Choosing a Digestive Supplement for Your Cat: What Do Probiotics, Fibre and Glutamine Do?

An owner searching for a "cat digestive supplement" meets a wide range of products: probiotic powders, fibre pastes, liquids containing glutamine, and mixtures of these. On the shelf they all sit under the "digestion" heading, but the ingredients they contain are different things, and what we know about each of them in cats is at a different level.

This article is a selection guide. We explained how the intestinal barrier works, the microbiota and glutamine's place in intestinal cells at the level of mechanism in our article on gut health in cats; we do not repeat that here. Instead we look for answers to these questions: What are probiotics, fibre and glutamine used for, and how much evidence is there in cats? Which form suits a cat? How do you read the ingredient list and the daily amount on the label? In which situations should you see a veterinarian before thinking about a supplement, and why should a supplement not be confused with a change of food?

Short answer: How do you choose a digestive supplement for a cat?

Start by answering "why". Supplements are used to complement the nutrition of a cat fed a complete and balanced food made for cats; the foundation of a cat's daily needs is its food (National Research Council, 2006). If there is ongoing vomiting, diarrhoea or loss of appetite, the first step is not a supplement but an assessment by your veterinarian. If you are going to choose a supplement, check four things: (1) it is made for cats, (2) the label clearly states each ingredient and its amount per daily serving, and for probiotics the strain name and the number of live microorganisms, (3) it comes in a form your cat will accept, (4) the daily amount is clearly given by weight or age. Probiotics, fibre and glutamine are not interchangeable; research in cats is limited for all three, and results vary by ingredient and, for probiotics, even by strain.

First, separate the paths: supplement, food change or veterinarian?

A concern about digestion can lead down three different paths, and not mixing them up is the most important step in choosing.

1. If there are signs, see your veterinarian first. Repeated vomiting, soft stools or a change in stool routine is not something to "try" a supplement on; its cause is determined by a veterinary examination. We covered what these signs may indicate in our article on digestive problems in cats. A veterinarian's nutritional assessment begins with a diet history that includes the cat's food, treats and all supplements it receives (WSAVA Nutritional Assessment Guidelines Task Force, 2011). It therefore helps to note the name and amount of every product you give your cat and bring the list to the appointment.

2. A change of food affects digestion on its own. The gut microbiota is shaped by the composition of the diet (Rochus et al., 2014). For this reason, sudden food changes can cause temporary changes in digestive routine, and food transitions are usually made gradually over 7–10 days. An owner who thinks "I changed the food, the stools got softer, let me add a supplement too" has in fact stacked two changes on top of each other.

3. One change at a time. If you start a new food and a new supplement in the same week, you cannot tell which of them caused a change you observe afterwards. Complete the food transition first, and once your cat's routine has settled, consider the supplement with your veterinarian's input.

For some digestive complaints your veterinarian may recommend a special-formula diet rather than a supplement. That is a different decision from choosing a supplement and rests on the veterinarian's assessment.

Probiotics: What do they do, and how much evidence is there in cats?

Definition. According to the consensus statement of the International Scientific Association for Probiotics and Prebiotics (ISAPP), probiotics are defined as "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host" (Hill et al., 2014). Two words in this definition matter for choosing: live and adequate amounts. The same statement also stresses that some effects are specific to particular strains; in other words, "probiotic" is not a single substance but a set of microorganisms, each of which needs to be assessed separately.

What do we know in cats? Probiotic studies in cats are few, and each was carried out with a specific strain:

In a double-blind, placebo-controlled study of 217 cats in an animal shelter, the proportion of cats with diarrhoea lasting two days or longer was 7.4% in the group given the Enterococcus faecium SF68 strain and 20.7% in the placebo group (Bybee et al., 2011). This result was obtained under shelter conditions and with a single strain; it does not mean the same result would be seen in a healthy cat living at home or with a different strain.
In a study of 15 healthy adult cats, the Lactobacillus acidophilus DSM13241 strain was recovered from the faeces and changes in the numbers of some faecal bacterial groups were reported (Marshall-Jones et al., 2006). The study was small, and the comparison was not a separate group of cats but the same cats before and after.

In short, the evidence on probiotics in cats rests on a small number of studies with specific strains. A product saying "contains probiotics" does not mean the strain in that product has been studied in cats.

Why does quality matter? In a study that examined 25 probiotic products sold for animals, only 4 of the 15 products (27%) that stated a specific number of live microorganisms on the label met that value, and at least one microorganism name was misspelled on 7 of the 22 products that listed microorganisms (Weese and Martin, 2011). This is why, when choosing a probiotic, how detailed the label is matters as much as the product itself.

What to look for on a probiotic label:

The genus, species and strain name of the microorganism (for example, not just "lactobacillus" but genus-species-strain).
The number of live microorganisms per daily amount (colony-forming units, CFU).
Whether this number is guaranteed at the time of manufacture or until the expiry date.
Storage conditions (some products are kept in the refrigerator) and preparation instructions. Unless the label says otherwise, do not mix a product containing live microorganisms into hot food.

