Source Naturals FOS Powder 100g 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.