The populations of dominant species within the human colonic microbiota can potentially be modified by dietary intake with consequences for health. Here we examined the influence of precisely controlled diets in 14 overweight men. Volunteers were provided successively with a control diet, diets high in resistant starch (RS) or non-starch polysaccharides (NSPs) and a reduced carbohydrate weight loss (WL) diet, over 10 weeks. Analysis of 16S rRNA sequences in stool samples of six volunteers detected 320 phylotypes (defined at 498% identity) of which 26, including 19 cultured species, each accounted for 41% of sequences. Although samples clustered more strongly by individual than by diet, time courses obtained by targeted qPCR revealed that 'blooms' in specific bacterial groups occurred rapidly after a dietary change. These were rapidly reversed by the subsequent diet. Relatives of Ruminococcus bromii (R-ruminococci) increased in most volunteers on the RS diet, accounting for a mean of 17% of total bacteria compared with 3.8% on the NSP diet, whereas the uncultured Oscillibacter group increased on the RS and WL diets. Relatives of Eubacterium rectale increased on RS (to mean 10.1%) but decreased, along with Collinsella aerofaciens, on WL. Inter-individual variation was marked, however, with 460% of RS remaining unfermented in two volunteers on the RS diet, compared to o4% in the other 12 volunteers; these two individuals also showed low numbers of R-ruminococci (o1%). Dietary non-digestible carbohydrate can produce marked changes in the gut microbiota, but these depend on the initial composition of an individual's gut microbiota.

Background: It has been proposed that the development of obesity in humans is influenced by the relative proportions of the two major phyla of bacteria (Bacteroidetes and Firmicutes) present in the large intestine. Objective: To examine the relationships between body mass index, weight loss and the major bacterial groups detected in fecal samples. Design: Major groups of fecal bacteria were monitored using fluorescent in situ hybridization (FISH) in obese and non-obese subjects under conditions of weight maintenance, and in obese male volunteers undergoing weight loss on two different reduced carbohydrate weight-loss diets given successively for 4 weeks each. Results: We detected no difference between obese and non-obese individuals in the proportion of Bacteroidetes measured in fecal samples, and no significant change in the percentage of Bacteroidetes in feces from obese subjects on weight loss diets. Significant diet-dependent reductions in a group of butyrate-producing Firmicutes were, however, detected in fecal samples from obese subjects on weight loss diets. Conclusions: Diets designed to achieve weight loss in obese subjects can significantly alter the species composition of the gut microbiota, but we find no evidence that the proportions of Bacteroidetes and Firmicutes among fecal bacteria have a function in human obesity.

Weight loss diets for humans that are based on a high intake of protein but low intake of fermentable carbohydrate may alter microbial activity and bacterial populations in the large intestine and thus impact on gut health. In this study, 19 healthy, obese (body mass index range, 30 to 42) volunteers were given in succession three different diets: maintenance (M) for 3 days (399 g carbohydrate/day) and then high protein/ medium (164 g/day) carbohydrate (HPMC) and high protein/low (24 g/day) carbohydrate (HPLC) each for 4 weeks. Stool samples were collected at the end of each dietary regimen. Total fecal short-chain fatty acids were 114 mM, 74 mM, and 56 mM (P < 0.001) for M, HPMC, and HPLC diets, respectively, and there was a disproportionate reduction in fecal butyrate (18 mM, 9 mM, and 4 mM, respectively; P < 0.001) with decreasing carbohydrate. Major groups of fecal bacteria were monitored using nine 16S rRNA-targeted fluorescence in situ hybridization probes, relative to counts obtained with the broad probe Eub338. No significant change was seen in the relative counts of the bacteroides (Bac303) (mean, 29.6%) or the clostridial cluster XIVa (Erec482, 23.3%), cluster IX (Prop853, 9.3%), or cluster IV (Fprau645, 11.6%; Rbro730 plus Rfla729, 9.3%) groups. In contrast, the Roseburia spp. and Eubacterium rectale subgroup of cluster XIVa (11%, 8%, and 3% for M, HPMC, and HPLC, respectively; P < 0.001) and bifidobacteria (4%, 2.1%, and 1.9%, respectively; P ‫؍‬ 0.026) decreased as carbohydrate intake decreased. The abundance of butyrate-producing bacteria related to Roseburia spp. and E. rectale correlated well with the decline in fecal butyrate.Low-carbohydrate diets in which carbohydrates are largely replaced by an increased proportion of dietary protein and/or fat have proved a popular weight loss strategy for humans (1,11,36). The potential health impacts associated with increased protein (34) or fat (27) intake have been controversial among nutritionists, but less attention has been paid to the consequences of low carbohydrate supply. It has been argued that a lower carbohydrate supply may be advantageous in ameliorating insulin insensitivity (9), although this may not occur with low-glycemic-index foods (21). Furthermore, in the context of overall dietary advice consideration also needs to be given to the role of carbohydrates in maintenance of gut health and function.Dietary carbohydrates include structural polysaccharides and oligosaccharides of plant origin plus resistant starch (14, 40) that are not digested in the small intestine and, instead, enter the colon. Here they can be fermented by the microbiota of the large intestine and normally provide the main energy supply to support microbial growth in the colon. Microbial fermentation may release as much as 10% of the dietary energy, mainly in the form of short-chain fatty acids (SCFA) that also act as energy sources for host cells (46). For example, butyrate is the preferred energy source for the epithelial cells of the colon (29, 50). Furthermore, butyr...