Fibre and prebiotics: When do they matter for a cat?

Definition. According to the ISAPP consensus statement, a prebiotic is defined as "a substrate that is selectively utilised by host microorganisms conferring a health benefit" (Gibson et al., 2017). So a prebiotic is not a live microorganism but a dietary component used by the microorganisms in the gut. Not every fibre is a prebiotic; in feline nutrition, fibres are divided into those that can be fermented in the large intestine and those that mainly add bulk to the stool (Rochus et al., 2014).

Fibre's place in the cat. The cat is an obligate carnivore; in nature most of its food is the protein and fat of prey animals (Zoran, 2002). Research in cats has shown that the fibre content of the diet can influence stool consistency and microbiota composition, but no "ideal" type or amount of fibre has been defined for cats (Rochus et al., 2014).

The limits of the evidence. Clinical data on fibre in cats come mostly from special-diet studies, not from supplements. For example, in two field trials involving 66 cats with constipation, a significant change in stool consistency was reported with a psyllium-enriched dry diet; however, these were pilot studies without a control group, supported by the diet's manufacturer, and the authors themselves recommended randomised controlled trials (Freiche et al., 2011). Constipation itself is also a condition that needs veterinary assessment.

In practice: Rather than adding a fibre source to the food on your own, talk to your veterinarian if you are considering a change. We covered the link between fibre and hairballs in our article on hairballs in cats.

Glutamine: An amino acid, not a probiotic or a fibre

Glutamine is an amino acid that is one of the main energy sources of the cells lining the small intestine (Kim and Kim, 2017). We explained the details in our gut health article. From a selection point of view, these are the things to know:

Glutamine is an amino acid; it is neither a probiotic nor a fibre. An owner who sees glutamine on an ingredient list should not assume the product directly provides microorganisms or fibre to the microbiota.
It is not essential for cats. The body can make glutamine itself; the amino acids a cat needs are met from the protein in a complete and balanced food for cats (National Research Council, 2006). In the literature glutamine is discussed as a "conditionally essential" amino acid whose requirement may rise in some states of intense stress (Kim and Kim, 2017).
Cat-specific evidence is limited. Most of what is known about glutamine comes from human and laboratory animal studies. In a model in which intestinal damage was induced experimentally in cats, adding glutamine to the diet made no difference to the intestinal permeability measurements examined (Marks et al., 1999).

Choosing a form: Liquid, powder, paste or tablet?

Remember that form is about practicality, not health: the best form is the one your cat will take without trouble every day.

Form	How it is given	Point to watch
Liquid / drops	Mixed into food or dropped into the mouth	The measuring tool (dropper, measuring cup) should be clear; scent-sensitive cats may refuse the food
Powder	Sprinkled over food and mixed in	Mixes more easily into wet food; may stay at the bottom of dry food
Paste	Directly by mouth or smeared on a paw	The cat needs to accept the taste
Tablet / capsule	By mouth or inside a treat	Many cats find tablets hard to take; check the label to see whether the capsule can be opened and mixed

If your cat will not eat food with a supplement mixed in, do not force it; if its food intake drops, consult your veterinarian. For the practical differences between drops and other forms, see our article on vitamin drops for cats.

Reading the label: 6 checks

Which species is it for? Cats' nutrient needs differ from those of dogs and humans (National Research Council, 2006; Zoran, 2002). Do not give your cat a product made for dogs or humans; look for "for cats" on the label.
Ingredient list and amounts. You cannot compare products using a label that only gives ingredient names without their amount per daily serving. Check how many mg (or, for probiotics, how many CFU) each ingredient provides per daily amount.
Strain and count for probiotics. Use the list above: strain name, live count, guarantee until the expiry date (Hill et al., 2014; Weese and Martin, 2011).
How the daily amount is described. Is it given by weight or age? Does a measuring tool come with the product?
Overlap. If your cat already takes another supplement, check whether the same ingredient appears in both products, and show all products to your veterinarian.
Traceability. The batch number, expiry date, manufacturer or importer details and a contact address should be on the label, and storage conditions should be clearly stated.

Reading the daily amount correctly

Do not exceed the daily amount stated on the label. "The more I give, the better" does not apply to supplements.
Calculate per cat, not per household. If you have more than one cat, do not put the product in a "shared bowl"; each cat's weight and amount are separate.
Note the start date. Record the day you start the supplement and your observations of your cat's stool consistency, appetite, vomiting and weight. The weekly observation list in our gut health article was prepared for exactly this.
Agree the review period with your veterinarian. Discussing in advance how long to continue a supplement and what the decision will be based on turns the question "is it working?" into something other than a guess.

When is it not a supplement but straight to the veterinarian?

In the following situations, set supplement selection aside and contact your veterinarian without waiting:

Repeated vomiting or diarrhoea lasting more than a day
Blood in the stool, very dark stools or noticeable mucus
Loss of appetite, lethargy or unexplained weight loss
Straining to pass stool, or not passing stool for several days
A newly started digestive change in an older cat
Your cat has a known chronic condition or is taking any medication

For the changing needs of an ageing cat, see our article on senior cat care.

