The proportion of bifidobacteria in the colonic microbiota drops following weaning and the introduction of solid food. In adults, they account for 1-5% of the total bacteria in feces. Although they form a slightly higher proportion of total bacteria in the caecum (117-121), the total numbers of bifidobacteria per gram of intestinal contents increases approximately 100-fold with passage from the caecum to the colon. In the feces of healthy adults bifidobacteria are found in numbers generally in the order of 108-1010 cells per gram. (10,122-125). While these figures represent the typical Bifidobacterium cell density, a proportion of healthy adults harbor considerably lower numbers of Bifidobacteria in their gut (by several orders of magnitude) without any discernable adverse effects (125-128).

It is yet to be determined how the total number of bifidobacteria within a stable microbiota influences the long-term health of the human host. In individuals with naturally low levels of bifidobacteria, other micro-organisms with similar functionalities may occupy a similar niche and fulfill a similar role in the intestinal tract.

It is clear from the number of human feeding studies reported to date that consumption of prebiotics can increase the numbers of bifidobacteria in the colon of adults. For NDOs, consumption of typically 10-15 g/day can induce 10- to 100-fold increases in Bifidobacterium numbers (129,130). However, a range of factors may influence the size of any increase in Bifidobacterium numbers, the most important being the initial size of the population within the intestinal tract. In comparing different trials conducted using fructo-oligosaccharides, Rao (130) observed that the size of the bifidogenic response was inversely proportional to the size of the initial Bifidobacterium population rather than showing a strong dose response. In individuals colonized with an already large population of bifidobacteria, prebiotic consumption appears not to increase the total Bifidobacterium population size further.

Bif. adolescentis, Bif.catenulatum/pseudocatenulatum, Bif. bifidum, and Bif. longum are the most frequently reported Bifidobacterium species in the intestines of adults, with considerable variation between individuals (121,125,126,131,132). To date, no clear rationale for promoting one species of Bifidobacterium over others has emerged. Indeed, it may be quite difficult to achieve major shifts within the population dynamic of bifidobacteria at the species level even if this was desirable. In one study to investigate this, feeding 8 g/day of galacto-oligosaccharides to healthy adult volunteers did not result in marked changes in the composition of their intestinal bifidobacteria! populations at the species level (133-135). Similarly, despite observing increases in total Bifidobacterium numbers, Harmsen et al. (136) also saw no changes in the species composition of bifidobacteria in a study where adult volunteers were fed 9 g/day of inulin. The species composition within intestinal bifidobacteria has been shown to remain fairly stable over many months in adults (10,121,125,137,138) suggesting that day-to-day fluctuations in diet have little impact on the species dynamic.

Even if they do not significantly alter the bacterial population dynamics in all individuals, prebiotics may still be effective in providing benefits to the consumer if they beneficially modulate the metabolic activity of the microbiota. Hypothetical examples might be increased production of SCFA or vitamins that benefit the health of the colonic epithelium, or synthesis of antagonistic metabolites that augment colonization resistance against pathogens. Tannock et al. (139) used molecular techniques to investigate both phylogenetic (DNA-DGGE) and metabolic (RNA-DGGE) changes in the intestinal microbiota induced by galacto- or fructo-oligosaccharides. While no discernable changes were observed in bacterial communities using DNA-DGGE (nor increases in total Bifidobacterium numbers by traditional culturing), RNA-DGGE analysis revealed that the prebiotics increased the activity of some bacterial groups including bifidobacteria. A current research need is to identify metabolic activities of the microbiota that affect the health of the host (positively or negatively) and to demonstrate that these can be specifically modulated with prebiotics in situ.

Prebiotics in the Treatment of Inflammatory Bowel Disease

A genetic predisposition to develop an over-zealous inflammatory immune response to components of the intestinal microbiota has been implicated in the etiology of IBD (140). Elimination of specific bacterial antigens, immunomodulation, and trophic effects of SCFA on the intestinal epithelium have all been proposed as mechanisms by which prebiotics could alleviate IBD (Fig. 2). The size of the intestinal Bifidobacterium population has been shown to be relatively small (141,142) in subjects afflicted with IBD, although cause and effect links between disease and a diminished intestinal Bifidobacterium population remain to be established. Interventions with prebiotics have shown some benefit in ameliorating inflammation in both animal and human feeding trials. Using differing rodent models of IBD, a number of research groups have demonstrated amelioration of inflammation using prebiotic interventions. These include studies with lactulose (143), inulin (140) and fructo-oligosaccharides (144). In contrast, Holma et al. (145) observed no reduction in inflammation by intervention with galacto-oligosacchar-ides despite an increase in Bifidobacterium numbers.

In addition to NDOs, larger polysaccharides with prebiotic potential have also been shown to have promise in the treatment of IBD. Resistant starch was demonstrated to ameliorate IBD in rodent models of disease (146,147), and in one study (147) outperformed a diet with an equivalent dose of fructo-oligosaccharides. Additionally, an arabinoxylan-rich germinated barley product has been reported to have benefits in the treatment of active IBD. This ingredient was shown to induce the proliferation of bifidobacteria in the human intestine (148), consistent with other in vitro and animal studies of the fermentation of arabinoxylans by intestinal bacteria (73,74,149). In rodent models of IBD, and in two small, non-blinded human studies of subjects with ulcerative colitis, consumption of the germinated barley product reduced inflammation (150-153).

These results suggest that prebiotics have at least some potential to benefit IBD sufferers. However, convincing evidence of a consistent clinical benefit in the treatment of IBD remains to be demonstrated in large, randomized, double-blind, placebo-controlled trials.

Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

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