The main role of diet is to provide enough nutrients to meet the requirements of a balanced diet, while giving the consumer a feeling of satisfaction and well-being. The most recent knowledge in bioscience supports the hypothesis that diet also controls and modulates various functions in the body, and, in doing so, contributes to the state of good health necessary to reduce the risk of some diseases. It is such an hypothesis which is at the origin both of the concept of ‘functional food’ and the development of a new scientific discipline of ‘functional food science’. In the context of this paper the potential ‘functional foods’ to be discussed are the prebiotics and the synbiotics. The prebiotics developed so far are the non-digestible oligosaccharides and especially the non-digestible fructans among which chicory fructans play a major role. The chicory fructans are β (2-1) fructo-oligosaccharides classified as natural food ingredients. They positively affect various physiological functions in such a way that they are already or may, in the future, be classified as functional food ingredients for which claims of functional effects or of disease risk reduction might become authorized. They are classified as prebiotic and have been shown to induce an increase in the number of bifidobacteria in human faecal flora. As part of a synbiotic-type product, they are already bifidogenic at a dose of 2·75 g/d and the effect lasts for at least 7 weeks. The other potential functional effects are on the bioavailability of minerals, but also, and more systemically, on the metabolism of lipids. Potential health benefits may concern reduction of the risk of intestinal infectious diseases, cardiovascular disease, non-insulin-dependent diabetes, obesity, osteoporosis and cancer. However, except for the prebiotic effect, and tentatively the improvement of calcium bioavailability, the evidence to support such effects is still missing in humans though hypotheses already exist to justify nutrition studies.

This paper reviews the evidence for the claims of health benefits derived from the use of probiotics. A brief history of probiotics and the types of probiotics currently used and the criteria for the selection of probiotics is discussed. The ability of probiotics to enhance the nutritional content and bioavailability of nutrients and the scientific evidence for the usefulness of probiotics in alleviating the symptoms of lactose intolerance and in enhancing growth development is examined. The remainder of the review focuses on studies of a specific probiotic, Lactobacillus GG which has been extensively investigated for its health benefits in humans and animals. These studies serve as a model for the potential benefits of probiotics. The ability of Lactobacillus GG to treat or prevent diarrhoeal disease, to serve as an adjuvant for vaccines, to prevent rotavirus-induced diarrhoea, to prevent milk-based allergic reactions, alcohol-induced liver disease and colon cancer are presented. The review concludes with a discussion of the data supporting the safety of probiotics.

The human colonic flora has both beneficial and pathogenic potentials with respect to host health. There is now much interest in manipulation of the microbiota composition in order to improve the potentially beneficial aspects. The prebiotic approach dictates that non-viable food components are specifically fermented in the colon by indigenous bacteria thought to be of positive value, e.g. bifidobacteria, lactobacilli. Any food ingredient that enters the large intestine is a candidate prebiotic. However, to be effective, selectivity of the fermentation is essential. Most current attention and success has been derived using non-digestible oligosaccharides. Types primarily being looked at include those which contain fructose, xylose, soya, galactose, glucose and mannose. In particular, fructose-containing oligosaccharides, which occur naturally in a variety of plants such as onion, asparagus, chicory, banana and artichoke, fulfil the prebiotic criteria. Various data have shown that fructo-oligosaccharides (FOS) are specifically fermented by bifidobacteria. During controlled feeding studies, ingestion of these prebiotics causes bifidobacteria to become numerically dominant in faeces. Recent studies have indicated that a FOS dose of 4g/d is prebiotic. To exploit this concept more fully, there is a need for assessments of (a) improved determination of the gut microbiota composition and activity; (b) the use of molecular methodologies to assess accurately prebiotic identities and develop efficient bacterial probing strategies; (c) the prebiotic potential of raw and processed foods; and (d) the health consequences of dietary modulation.

Curiosity about probiotic bacteria is high. This is not surprising given the rapid growth of the natural products market in the USA, which was 22% in 1995 (New Hope Natural Media, Boulder, CO). Although probiotic cultures are only one component of that market, this trend shows that consumers in the USA are increasingly taking a pro-active stance towards their health and are purchasing products not only to eliminate what is perceived as dietary negatives but to increase the levels of dietary positives (Wrick, 1995). This shift in consumer attitude bodes well for the development of the American probiotic market. This article describes the composition, promotion and labelling of probiotic-containing products, discusses attributes of commercial probiotic bacteria, and estimates the size of the probiotic market in the USA. This market information was obtained from several published (Anon, 1996a,b)and unpublished but commercially available (SPINS Information Services, San Francisco, CA and New Hope Natural Media, Boulder, CO) sources.

