrition: Health Benefits and Practical Applications

Dietary fiber plays a valuable role in veterinary nutrition, but the term “fiber” is often oversimplified. Its definition can vary according to specific characteristics and how it interacts with the digestive system. Fiber is commonly used to manage conditions such as constipation, diarrhea, and anal gland irritation, among others. Depending on the type, fiber can have other attributes, such as acting as a prebiotic to support microbiome health.

When recommending fiber, consider the specific goal or intended outcome and choose a fiber type that aligns with that goal. Fiber is categorized by its fermentability, solubility, and viscosity, which influence its effect in the body. Whether evaluating a diet or adding fiber to one, these 3 properties should be matched to the condition being managed. Given the diversity of fiber properties and physiological effects, dietary fiber should be evaluated on an individual basis rather than classified broadly as universally beneficial or detrimental. Finding the right balance and selecting fiber that aligns with the patient’s specific needs are essential.

Description of Fiber

Fiber is the nondigestible portion of carbohydrates. Because it is not digested, fiber passes through the small and large intestines, where it can be fermented by bacteria or affect the bulk and consistency of the feces. Fiber plays a role in supporting gastrointestinal (GI) health due to its effects on the microbiome, and it is a fuel source for enterocytes; however, not all patients respond to fiber in the same way, and thus the amount and type of fiber that is fed should be considered on an individual basis. Fiber can have several effects on the GI tract, such as altering the digestibility of a diet or transit time.¹

Categories of Fiber

Fiber categorization is generally based on its characteristics and how it behaves in the intestinal tract. The main categories are fermentability, solubility, and viscosity (TABLE 1).

Fermentability

Fermentability refers to how easily GI bacteria can ferment a type of fiber. Fermentable fiber may produce beneficial effects in some patients but induce GI signs or intolerance in others (BOX 1).

Fermentable fiber supports healthy bacteria within the GI tract and can help promote balance within the microbiome.⁴ When fiber is fermented, bacteria produce short-chain fatty acids (SCFAs), which serve as an energy source for cells in the intestinal tract. Fermentable fibers such as inulin and fructooligosaccharides produce several SCFAs but tend to generate higher concentrations of the SCFA butyrate compared with other types of fermentable fiber. Butyrate is considered one of the preferred sources of energy for enterocytes and has been shown to promote a healthy intestinal barrier and improve water and electrolyte absorption.⁵ Butyrate has also been shown to be beneficial for humans with ulcerative colitis due to its anti-inflammatory effects, which inhibit activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) in macrophages.⁶ When NF-κB is activated, it can promote inflammation; its activation has been shown to promote inflammation within the lamina propria in dogs with chronic enteropathy.⁷ Using fermentable fiber to modulate inflammation in patients with chronic enteropathy is an example of how diet can be used to support intestinal health.

Although highly fermentable fiber offers many benefits, it can also increase gas production, which may lead to undesirable side effects such as intestinal distension, discomfort, and flatulence. For patients with excessive flatulence, it is helpful to review the diet ingredient list for fermentable fiber sources that may be contributing to the flatulence. Another consideration is that highly fermentable fiber can increase methane production, which may worsen signs of colitis.

Solubility and Viscosity

Fiber is also categorized by solubility (soluble or insoluble), which is associated with its viscosity as well. Viscosity refers to the ability of fiber to thicken or form a gel when mixed with liquid and has been suggested as an alternative way to describe soluble fiber.² Soluble fiber, or that with higher viscosity, has a greater water-holding capacity than insoluble fiber and is generally more susceptible to bacterial fermentation. However, although it may seem logical to link solubility with fermentability, the 2 properties are not always directly associated.² For example, beet pulp is considered an insoluble fiber, but it is also considered to be fermentable. To avoid confusion, solubility and fermentability should generally be considered separately when evaluating fiber sources (TABLE 1).