Dietary carbohydrates have the potential to influence diverse functional groups of bacteria within the human large intestine. Of 12 Bifidobacterium strains of human gut origin from seven species tested, four grew in pure culture on starch and nine on fructo-oligosaccharides. The potential for metabolic cross-feeding between Bifidobacterium adolescentis and lactate-utilizing, butyrate-producing Firmicute bacteria related to Eubacterium hallii and Anaerostipes caccae was investigated in vitro. E. hallii L2-7 and A. caccae L1-92 failed to grow on starch in pure culture, but in coculture with B. adolescentis L2-32 butyrate was formed, indicating cross-feeding of metabolites to the lactate utilizers. Studies with [13 C]lactate confirmed carbon flow from lactate, via acetyl coenzyme A, to butyrate both in pure cultures of E. hallii and in cocultures with B. adolescentis. Similar results were obtained in cocultures involving B. adolescentis DSM 20083 with fructo-oligosaccharides as the substrate. Butyrate formation was also stimulated, however, in cocultures of B. adolescentis L2-32 grown on starch or fructo-oligosaccharides with Roseburia sp. strain A2-183, which produces butyrate but does not utilize lactate. This is probably a consequence of the release by B. adolescentis of oligosaccharides that are available to Roseburia sp. strain A2-183. We conclude that two distinct mechanisms of metabolic cross-feeding between B. adolescentis and butyrate-forming bacteria may operate in gut ecosystems, one due to consumption of fermentation end products (lactate and acetate) and the other due to cross-feeding of partial breakdown products from complex substrates.Microbial growth and metabolism in the human large intestine depend to a large extent on the supply of dietary carbohydrates that resist digestion in the upper gut. The fermentation of these compounds, which include plant cell wall polysaccharides and some storage polysaccharides and oligosaccharides, has a major influence on health (9, 20, 43). Indeed, specific carbohydrates are now widely used as functional foods and as prebiotics, based on the concept that they stimulate particular gut bacteria that promote gut health (18) and, at the same time, reduce the populations of nonutilizing bacteria through competition. Inulin and fructo-oligosaccharides (FOS), for example, were originally proposed as prebiotics that selectively stimulate bifidobacteria. While there is evidence that this occurs (11,19,26,45), other studies, using molecular techniques, have revealed that a variety of other bacterial groups, including clostridium-related species, also respond to inulin or FOS supplied as prebiotics in either fermentor experiments or animal models (13, 25).Among the possible explanations for this diversity in response to prebiotics is that complex gut microbial communities involve extensive metabolic interactions (10, 46). Metabolic products produced from dietary prebiotics by one bacterial species may then provide substrates to support growth of other populations, and this i...

After 4 wk, weight-loss diets that were high in protein but reduced in total carbohydrates and fiber resulted in a significant decrease in fecal cancer-protective metabolites and increased concentrations of hazardous metabolites. Long-term adherence to such diets may increase risk of colonic disease.