Fiber in pet diets: diarrhea, weight loss and hairballs

Fiber is the part of a dog's or cat's food that its own digestive enzymes cannot break down. It changes how much water and bulk the stool holds, how fast food moves through the gut, how full a meal feels and what the bacteria in the colon have to eat. Clinical studies support extra fiber for dogs with large bowel diarrhea and for weight loss. For diabetic pets, and diabetic cats in particular, the older advice to feed high fiber has not held up well.

Types of fiber

Labels and older guides split fiber into soluble and insoluble. That division is useful but incomplete. A 2022 review for veterinarians describes fermentability, solubility and viscosity as the features that decide what a fiber does in the gut, and lists its main roles as altering the digesta physically, affecting appetite and satiety, regulating digestion and feeding gut microbes.

Soluble and fermentable fibers

Soluble fibers dissolve or disperse in water. Some, such as psyllium husk, form a thick gel that holds water in the stool. Others, such as inulin and fructooligosaccharides (often from chicory root), are fermented quickly by colon bacteria. Beet pulp, common in dry foods, sits in between: partly soluble and moderately fermented.

When bacteria ferment fiber they produce short-chain fatty acids, including butyrate, which the cells lining the colon use as fuel. That is one reason fermentable fiber is used in diets for colon problems.

Insoluble fibers

Insoluble fibers such as cellulose pass through largely unfermented. They add bulk to stool, hold some water, and tend to speed movement through the large intestine. They also dilute the calories in a food, which matters for weight-loss diets.

Diarrhea from the large bowel

Large bowel (colonic) diarrhea typically means frequent small stools, urgency, straining and sometimes mucus or fresh blood. Fiber is one of the standard dietary tools for it, and two recent studies give a sense of what it can do.

In 22 police working dogs with chronic large bowel diarrhea and no other identified disease, psyllium husk was added to the food for a month. Response was rated good or very good in 90% of the dogs, stools became consistently formed in 90% and defecation frequency fell, and the improvement largely held through a further month without psyllium. There was no control group, so some improvement may have happened anyway.

A randomized study in shelter dogs with acute colitis compared a diet high in both soluble and insoluble fiber with a standard diet. Loose stools made up 16% of samples from dogs on the high-fiber diet, against 60% on the standard diet, and the authors supported feeding the high-fiber diet for acute large bowel diarrhea. Only 11 dogs per group completed the protocol, which limits how far the numbers can be pushed.

Diarrhea that lasts more than a few days, contains a lot of blood, or comes with vomiting, weight loss or lethargy needs a vet before any diet change. Fiber manages symptoms; it does not identify parasites, infections or other causes.

Constipation and anal sacs

Fiber is also used for mild constipation, but the right type and amount depend on the cause, and a dog or cat that strains without passing stool should be examined first. A cat, especially a male, straining in the litter tray may have a blocked bladder rather than constipation, which is an emergency, so it needs a vet the same day.

Fiber is often recommended for dogs with recurrent anal sac problems, on the reasoning that firm, bulky stools press the sacs empty during defecation. Direct evidence for fiber on its own is limited.

Weight management

Hunger and begging make weight-loss plans hard to stick to. Fiber adds volume with few calories and can make restricted portions easier to live with. In client-owned obese dogs, those fed a high-protein, high-fiber diet lost a greater percentage of body weight, and lost it faster, than matched dogs on a high-protein diet with moderate fiber and the same caloric density. They also lost more body fat.

Adding vegetables to a normal food is not the same thing. Replacing part of a complete food with green beans or similar cuts protein, vitamins and minerals along with the calories, so a long weight-loss plan is better built on a food designed for it.

Diabetes

High-fiber diets have often been recommended for diabetic dogs and cats, on the theory that fiber slows glucose absorption. Controlled studies have been less convincing.

• Dogs. In 12 dogs with stable diabetes, high-fiber, moderate-carbohydrate diets offered no significant advantage in insulin dose or glucose control over a commercial diet with moderate fiber and low carbohydrate. Dogs lost weight on one of the high-fiber diets, and the authors advised against routinely using such diets in thin diabetic dogs.
• Cats. In a randomized trial of 63 diabetic cats, 68% of cats fed a low-carbohydrate, low-fiber canned food no longer needed insulin at 16 weeks, compared with 41% on a moderate-carbohydrate, high-fiber canned food.

Diet for a diabetic pet should be set with the vet treating it, because a change in food can change the insulin dose needed.

Hairballs in cats

Swallowed hair normally passes in the stool. Fiber is used in hairball diets to move more of it through. In a controlled feeding study, long-haired cats excreted 81% and 113% more hair in their feces on diets with psyllium and 11% or 15% total dietary fiber than on a 6% fiber diet over 14 days. Short-haired cats showed no change. The study measured hair in feces rather than hairballs vomited. Regular brushing reduces how much hair is swallowed in the first place, and a cat that vomits hairballs often, retches without producing anything or stops eating should see a vet.

Reading fiber on a label

The guaranteed analysis lists crude fiber as a maximum. The crude fiber method recovers mainly insoluble fiber and misses much of the soluble fraction, so it understates the total. Total dietary fiber, which some therapeutic diets and studies report, is the more complete figure, and the two numbers cannot be compared directly.

More is not always better. High-fiber foods produce more stool and can lower the digestibility of other nutrients, and rapidly fermented fibers can cause gas and loose stools when introduced quickly.

Adding fiber at home

• Psyllium husk is the supplement used in the police dog study. Start with a small amount mixed into moist food with extra water, and ask the vet for a dose suited to the animal's size; the police dogs above were working dogs given 4 tablespoons a day.
• Plain canned pumpkin is widely used. Pie filling is not a substitute, as it contains sugar and spices.
• Therapeutic high-fiber diets are usually more consistent than home additions for ongoing problems, because the fiber type and amount are fixed and the rest of the diet stays balanced.

Change fiber gradually over several days, keep fresh water available, and judge the result by stool quality and body condition rather than by the label.

Sources

1. Dietary fiber aids in the management of canine and feline gastrointestinal disease. Journal of the American Veterinary Medical Association, 2022.
2. The use of soluble fibre for the management of chronic idiopathic large-bowel diarrhoea in police working dogs. BMC Veterinary Research, 2021.
3. Efficacy of feeding a diet containing a high concentration of mixed fiber sources for management of acute large bowel diarrhea in dogs in shelters. Journal of Veterinary Internal Medicine, 2022.
4. A high protein high fibre diet improves weight loss in obese dogs. The Veterinary Journal, 2010.
5. Lack of advantage of high-fibre, moderate-carbohydrate diets in dogs with stabilised diabetes. Journal of Small Animal Practice, 2009.
6. Comparison of a low carbohydrate-low fiber diet and a moderate carbohydrate-high fiber diet in the management of feline diabetes mellitus. Journal of Feline Medicine and Surgery, 2006.
7. Influence of the dietary fibre levels on faecal hair excretion after 14 days in short and long-haired domestic cats. Veterinary Medicine and Science, 2015.