Bananas, onions, garlic, leeks, asparagus, and artichokes, plus soybeans and whole-wheat foods are a good source of prebiotic fibre. Prebiotics are not digested by the small intestine, but are used as fuel by colon bacteria. This fermentation process helps to increase the number of desirable or "good" bacteria in our digestive systems that are associated with better health and reduced irritable bowel syndrome.