Oligofructose and inulin, selective fermentable chicory fructans, have been shown to stimulate the growth of bifidobacteria which are regarded as beneficial strains in the colon. Studies were designed to evaluate inulin (Raftiline) and oligofructose (Raftilose), for their potential inhibitory properties against aberrant crypt foci (ACF) formation in the colon of rats. ACF are putative preneoplastic lesions from which adenomas and carcinomas may develop. The results of this study demonstrate that dietary administration of oligofructose and inulin inhibits the formation of preneoplastic lesions in the colon suggesting the potential colon tumour inhibitory properties of chicory fructans. Since these prebiotics selectively stimulate the growth of bifidobacteria, tumour inhibitory activity of lyophilized cultures of Bifidobacterium longum (BL) against azoxymethane (AOM)-induced colon carcinogenesis in rats and modulating effect of these cultures on colonic tumour cell proliferation, ornithine decarboxylase (ODC) activity, and ras-p21 oncoprotein expression were investigated. Dietary administration of lyophilized cultures of BL strongly suppressed AOM-induced colon tumour development. Inhibition of colon carcinogenesis was associated with a decrease in colonic mucosal cell proliferation and colonic mucosal and tumour ODC and ras-p21 activities.

Since the early work of Mann and Spoerry, probiotics in the form of fermented milk products have been reputed to have cholesterol-lowering properties in humans. However, studies conducted since the early 1970s have produced equivocal findings, with interpretation of the outcomes complicated by use of excessive quantities of product, inadequate sample sizes, failure to control nutrient intake and energy expenditure and variations in baseline blood lipids. More recent studies are of better quality, but fail to provide convincing evidence that ‘live’ fermented milk products have cholesterol-lowering efficacy in man. Future studies using probiotics should ensure adequate sample sizes sufficient to detect relatively small changes in blood cholesterol and should be conducted over longer periods of time. The recent introduction of the concept of prebiotics has directed attention towards the possibility that alterations in gut microflora induced by the fermentation of non-digestible components of the diet may also have the potential to influence systemic lipid metabolism. This possibility has been strengthened by the observation that in animals, dietary oligofructosaccharides cause suppression of hepatic triglyceride and VLDL synthesis, resulting in marked reductions in triglyceride, and to a lesser extent cholesterol, levels. Evidence for similar effects in humans is sparse and more studies are needed, particularly with respect to effects on postprandial triglyceride concentrations.

A growing number of food manufacturers in western Europe are beginning to explore the commercial opportunities for foodstuffs containing health-promoting microbial food supplements (probiotics) and health-promoting non-digestible food ingredients (prebiotics). A prebiotic is considered to affect the host beneficially by selectively stimulating the growth and/or activity of one or a limited number of naturally present or introduced bacterial species in the colon, also leading to a claimed improvement in host health. Increasingly, probiotics and prebiotics are used in combination, this being termed a synbiotic (Gibson & Roberfroid, 1995). Throughout European history, fermented milk products in particular have been considered beneficial to health, but only in recent years has there been scientific support for these beliefs. Issues considered important to the continuing development of this growing market are proof of safety, proof of efficacy, consumer education, market positioning, price and appropriate health claims strategies. Until recently, much of the innovation in the use of probiotics and prebiotics has been in the dairy cabinet, with an ever-growing number and range of ‘health-promoting’ yoghurts and yoghurt-type fermented milk being made available to the European consumer, a market which is currently estimated to be worth in excess of $US2 billion per annum (Hilliam et al. 1997). However, prebiotics are beginning to find increasing application outside the dairy sector, particularly in baked goods. A key driver behind the broadening application of prebiotics has been the pro-active stance taken by key prebiotic suppliers such as Beghin-Say, Orafti and Cosucra. To date, market activity in probiotic- and prebiotic-containing foods has centred around three health propositions, namely improving general gut health, lowering blood cholesterol and improving the body's natural defences.