When we read the labels on pet food for our four-legged friends, we often find references to the intestinal health of dogs and cats. Among the ingredients that most beneficially support digestion, prebiotics, particularly FOS and MOS, are essential. But what are they, and how do they act on the body?
What are prebiotics?
The abbreviations FOS and MOS refer to fructo-oligosaccharides and mannan-oligosaccharides. These two ingredients, often found in pet food, are beneficial fibers belonging to the large group of prebiotics.
Prebiotics—completely different from probiotics —are special sugars that support intestinal health and more, thanks to their nutraceutical function (which combines 
nutrition and pharmaceuticals). They mainly include fiber, namely the carbohydrates that line plant cell walls and consist of various units of glucose and other simple sugars, such as fructose or mannose. Their composition makes them completely indigestible in the stomach and the first section of the intestine, but this is not a negative aspect. In fact, prebiotics can therefore reach the large intestine intact, where they encounter microorganisms (bacteria, fungi, protozoa) that have beneficial effects on the health of the digestive system and the entire body. However, these bacteria need nourishment to reproduce and perform their functions. Prebiotics provide them with the nourishment and conditions necessary for their survival and reproduction.
Without a strong and healthy microbial flora, the intestine does not function properly and the entire body suffers as a result, as we will now see. In addition to competing with “bad” microorganisms, these bacteria can ferment fiber, producing many beneficial substances for themselves and the host organism, especially volatile fatty acids (acetic, propionic and butyric acid), which are absorbed by the intestine. Volatile fatty acids are essential for the cells lining the intestine because they allow them to maintain their integrity, ensuring proper digestive function. These fatty acids also keep intestinal pH fairly low, preventing certain groups of “bad” bacteria (such as Clostridia) from surviving.
But that is not all, because beneath these cells are others that form part of the immune system and can intervene in the event of an attack by bacteria, viruses and other “bad” microorganisms. These cells also need plenty of nourishment to multiply and act as they should.
By contrast, probiotics (to which we have dedicated a specific article) are what we commonly call “lactic acid bacteria”, namely all those “good” bacteria we take with yogurt or specific supplements when our intestines are somewhat upset. In this way, we can “replenish” the population of microorganisms normally present in the intestine, for example after antibiotic therapy or when symptoms such as diarrhea appear.
Prebiotics and probiotics, although they have different functions, are both fundamentally important. Without an adequate supply of prebiotics—from the diet or supplements—“good” microorganisms cannot nourish themselves and multiply as they should.
FOS and MOS
Let us look in detail at the two most important prebiotics: FOS and MOS.
FOS are soluble fibers (they can dissolve in water), whose main example is inulin, found in many plants such as chicory, asparagus, artichokes, legumes, garlic and onion, as well as bananas and wheat. They are fairly small sugars, consisting of a maximum of 5 units of glucose and fructose. The most important characteristic of FOS is that they provide nourishment exclusively to “good” bacteria, allowing them to reproduce at a good rate. When the number of these bacteria is high, they compete with “bad” bacteria and other microorganisms, which cannot multiply as they would like in the intestine because the “good” bacteria keep all the available nutrients for themselves. 
MOS have the same beneficial effects on intestinal health, but achieve them through a mechanism different from that of FOS. Mannan-oligosaccharides, which derive from yeast cell walls, consist of two different sugars (glucose and mannose) capable of adhering to “bad” bacteria, eliminating them through the feces.
However, like all fibers, FOS and MOS can also cause minor problems if taken in excess. Since they are soluble fibers, they swell on contact with the water present in the intestine and create a gelatinous mass that accelerates transit. This is very useful in cases of constipation, but could cause diarrhea and flatulence if consumed in large amounts.
Most pet foods already contain a certain amount of FOS and MOS. However, for dogs with specific needs (antibiotic therapy, gastrointestinal disorders) or when following homemade diets, it may be necessary to use prebiotic supplements, particularly FOS and MOS.