By  Dr Anton C Beynen*

Prebiotics can be defined as dietary arbohydrates that resist the cats’ digestive enzymes and pass to the hindgut where they fall prey to bacteria. In other words, prebiotics are indigestible, fermentable carbohydrates. Most plant ingredients in cat foods carry with them variable types and amounts of prebiotics. Wet food may contain thickening agents with prebiotic properties, such as guar gum and carrageenan. The focus here is on high-purity prebiotics that are added to cat foods to support a claim for gut health.

Cat foods promising gut health mostly contain added FOS (fructo-oligosaccharides), beet pulp, MOS (mannanoligosaccharides), XOS (xylo-oligosacharides) or lactosucrose as highlighted ingredient. Those foods generally purport to stimulate growth of the good gut bacteria toward a healthier community and to aid in the cat’s digestion. Uncurbed intakes of prebiotics, meanwhile, expand fecal bulk, due to more water and solids, while appearing as loose stool.

The research literature provides limited information on the effects of prebiotics in cats, but some tentative comments can be made. High intakes of FOS, beet pulp or guar gum depress the net digestive efficiency for dietary dry substances, whereas MOS acts augmentative. Digestive measures appear unreported for XOS, lactosucrose and carrageenan. Dietary FOS and lactosucrose, rather than XOS, may enhance putative good bacteria in the large intestine.
Whether or not cat food featuring a certain prebiotic lives up to its promise of boosting gut health, depends on the backing evidence. Claim substantiation should be based on controlled research in cats fed the same amount and type of the prebiotic in the marketed food. That information seems unavailable in the public domain. Cat lovers may contact the appropriate food-brand proprietor for proof of efficacy.

General considerations
The indigestibility of polysaccharide prebiotics is based on the principle that their glycosidic bonds cannot be hydrolysed by cat’s pancreatic α-amylase. Colonic fermentation of specific prebiotics is deduced from their breakdown by feline feces as microbial inoculum (1-3). For guar, beet pulp and FOS, respectively, there was an ascending degree of fermentation. Beet pulp fermentation by fecal microflora from cats fed either beet pulp or corn was determined, but undisclosed (2).
In dogs, dietary FOS and MOS increased apparent ileal digestion of dietary dry matter (4, 5), but the underlying mechanism is obscure. Instead, beet pulp depressed digestion (6), perhaps because its pectin component raises viscosity of ileal digesta (cf. 7). In the feline colon, prebiotics stimulate bacterial growth. Fecal, bacterial protein increases (cf. 8), which in itself diminishes apparent, total-tract protein digestibility. Microbial-derived, colonic and fecal short-chain fatty acids mount up, which lowers fecal pH and, through an osmotic effect in the colon, raises fecal water content and volume.
Clearly, ingestion of prebiotics can affect feline digestive and fecal characteristics. However, the effect direction and size depend on prebiotic type and amount, and also on the control treatment. All of those factors, and outcome reproducibility in controlled studies, get to practical application of prebiotics in cat food.

Legume seeds
Peas, lentils and soybeans contain a total of 6-8% α-galactoside prebiotics in the form of raffinose, stachyose and verbascose (9, 10). Cats were fed balanced, dry, extruded diets with 35% starch, but having either brewer’s rice, peas or lentils as the only starch source (11). The two legume seeds, at the impractically high level of 66%, raised the output of fecal wet and dry weight by 91 and 42%, and reduced fecal pH by 1.2 units. Those effects were smaller for corn as control. Regardless of the control, peas and lentils left fecal condition scores unchanged.
Cats received isonutrient, dry kibbled foods with 29% micronized, whole soybeans or 17% corn gluten meal plus extra corn starch and soybean oil (12). Soybean feeding increased wet and dry feces production by 26 and 4%, but gave feces a slightly drier look.

Beet pulp
Sugar-beet pulp in practically excessive concentrations, 12.5 % in dry food or 10% in the dry matter of ground chicken carcass, markedly lowered the dry matter percentage of cat feces (3, 13). Beet pulp in the dry food reduced apparent total-tract digestibility of dry matter, but surprisingly it did not so when added to the chicken mince.

Based on two and four cat trials, 4.4% added FOS in dry food reduced apparent dry matter digestibility by 1.4% units, increased fecal wet weight by 19% and fecal dry weight by 12% (8, 14-16). Dry food with additional FOS (0.7 or 4%) strikingly increased the number of fecal lactobacilli and reduced that of clostridia (16, 17). FOS inclusion in practice normally is way below 1%.

MOS, guar gum, lactosucrose, XOS
Data on the four prebiotics in the subheading are unconfirmed and fragmentary. At 0.4% in the dry matter fraction of wet(ted) food, MOS increased apparent digestion of dry matter in cats (18). At 4% in dry cat food, viscous guar gum versus insoluble cellulose increased wet and dry feces excretion by 24 and 21% (19).
Lactosucrose is a non-digestible trisacchararide consisting of a galactose, glucose and fructose unit. Administration of lactosucrose to cats, at the equivalent of 0.3% in dry food, produced a 7-fold increase in fecal lactobacilli (20). In cats, XOS feeding (0.04 or 0.4% in the diet) lowered fecal Bifidobacteriales and increased Clostridiales in dose-dependent fashions (21).

* List of references is available on request from the author (