A product example within this framework

The ingredients of products sold under the digestion heading differ from one another, so you need to look at the ingredient list rather than the category name. For example, Nutrasion L-Glutamine Cat Digestive Supplement is a 120 ml liquid product prepared for cats aged 7 and over, and the daily amount stated on the label is given mixed into food. L-glutamine appears in the ingredient list on the product page; no probiotic or fibre ingredient is listed. Supplements are used to complement nutrition; your cat's daily needs are met by its main food. Before starting any supplement, talk to your veterinarian, especially if your cat has a known health problem or takes any medication.

Summary

A supplement complements the nutrition of a cat fed a complete and balanced food; with an ongoing digestive complaint, the first step is your veterinarian.
Do not start a food change and a supplement at the same time; complete a gradual food transition first.
Probiotics are live microorganisms and their effects vary by strain; the evidence in cats rests on a small number of studies with specific strains.
On a probiotic label, look for the strain name, the daily live count and a guarantee until the expiry date; label accuracy problems have been reported in animal products.
Fibre and prebiotics are dietary components used by microorganisms; no ideal amount of fibre is defined for cats, and clinical data come mostly from special-diet studies.
Glutamine is an amino acid; it is not essential for cats, and cat-specific evidence is limited.
The best form is the one your cat will take without trouble every day; do not exceed the daily amount on the label.

Frequently asked questions

Does my cat need a digestive supplement?

A healthy cat fed a complete and balanced food made for cats gets its daily nutrient needs from its food. A supplement is used to complement nutrition and is not necessary for every cat. If there is vomiting, diarrhoea, loss of appetite or weight loss, contact your veterinarian before choosing a supplement.

What is the difference between probiotics, prebiotics and fibre?

Probiotics are live microorganisms that confer a health benefit when given in adequate amounts. A prebiotic is a dietary component selectively used by the microorganisms in the gut. Fibre is a broader group; some fibres are fermented and may have prebiotic properties, while others mainly add bulk to the stool.

What should I look for on the label when choosing a probiotic for my cat?

Look for the genus, species and strain name of the microorganism, the number of live microorganisms per daily amount (CFU), whether that number is guaranteed until the expiry date, and the storage conditions. Studies have shown that the live count stated on the label is not always met in probiotics sold for animals.

Is glutamine a probiotic?

No. Glutamine is an amino acid; it is neither a live microorganism nor a fibre. It is one of the main energy sources of small intestine cells, but it is not an essential amino acid for cats, and cat-specific research is limited.

Can I start a supplement at the same time as changing food?

It is not recommended. A change of food can affect digestive routine on its own. If you start both at the same time, you cannot tell which one caused a change you observe afterwards. Complete the food transition first, spreading it over 7–10 days, and consider the supplement afterwards with your veterinarian's input.

References

Hill C, Guarner F, Reid G, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology, 2014;11(8):506–514.
Gibson GR, Hutkins R, Sanders ME, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nature Reviews Gastroenterology & Hepatology, 2017;14(8):491–502.
Bybee SN, Scorza AV, Lappin MR. Effect of the probiotic Enterococcus faecium SF68 on presence of diarrhea in cats and dogs housed in an animal shelter. Journal of Veterinary Internal Medicine, 2011;25(4):856–860.
Marshall-Jones ZV, Baillon ML, Croft JM, Butterwick RF. Effects of Lactobacillus acidophilus DSM13241 as a probiotic in healthy adult cats. American Journal of Veterinary Research, 2006;67(6):1005–1012.
Weese JS, Martin H. Assessment of commercial probiotic bacterial contents and label accuracy. Canadian Veterinary Journal, 2011;52(1):43–46.
Rochus K, Janssens GPJ, Hesta M. Dietary fibre and the importance of the gut microbiota in feline nutrition: a review. Nutrition Research Reviews, 2014;27(2):295–307.
Freiche V, Houston D, Weese H, et al. Uncontrolled study assessing the impact of a psyllium-enriched extruded dry diet on faecal consistency in cats with constipation. Journal of Feline Medicine and Surgery, 2011;13(12):903–911.
Kim MH, Kim H. The roles of glutamine in the intestine and its implication in intestinal diseases. International Journal of Molecular Sciences, 2017;18(5):1051.
Marks SL, Cook AK, Reader R, et al. Effects of glutamine supplementation of an amino acid-based purified diet on intestinal mucosal integrity in cats with methotrexate-induced enteritis. American Journal of Veterinary Research, 1999;60(6):755–763.
Zoran DL. The carnivore connection to nutrition in cats. Journal of the American Veterinary Medical Association, 2002;221(11):1559–1567.
WSAVA Nutritional Assessment Guidelines Task Force Members; Freeman L, Becvarova I, Cave N, et al. WSAVA Nutritional Assessment Guidelines. Journal of Small Animal Practice, 2011;52(7):385–396.
National Research Council. Nutrient Requirements of Dogs and Cats. National Academies Press, 2006.

This article is for information purposes; consult your veterinarian for diagnosis and treatment. Supplements complement nutrition and do not replace medication.