Probiotic Enhancer. Source Naturals FOS is a complex of fructooligosaccharides (FOS) - a group of naturally occurring carbohydrates that can help promote the growth of beneficial flora in the gastrointestinal tract. The body normally maintains a balance of "friendly" flora in the intestines; unfortunately factors such as improper diet stress gastrointestinal problems and use of antibiotics may disrupt this delicate balance. FOS are long chain sugars which are indigestible by humans but serve as the "food" for friendly flora helping to increase their numbers in the body. Scientific research conducted in Japan has shown that FOS are selectively used by friendly flora such as bifidobacteria and lactobacilli.