If you asked me to name the nutrient that causes the most confusion for pet owners and veterinarians alike, I’d have to say fiber. Fiber is carbohydrate from plants that cannot be digested by the enzymes that mammals make and use in our gastrointestinal tract.

Fiber is associated with health benefits in both people and pets. For healthy pets, fiber can affect stool quality and regularity and promote a healthy population of gut bacteria. It can also have benefits for a number of health concerns in pets such as gastrointestinal issues, diabetes, high cholesterol and triglycerides, and for weight loss.

There are two properties based on which fiber can be classified: solubility and fermentability. Solubility is a measure of how thoroughly the fiber dissolves in water. Insoluble fibers do not dissolve while highly soluble fibers dissolve completely. Insoluble fibers basically enter and exit the body virtually unchanged. They tend to increase stool volume and frequency. The most common type of insoluble fiber found in pet foods is cellulose. Wheat bran is a good source of insoluble fiber that may be supplemented for people or pets.

Soluble fibers include pectins, gums, and fructans. Pure soluble fibers will dissolve in water and form gels if in the right proportions (pectins are used to make jam). Soluble fiber supplements for people are often advertised as being able to be mixed in with any kind of food or beverage while producing “no gritty residue”. Common soluble fiber sources in pet foods include guar gum and inulin (often derived from Jerusalem artichoke or chicory root). Soluble fibers tend to slow digestion and pull water into the gut.

Fibers can also be categorized on the basis of the ability of gut bacteria to use them to produce nutrients for the bacteria as well as the cells that line the colon. This process is called fermentation. Generally, the more soluble the fiber source, the more rapidly and completely fermentable it is. While the ability to be fermented by gut bacteria can provide health benefits to the gut and the bacteria (which appear to be closely tied to overall health), too much highly fermentable fiber can interfere with digestion and can lead to excessive production of gas and associated gastrointestinal discomfort. Fermentable, soluble fibers are often called “prebiotic” fibers because of their benefit to the gut bacteria.

Not all fiber is completely soluble or insoluble. Some is a mixture – examples include beet pulp, psyllium seed husk, and oat bran. These fibers will have an effect related to their ratios of soluble to insoluble components as well as their fermentability.

Comparing fiber content between foods

One of the biggest challenges of fiber is that it can be very difficult to compare the amounts of each type of fiber or even the total fiber between commercial pet foods. The fiber amount listed on the pet food label in the Guaranteed Analysis is the maximum “crude fiber”, which includes most, but not all, insoluble fiber, but does not include any soluble fiber. Plus, because it is a maximum, the actual crude fiber in the diet could be much lower. In general, the crude fiber on the pet food label is not a very good representative of the actual amount of fiber in a pet food.

Another type of fiber analysis, total dietary fiber (TDF), measures soluble fiber and the most common types of insoluble fibers in pet foods and is a more accurate measurement of the total fiber in a food. The TDF analysis (used for human foods) is more time-consuming and considerably more expensive than crude fiber measurement. Because of these limitations and because it is not required of pet food companies, TDF is rarely available for commercial pet foods.

Because the crude fiber only includes insoluble fiber, total fiber in the diet may be much higher than the crude fiber value listed on the label if the diet is high in soluble fiber. This discrepancy is likely why it can be so hard to match up fiber amounts in pet foods – two diets that are similar in crude fiber could be very different in terms of the total amount and the types of fiber and these variations could have a dramatically different effect in the pet.

Differences in fiber type and amount likely result in many of the gastrointestinal issues that pet owners see that vary with diet – they can often explain a pet who has poor stool quality on one diet but perfect stool on another. Too much or too little or the wrong mixture of fiber is a much more likely reason for a pet to not do well on a specific diet than a food allergy, despite much misinformation to the contrary.

Adjusting the fiber in your pet’s diet

If you think your pet might benefit from more fiber, touch base with your veterinarian. If she agrees, you have the option of feeding a higher fiber diet, or adding fiber supplements to the existing diet. If you’re going to try a higher fiber diet, it’s best to use one from your veterinarian – therapeutic high fiber diets will have more accurate measurement of the types and amount of fiber than over-the-counter diets, making it much easier to sort things out.

Common fiber supplements that may be added to pet foods include psyllium seed husk, wheat bran, cellulose, and inulin, all of which are available commercially for humans. Many pet owners also add vegetables such as pumpkin or green beans to the diet as fiber sources. The challenge with these foods is that while they are healthy and safe for most pets, the fiber they contain is variable and typically low compared to the amount that could be obtained from a small amount of a concentrated fiber supplement.

Which type of fiber to use and how much is going to vary based on the goals, size of pet, type of pet (cat vs dog), and to a good extent some trial-and-error. There is limited guidance for standardized dosages of fiber to add to an existing commercial diet. One study found benefits in dogs with a median dosage of 2 tablespoons per day of a common human psyllium supplement, although there was a range of 0.25 to 6 tablespoons per day.

If supplemented, fiber should always be given gradually over a few days to weeks until the stool reaches the desired composition or other desired benefit is reached (or it becomes clear that it is not helping). When choosing a fiber supplement, take care that any products intended for people do not contain xylitol or other artificial sweeteners. It is generally best to select pure sources with no added flavors, colors, or sweeteners.

The table below compares the TDF as well as the soluble and insoluble breakdown of several fiber sources commonly included in pet foods or supplemented in addition to pet foods.

Common fiber supplements used for pets (nutrients per tablespoon)

Fiber	Calories	Total Dietary Fiber (g)	Soluble fiber (g)	Insoluble fiber (g)
Canned pumpkin	5	0.4	0.1	0.4
Wheat bran	8	1.6	0.1	1.6
Oat bran	14	0.9	0.4	0.5
Psyllium seed husk	17	3.5	3.0	0.5
Inulin	5	2.5	2.5	0