What are prebiotics?

The scientific definition of prebiotics is: non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon.

Source: Gibson, G.R. and Roberfroid, M.B., “Dietary Modulation of the Human Colonic Microbiota: Introducing the Concept of  Prebiotics,” Journal of Nutrition, 125: 1401-1412 (1995).