Prebiotics are currently being promoted for gut health and IBS. They are poorly absorbed carbohyrates - e.g short chains of fructose or resistant starch. They are fermented by colonic bacteria. The breakdown products result in substrate (food) for the colon and also encourage the growth of "good" bacteria such as Lactobacillus.These beneficial effects have theoretical value but remain unproven in IBS. The major disadvantage is the increase in bloating and flatulence - often troublesome symptoms in IBS