The specificity of the interaction with intestinal bacterial fermentation by prebiotics determines their physiological efficacy.

The concept of prebiotic food ingredients is an important recent development in nutrition. The concept has attracted a great deal of attention, and many food ingredients (mainly dietary carbohydrates) have been claimed to be 'prebiotic'. It is emphasised that in order to be called prebiotic, a compound should be: (1) non-digestible; (2) fermentable; (3) fermentable in a selective way. These properties should be demonstrated in human volunteers in at least two independent dietary intervention trials. On the basis of published and unpublished results, it is shown in the present paper that the way in which a prebiotic influences intestinal fermentation is the key to its physiological properties. This statement is illustrated mainly by considering an established group of prebiotics, the beta(2-1) fructans. These linear molecules show a strong discontinuity in physicochemical properties as the chains become longer. The beta(2-1) fructans with a chain length of up to ten monomer units are very soluble and are particularly 'bifidogenic'. Longer chains (ten to sixty-five monomer units) are poorly soluble in water, they have less pronounced bifidogenic properties, and they are fermented more slowly. It was observed that a combination of short-chain and long-chain fructans (Synergy 1) is physiologically (for example, increasing mineral absorption, suppressing carcinogenesis, modulating lipid metabolism, etc) more active than the individual fractions. A possible mechanism is described in the present review. From an in-depth overview of the literature it is confirmed that for prebiotic action, the 'selectivity principle' for intestinal fermentation is determinative for the type and for the efficiency of physiological activity. It is confirmed that prebiotics act through their influence on intestinal fermentation.