Fiber with higher viscosity can increase the water content and bulk of feces. Highly viscous fiber can also slow gastric emptying and intestinal transit time. However, the effects of fiber on transit time reported in the literature can vary depending on the study, which may be influenced by the amount of fiber used. For example, 1 study found that adding 30 g of purified cellulose to dog food slowed intestinal transit time, whereas another study found that feeding a diet containing 9% α-cellulose (as fed) resulted in faster intestinal transit time compared with that in controls.² It is generally accepted that soluble and viscous fiber typically delays gastric emptying and, conversely, insoluble fiber tends to speed up transit time through the small intestine and colon, meaning that contents pass more quickly; however, variations are possible.³

Fiber also affects digestibility. It has been shown that higher-fiber diets tend to decrease total dry matter digestibility and may influence protein digestibility as well.^8,9 Thus, therapeutic diets that are formulated to be highly digestible tend to be lower in fiber, but every diet should be examined individually as fiber variations between highly digestible diets certainly exist (TABLE 2).

Fiber in Pet Food

Determining how much fiber a diet provides can be challenging. Dietary fiber is not considered an essential nutrient by AAFCO and does not have a minimum requirement. On pet food labels, fiber is reported as a maximum percentage of crude fiber. However, crude fiber mainly reflects insoluble fiber and does not indicate whether the diet contains soluble or fermentable fiber. Because of this, crude fiber can significantly underestimate total dietary fiber (TDF).

The most accurate way to compare dietary fiber is to evaluate TDF in a diet, which indicates both soluble and insoluble fiber. The AAFCO Pet Food Label Modernization initiative will eventually require that TDF be included on labels, but this will take time.¹⁰ TDF analysis is more involved and requires laboratories to be able to process large numbers of samples.

The guaranteed analysis on labels provides percent crude fiber, which is a percentage of fiber per 100 g of food. Because this percentage is based on weight, comparing it between diets that vary in moisture and energy density is difficult. The preferred method is to compare fiber content on a caloric basis, such as grams per 1000 kcal. Grams per 1000 kcal from percent crude fiber provided on the label can be estimated as follows:

• Divide the crude fiber % by the kcal/kg of the food.
• Multiply the result by 10 000.

Example: For a diet that contains 1.5% crude fiber and 3900 kcal per kg

• Step 1: 1.5 / 3900 = 0.000384
• Step 2: 0.000384 × 10 000 = 3.84 g crude fiber per 1000 kcal

When TDF is available, comparing grams of TDF per 1000 kcal is the preferred method (TABLE 3).

Fiber for Constipated Patients

Indications for Adding Fiber

If a constipated patient has normal colonic function and motility, fiber can offer several benefits that help reduce the risk for constipation. When managing constipation, the type of fiber is relevant. Soluble fiber, such as psyllium, is a viscous fiber that helps draw water into the feces, making it softer and easier to pass. Although insoluble fiber is not effective for dry, hardened feces, it can help maintain fecal bulk and stimulate colonic motility.

Contraindications for Adding Fiber

For patients that have dry, hard feces and are unable to defecate (e.g., those with obstipation), adding fiber is contraindicated until the patient is deobstipated. For patients with megacolon, when the colon has lost its ability to contract and move feces due to poor motility, a high-fiber diet can be harmful. Adding insoluble fiber to the diet of these patients increases fecal bulk, which the colon cannot move, thereby worsening the condition. Even soluble fiber may be ineffective because the underlying issue is the inability of the colon to contract and evacuate feces. For these patients, a highly digestible, low-residue, low-fiber diet is preferred.

Fiber for Patients with Diarrhea

Diarrhea may be lessened by modifying dietary fiber, depending on the source of the diarrhea.

