Whole plant foods, including fruit, vegetables, and whole grain cereals, protect against chronic human diseases such as heart disease and cancer, with fiber and polyphenols thought to contribute significantly. These bioactive food components interact with the gut microbiota, with gut bacteria modifying polyphenol bioavailability and activity, and with fiber, constituting the main energy source for colonic fermentation. This paper discusses the consequences of increasing the consumption of whole plant foods on the gut microbiota and subsequent implications for human health. In humans, whole grain cereals can modify fecal bacterial profiles, increasing relative numbers of bifidobacteria and lactobacilli. Polyphenol-rich chocolate and certain fruits have also been shown to increase fecal bifidobacteria. The recent FLAVURS study provides novel information on the impact of high fruit and vegetable diets on the gut microbiota. Increasing whole plant food consumption appears to up-regulate beneficial commensal bacteria and may contribute toward the health effects of these foods.

Obesity is now considered a major public health concern globally as it predisposes to a number of chronic human diseases. Most developed countries have experienced a dramatic and significant rise in obesity since the 1980s, with obesity apparently accompanying, hand in hand, the adoption of "Western"-style diets and low-energy expenditure lifestyles around the world. Recent studies report an aberrant gut microbiota in obese subjects and that gut microbial metabolic activities, especially carbohydrate fermentation and bile acid metabolism, can impact on a number of mammalian physiological functions linked to obesity. The aim of this review is to present the evidence for a characteristic "obese-type" gut microbiota and to discuss studies linking microbial metabolic activities with mammalian regulation of lipid and glucose metabolism, thermogenesis, satiety, and chronic systemic inflammation. We focus in particular on short-chain fatty acids (SCFA) produced upon fiber fermentation in the colon. Although SCFA are reported to be elevated in the feces of obese individuals, they are also, in contradiction, identified as key metabolic regulators of the physiological checks and controls mammals rely upon to regulate energy metabolism. Most studies suggest that the gut microbiota differs in composition between lean and obese individuals and that diet, especially the high-fat low-fiber Western-style diet, dramatically impacts on the gut microbiota. There is currently no consensus as to whether the gut microbiota plays a causative role in obesity or is modulated in response to the obese state itself or the diet in obesity. Further studies, especially on the regulatory role of SCFA in human energy homeostasis, are needed to clarify the physiological consequences of an "obese-style" microbiota and any putative dietary modulation of associated disease risk.

Sugars (lactose, glucose, and galactose), nonvolatile acids (citric, orotic, piruvic, lactic, ossalic, and hippuric), some free fatty acids (formic, acetic, propionic, butyric, isobutyric, valeric, and isovaleric), diacetyl, and acetoin were separated on an Aminex HPX-87H column using a simple isocratic HPLC method and identified by retention times with ultraviolet and refractive index detectors. With the proposed technique it is possible to evaluate the development of microbial fermentations and, at the same time, degree of cheese ripening.

Apples are a rich source of polyphenols and fiber. A major proportion of apple polyphenols escape absorption in the small intestine and together with non-digestible polysaccharides reach the colon, where they can serve as substrates for bacterial fermentation. Animal studies suggest a synergistic interaction between apple polyphenols and the soluble fiber pectin; however, the effects of whole apples on human gut microbiota are less extensively studied. Three commercial apple varieties—Renetta Canada, Golden Delicious and Pink Lady—were digested and fermented in vitro using a batch culture colonic model (pH 5.5–6.0, 37 °C) inoculated with feces from three healthy donors. Inulin and cellulose were used as a readily and a poorly fermentable plant fiber, respectively. Fecal microbiota composition was measured by 16S rRNA gene Illumina MiSeq sequencing (V3-V4 region) and Fluorescence in Situ Hybridization. Short chain fatty acids (SCFAs) and polyphenol microbial metabolites were determined. The three apple varieties significantly changed bacterial diversity, increased Actinobacteria relative abundance, acetate, propionate and total SCFAs (p < 0.05). Renetta Canada and Golden Delicious significantly decreased Bacteroidetes abundance and increased Proteobacteria proportion and bifidobacteria population (p < 0.05). Renetta Canada also increased Faecalibacterium prausnitzii, butyrate levels and polyphenol microbial metabolites (p < 0.05). Together, these data suggest that apples, particularly Renetta Canada, can induce substantial changes in microbiota composition and metabolic activity in vitro, which could be associated with potential benefits to human health. Human intervention studies are necessary to confirm these data and potential beneficial effects.

PurposeOlive pomace is a major waste product of olive oil production but remains rich in polyphenols and fibres. We measured the potential of an olive pomace-enriched biscuit formulation delivering 17.1 ± 4.01 mg/100 g of hydroxytyrosol and its derivatives, to modulate the composition and metabolic activity of the human gut microbiota.MethodsIn a double-blind, controlled parallel dietary intervention 62 otherwise healthy hypercholesterolemic (total plasma cholesterol 180–240 mg/dl) subjects were randomly assigned to eat 90 g of olive pomace-enriched biscuit (olive-enriched product, OEP) or an isoenergetic control (CTRL) for 8 weeks. Fasted blood samples, 24-h urine and faecal samples were collected before and after dietary intervention for measurement of microbiota, metabolites and clinical parameters.ResultsConsumption of OEP biscuits did not impact on the diversity of the faecal microbiota and there was no statistically significant effect on CVD markers. A trend towards reduced oxidized LDL cholesterol following OEP ingestion was observed. At the genus level lactobacilli and Ruminococcus were reduced in OEP compared to CTRL biscuits. A trend towards increased bifidobacteria abundance was observed after OEP ingestion in 16S rRNA profiles, by fluorescent in situ hybridization and by qPCR. Targeted LC–MS revealed significant increases phenolic acid concentrations in 24-h urine following OEP ingestion and 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid, derivatives of hydroxytyrosol, were elevated in blood. A sex effect was apparent in urine small phenolic acid concentrations, and this sex effect was mirrored by statistically significant differences in relative abundances of faecal bacteria between men and women.ConclusionIngestion of OEP biscuits led to a significant increase in the metabolic output of the gut microbiota with an apparent sex effect possibly linked to differences in microbiota makeup. Increased levels of homovanillic acid and DOPAC, thought to be involved in reducing oxidative LDL cholesterol, were observed upon OEP ingestion. However, OEP did not induce statistically significant changes in either ox-LDL or urinary isoprostane in this study.