Prebiotics are non-digestible dietary fibres that enhance the growth of "healthy" bacteria in the intestines when given in adequate amounts. Prebiotics are relatively stable, can be stored at room temperature and can be easily added to foods or drinks. They are not degraded by enzymes and acids in the digestive tract; therefore they reach the colon relatively unchanged, where they are metabolised by specific bacteria, thus promoting their beneficial effects. Prebiotics can be found in very small proportions in vegetables and fruits such as leeks, onions, tomatoes, garlic, etc., but this naturally occurring quantity is generally insufficient to promote a significant growth of beneficial bacteria. Therefore, large quantities of purified prebiotics are available as powder supplements and sold in packages of the appropriate dose.