• Small bowel diarrhea: Large volume, watery, normal frequency
• Large bowel diarrhea: Smaller volume, increased frequency, may include mucus and/or blood, patient may exhibit straining (tenesmus)

For acute small bowel diarrhea, a highly digestible diet is usually recommended as the first step. If that does not lead to a response, clinicians can explore therapeutic diets containing novel or hydrolyzed proteins. If there is still no response, it may be appropriate to trial a higher-fiber diet. Large bowel diarrhea often responds well to a higher-fiber diet; for these patients, it can be helpful to begin by increasing the fiber in the diet. If that is unsuccessful, considering a highly digestible, novel or hydrolyzed diet is often a next step in addition to increasing the fiber.

Although it is still appropriate to consider dietary fiber supplementation for either type of diarrhea, taking a methodical approach to management with fiber can help the clinician determine the best next step. Before deciding to modify the fiber in the diet, the amount of fiber in the current diet must be considered. When managing chronic enteropathy in dogs or cats, evaluating the TDF content of the diet can be helpful. Many hydrolyzed or limited-ingredient diets are relatively low in fiber, although exceptions exist. If a patient has not responded to a diet trial for diarrhea and the current diet is low in fiber, a higher-fiber limited-ingredient or hydrolyzed protein diet may be beneficial (TABLE 2).

Comorbidities and Fiber

When managing patients with comorbidities, evaluating the fiber content of the diet is particularly valuable. A diet that provides the appropriate nutrient profile for a patient’s medical condition(s) may not contain the amount or type of fiber needed to address concurrent GI concerns.

For example, a cat with fiber-responsive diarrhea and renal disease may require a dietary plan that includes supplemental fiber. Although it can be challenging to match the fiber content of a high-fiber diet by using supplements alone, doing so may not be necessary. Both the type and amount of fiber play roles in managing fiber-responsive diarrhea. Evaluating the TDF content of therapeutic renal diets by different pet food manufacturers can help identify formulations that are higher in fiber and may better support patients with multiple conditions (TABLE 4).

Adding enough supplemental fiber to match that of many of the high-fiber diets can be difficult. High-fiber diets may provide 40 to 60 g TDF per 1000 kcal, whereas 2 tbsp of psyllium add only 20 mg of TDF. If a dog or cat requires more dietary fiber than supplementation can provide, feeding a high-fiber therapeutic diet can be considered.

Some clients may choose to add human food to increase dietary fiber; however, the amount needed to make a meaningful effect is often impractical to feed on a regular basis. For example, 1 cup of canned pumpkin contains approximately 7 g of total dietary fiber. If a dog is consuming 1000 calories per day and the goal is to match a therapeutic diet that provides 40 g of TDF per 1000 kcal, the dog would need about 5 ¾ cups of canned pumpkin daily, an amount that is neither practical nor recommended. That said, the amount of fiber needed to achieve a clinical response varies by individual patient. Some dogs may show improvement with as little as 1 to 2 tbsp of canned pumpkin added to their diet, highlighting the value of tailoring the fiber “dose” to the patient rather than aiming for a specific target number. However, the key takeaway is that if a client has tried adding a small amount of pumpkin and the dog’s condition did not improve, it does not necessarily mean that the dog will not respond to fiber; it may simply mean that the amount added was insufficient to achieve the desired effect.

If fiber is added and the condition does not improve, consider the type of fiber (BOX 2).

• Psyllium: mostly soluble, some insoluble
• Insoluble fiber: If psyllium is not effective, consider adding more insoluble fiber (e.g., Balance IT Firming Fiber [cellulose], balanceit.com; Nutramax Proviable fiber supplement powder [combination],
  proviable.com).

Different types of fiber work differently; therefore, lack of improvement with one does not mean that fiber will not help. Choosing the right type is key.

Summary

Dietary fiber is commonly used to manage GI problems (e.g., constipation, diarrhea, anal gland irritation). However, this nondigestible portion of carbohydrates is not a “one size fits all” nutrient. Rather, its uses depend on the properties of the fiber (e.g., fermentability, solubility, viscosity). Not all patients respond to fiber in the same way; thus, the amount and type of fiber fed should be based on the individual patient and not generalized as being universally beneficial or detrimental.