Oligosaccharides are considered as one of the most valuable sources of prebiotic fibers in diets for young, non-ruminant animals such as broilers, pullets, piglets and calves. Nevertheless, their level of use does not reflect their true potential and wanes as marketing efforts reach their budget limits. Like an additive pertaining to digestive function and health, understanding its role and mode of action is rather elusive, contributing to its underuse by the feed industry. In essence, however, oligosaccharides are just food for beneficial gut bacteria, but this requires some further explanation.
To understand better how oligosaccharides work, we can use inulin as an example, if only because there is ample literature on this additive. Inulin, which abounds in root plants such as artichokes, chicory, onions, leeks and asparagus, is nothing but a blend of short- and medium-length chains of fructose molecules. These chains are non-digestible by non-ruminant animals due to the way the fructose molecules are linked to each other by a special beta-2-1-glycosidic bond. In contrast, this specific bond gives inulin all its properties as a prebiotic fiber. Chicory being the richest source of inulin in nature (about 16 percent inulin) is the primary source of commercial inulin extracts, which are derived by extraction with hot water. As such, crystalline inulin, which is slightly sweet, is highly soluble in warm water, making it an ideal ingredient for certain types of milk replacers. It is totally soluble under gut conditions as soon as it reaches the stomach. Of course, one is not limited in purified inulin, as chicory pulp is available at a reduced cost, albeit variability in concentration is higher than in a standardized product.
In the stomach
One of the main functions of the stomach is as a barrier against the entrance of pathogenic bacteria — a function achieved through the secretion of hydrochloric acid that creates a very acidic (unfavorable) environment for most pathogens. In young animals, however, this mechanism is not yet fully developed, with ensuing diarrheas being the most common observation. In suckling pigs, for example, milk provides lactose that, apart from feeding the piglet, also feeds lactic bacteria that produce lactic acid, which replaces the function of hydrochloric acid.
Fully understanding the mechanisms of proper nourishment for beneficial bacteria is the next frontier in nutrition.
In weaned pigs, however, not only are milk products expensive (and thus included in restricted amounts in their diets) but also feed intake is rather limited. Thus, lactose reaching the stomach is often not sufficient to provide nourishment for lactic bacteria, leading to a less acidic environment. Providing inulin, however, may stimulate the growth of acid producing bacteria in the stomach and yields the same beneficial effects as natural milk in terms of providing for an acidic barrier to pathogens. In fact, certain commercial lactose-replacement products often include inulin to account for the prebiotic effect of lactose. The same goes for liquid milk-replacer formulas.
In the small intestine
Upon leaving the stomach, we encounter an almost pH-neutral environment, which is regulated by the animal with utmost rigidity. Here it is almost impossible to lower the acidity of the gut contents in order to harm pathogens. In contrast, another mechanism comes into play that limits the chances of survival for pathogenic bacteria: competitive exclusion.
In simple words, all bacteria have to compete with each other for available nutrients, while at the same time secreting their own "antibiotics" to eliminate competitors. Thus, when lactic bacteria and bifidobacteria, to name a couple beneficial ones, outgrow coliforms and Salmonella, some of the most common pathogens, then the chances of such opportunistic pathogenic bacteria becoming a real problem greatly diminishes. If young animals are not provided with a source of prebiotic fiber (that preferentially feeds the beneficial bacteria), then, as is often the case, such diets (which are rather high in protein) favor the pathogens.
In the large intestine
Fermentation, that is uptake of nutrients by beneficial bacteria for use as fuel for their own needs, yields not only lactic acid, as is the case with lactic bacteria, but also a number of volatile fatty acids (VFA). In a recent study, it was demonstrated that oligofructose supplementation increased total VFA production from 83 mmol/g digesta to 108 mmol/g, an increase of 30 percent! These VFA have a double role. First, they provide a small amount of energy to the animal as they are being absorbed by the intestinal epithelium. Although this amount of energy accounts for less than 5 percent of total energy needs, for young animals that are always in an energy-deficient state, each small contribution is significant.
The second and most important role of VFA is, of course, as a deterrent against pathogenic bacteria, which prefer a less acidic environment. In fact, such acids as acetic acid and propionic acid are often used as feed ingredients for the same purpose. In addition, a certain VFA, butyric acid, plays a key role in stimulating the regeneration process in damaged epithelial tissue. In broilers, butyric acid is used extensively for this purpose. As this organic acid is extremely expensive, one can only wonder whether oligosaccharides can provide such acid at the level required and at a fraction of the cost.
The importance of understanding
In the post-antibiotic era, it is important to have a complete understanding of how gut bacteria grow and interact with each other and with the animal. Today, we place significant value on establishing a beneficial microflora right after birth. Fully understanding the mechanisms of proper nourishment for beneficial bacteria is the next frontier in nutrition. In this regards, oligosaccharides and other functional fibers have a tremendous role to play.