Fructooligosaccharides (FOS) play an important role in maintaining a healthy digestive tract. FOS is a non-digestible sugar that acts as a food source for the beneficial bacteria that live in your pet’s large intestine. The bacteria ingest the FOS by fermentation. A healthy beneficial bacteria colony forms a good defense against harmful bacteria. Misleading information has circulated about the FOS fermentation process claiming that it can cause diarrhea, gas, and nutrient loss.Those negative side effects could only happen if too much FOS was added to the pet food formula.