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Prebiotics have been defined as 'non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth, and/or activity, of one or a limited number of beneficial bacteria in the colon and thus improve host health'(1). Research on the potential health benefits of prebiotics has occurred over the last 15 years or so, with a recent interest in the effects on the immune system, the host's ability to fight infection, and inflammatory processes and conditions. These effects have been reviewed several times(2-7) but to our knowledge there are no reviews that bring together all of the available studies in all of the these areas. Thus, the aim of the present article is to describe the structure and dietary sources of prebiotics, and to summarise and evaluate studies investigating the influence of prebiotics on immunity, host defence, and inflammatory processes and conditions.
Structure of prebiotics
[beta]2-1 Fructans, which include inulin (IN) and fructo-oligosaccharides (FOS), fulfil the criteria for prebiotics(8). Other carbohydrates including galacto-oligosaccharides (GOS), gluco-oligosaccharides, isomalto-oligosaccharides, lactulose, mannanoligosaccharides (MOS), nigero-oligosaccharides, oat [beta]-glucans, raffinose, soyabean oligosaccharides, transgalacto-oligosaccharides and xylo-oligosaccharides are considered as candidate prebiotics. Only studies with [beta]2-1 fructans will be considered in the present review, as these are the most widely studied with regard to potential modulation of the immune system, and relatively little information is available on the immunomodulatory properties of the other candidate prebiotics.
IN is a linear carbohydrate molecule which contains [beta]-(2 [arrow right] 1) fructosyl-fructose linkages with a terminal glucose(9). IN may contain between two and sixty fructose residues (Fig. 1), with an average of twelve. Partial enzymatic hydrolysis of IN yields a FOS known as oligofructose (OF), which can have a terminal glucose or fructose residue (Fig. 1). In OF there can be two to eight (average five) fructose residues with a terminal glucose residue or a chain of three to eight (average five) fructose residues(10). Thus IN and OF differ according to degree of polymerisation (Fig. 1). Short-chain FOS may also be derived by enzymatic addition of fructose residues to sucrose (Fig. 1); the products formed contain two to four fructose residues...