Prebiotic carbohydrates, known as oligosaccharides, are found naturally in certain fruit and vegetables, including bananas, asparagus, garlic, wheat, tomatoes, Jerusalem artichoke, onions and chicory. They are not digested but go straight to the gut where they are seized on by good bacteria, stimulating the bacteria's growth.

"Because you have more chances of getting prebiotics to the large intestine, you see a bigger response than you would possibly expect with a probiotic," says Gibson.

Substances that stimulate the growth of specific bacteria in the colon are referred to as "prebiotics," and are at the forefront of nutritional research.

An alternative approach to the manipulation of the gut microflora is the use of prebiotics. These are food ingredients that selectively target the colon and may beneficially affect the host by stimulating the growth or activity of specific resident bacteria. Again, the goal is to realise potential health benefits. Dr Glenn Gibson of the UK's Medical Research Council, Dunn Clinical Nutrition Centre, discussed the functional use of prebiotics, referring to the non-digestible oligosaccharides and their influence on bifidobacteria.