Lipid Lowering with Soluble Dietary Fiber

The involvement of the gut microbiota in metabolic disorders, and the ability of whole grains to affect both host metabolism and gut microbial ecology, suggest that some benefits of whole grains are mediated through their effects on the gut microbiome. Nutritional studies that assess the effect of whole grains on both the gut microbiome and human physiology are needed. We conducted a randomized cross-over trial with four-week treatments in which 28 healthy humans consumed a daily dose of 60 g of whole-grain barley (WGB), brown rice (BR), or an equal mixture of the two (BR+WGB), and characterized their impact on fecal microbial ecology and blood markers of inflammation, glucose and lipid metabolism. All treatments increased microbial diversity, the Firmicutes/Bacteroidetes ratio, and the abundance of the genus Blautia in fecal samples. The inclusion of WGB enriched the genera Roseburia, Bifidobacterium and Dialister, and the species Eubacterium rectale, Roseburia faecis and Roseburia intestinalis. Whole grains, and especially the BR+WGB treatment, reduced plasma interleukin-6 (IL-6) and peak postprandial glucose. Shifts in the abundance of Eubacterium rectale were associated with changes in the glucose and insulin postprandial response. Interestingly, subjects with greater improvements in IL-6 levels harbored significantly higher proportions of Dialister and lower abundance of Coriobacteriaceae. In conclusion, this study revealed that a short-term intake of whole grains induced compositional alterations of the gut microbiota that coincided with improvements in host physiological measures related to metabolic dysfunctions in humans.

Over the last 20 years there has been an increasing interest in the influence of the gastrointestinal tract on appetite regulation. Much of the focus has been on the neuronal and hormonal relationship between the gastrointestinal tract and the brain. There is now mounting evidence that the colonic microbiota and their metabolic activity have a significant role in energy homeostasis. The supply of substrate to the colonic microbiota has a major impact on the microbial population and the metabolites they produce, particularly short chain fatty acids (SCFAs). SCFAs are produced when non-digestible carbohydrates, namely dietary fibres and resistant starch, undergo fermentation by the colonic microbiota. Both the consumption of fermentable carbohydrates and the administration of SCFAs have been reported to result in a wide range of health benefits including improvements in body composition, glucose homeostasis, blood lipid profiles and reduced body weight and colon cancer risk. However, published studies tend to report the effects that fermentable carbohydrates and SCFAs have on specific tissues and metabolic processes, and fail to explain how these local effects translate into systemic effects and the mitigation of disease risk. Moreover, studies tend to investigate SCFAs collectively and neglect to report the effects associated with individual SCFAs. Here, we bring together the recent evidence and suggest an overarching model for the effects of SCFAs on one of their beneficial aspects: appetite regulation and energy homeostasis.

Summary Background Application of modern rapid DNA sequencing technology has transformed our understanding of the gut microbiota. Diet, in particular plant‐based fibre, appears critical in influencing the composition and metabolic activity of the microbiome, determining levels of short‐chain fatty acids (SCFAs) important for intestinal health. Aim To assess current epidemiological, experimental and clinical evidence of how long‐term and short‐term alterations in dietary fibre intake impact on the microbiome and metabolome. Methods A Medline search including items ‘intestinal microbiota’, ‘nutrition’, ‘diet’, ‘dietary fibre’, ‘SCFAs’ and ‘prebiotic effect’ was performed. Results Studies found evidence of fibre‐influenced differences in the microbiome and metabolome as a consequence of habitual diet, and of long‐term or short‐term intervention (in both animals and humans). Conclusions Agrarian diets high in fruit/legume fibre are associated with greater microbial diversity and a predominance of Prevotella over Bacteroides. ‘Western’‐style diets, high in fat/sugar, low in fibre, decrease beneficial Firmicutes that metabolise dietary plant‐derived polysaccharides to SCFAs and increase mucosa‐associated Proteobacteria (including enteric pathogens). Short‐term diets can also have major effects, particularly those exclusively animal‐based, and those high‐protein, low‐fermentable carbohydrate/fibre ‘weight‐loss’ diets, increasing the abundance of Bacteroides and lowering Firmicutes, with long‐term adherence to such diets likely increasing risk of colonic disease. Interventions to prevent intestinal inflammation may be achieved with fermentable prebiotic fibres that enhance beneficial Bifidobacteria or with soluble fibres that block bacterial–epithelial adherence (contrabiotics). These mechanisms may explain many of the differences in microbiota associated with long‐term ingestion of a diet rich in fruit and vegetable fibre.