FOS (fructo-oligosaccharides) refers to a group of carbohydrates consisting of bonded fructose units. Labels of dog foods with added FOS declare it as such, as inulin, or as chicory root, extract or powder. Inulins present in or isolated from chicory roots are FOS mixtures with longer fructose chains. FOS blends comprising shorter chains are made from inulin or table sugar. Admixed preparations in relevant dry dog foods provide 0.025 to 1.0% FOS, while the amount of FOS brought along by the base ingredients is unknown.

Foods with added FOS claim to balance the dog’s intestinal, bacterial composition. FOS is purported to selectively feed the good gut bacteria toward a healthier community. A compilation of 11 and 9 dog experiments shows that 0.6% supplemental FOS in dry food is required to produce a two- and three-fold increase in the populations of the putative good bacteria, bifidobacteria and lactobacilli.

Some promotional materials also assert that FOS aids in canine digestion. Research data indicate that adding 0.6% FOS to dry food increases net uptake of total, dietary dry substances by the small intestine, but slightly decreases the uptake along the total intestinal tract, or from mouth to anus. Despite its latter effect, 0.6% added FOS in dry food does not noticeably raise stool volume.

To sum up, 0.6% added FOS in dry food appreciably enhances beneficial gut bacteria and small-intestinal digestion. That inclusion level is higher than the amounts mixed into many dog foods featuring FOS. Dog owners wishing to feed effective, FOS-enhanced dry food, may consult product labels and/or contact manufacturers to verify that the potential purchase contains sufficient, added FOS. Noteworthily, there is no evidence that dietary FOS maintains or improves visible health and extends longevity in dogs.

Fructo-oligosaccharides
Within FOS, a subgroup of the fructans, the degree of polymerization ranges from 2-60 for the inulins and from 2-10 for oligofructose/short-chain FOS preparations. Inulins, which have an α(1-2)-linked glucose end unit, are extracted from chicory root. FOS with shorter chains is made by enzymatic hydrolysis of inulin or fructosylation of sucrose, yielding products without/with or with terminal glucose, respectively (1).
FOS is a plant storage carbohydrate. Chicory root contains 15-20 percent inulin and 5-10 percent oligofructose (1). Wheat and barley may hold 1-4 and 0.5-1 percent FOS (2). As the fructose molecules in FOS have β(2-1) linkages, they cannot be hydrolysed by canine pancreatic α-amylase. Ingested FOS is broken down and fermented by colonic bacteria. FOS is efficiently converted into short-chain fatty acids by canine feces as microbial inoculum (3-5).

Fecal bacteria

A 1994 paper concluded that FOS feeding changed the bacterial composition in duodenal/proximal jejunal fluid of dogs with intestinal bacterial overgrowth (6), but the control and test animals were incomparable. Even so, the results were interpreted as indicative of a positive health response (7), inspiring further research.

Taken together, the results of 7-11 trials show that adding 0.6 percent FOS to dry food doubled and tripled the fecal numbers of desirable Bifidiobacterium and Lactobacillus, while lowering the viable cell counts of the undesirable Clostridia perfringens and Escherichia coli by about 20 percent (8-17).

Food digestion
Mixing inulin into dogs’ food, at 7 percent of dietary dry matter, increased wet feces quantity by 46 percent and reduced apparent protein digestibility by 2.6 percent units (18). The extra feces contained 89 percent moisture, pointing to an osmotic effect of colonic solutes generated by enhanced bacterial activity. Inulin acts as microbial substrate (3-5), thereby expanding fecal, bacterial mass, including protein (19). The latter explains, at least partly, that inulin lowered apparent protein digestion (18).
Averaged over five trials, 0.48 percent added FOS in dry food raised the apparent ileal digestibilities of dry matter and protein by 3.2 and 4.0 percent units. In contrast, for 11 trials, 1.0 percent FOS decreased mean apparent fecal digestibilities of dry matter and protein by 0.9 and 1.4 percent units (8, 10, 12, 14, 15, 17, 20, 21). Within trials (14, 17, 20), the dose-response relationships were erratic.

For 12/13 trials, mean wet stool production, either expressed as g/day or g/100 g dry matter intake, was unaltered by 1.2 percent added FOS in the diet, while dry matter content and pH of feces were unaffected (35.9 versus 35.4 percent) and slightly decreased (from 6.5 to 6.4) (9, 10, 12-15, 17, 20-23). Clearly, the impact of FOS on apparent digestion and colonic bacteria hardly altered fecal characteristics.

Immunity and fermentative products
In dogs fed diets with various FOS preparations, indicators of the immune response and bacterial degradation of intestinal proteins were measured, which produced inconsistent outcomes. FOS feeding raised immunoglobulin A in serum and ileal content (10, 21) or had no effect (15-17, 21), while the amount in feces was increased (21) or decreased (10, 21). FOS also unsystematically influenced fecal concentrations of isovalerate, indole, putrescine and hydrogen sulfide (10, 12, 14, 16, 17, 20, 22, 24).

Salmonella challenge
Puppies were fed dry food without or with 1 percent added short-chain FOS or inulin (25). After 14 days, half of them received an oral dose of Salmonella enterica. One and two days later, the infection-induced decrease in food intake was reduced by inulin only. The increase in body temperature was lessened by both FOS preparations. In rats, dietary FOS inhibited colonization, but stimulated translocation of salmonella (26, 27).

List of references is available on request from the author (beynen@freeler.nl)