Obesity and the gut microbiota: does up-regulating colonic fermentation protect against obesity and metabolic disease?

Gut microbial adaptation to dietary consumption of fructose, artificial sweeteners and sugar alcohols: implications for host-microbe interactions contributing to obesity

The Western diet, comprised of highly refined carbohydrates and fat but reduced complex plant polysaccharides, has been attributed to the prevalence of obesity. A concomitant rise in the consumption of fructose and sugar substitutes such as sugar alcohols, artificial sweeteners, even rare sugars, has mirrored this trend, as both probable contributor and solution to the epidemic. Acknowledgement of the gut microbiota as a factor involved in obesity has sparked much controversy as to the cause and consequence of this relationship. Dietary intakes are a known modulator of gut microbial phylogeny and metabolic activity, frequently exploited to stimulate beneficial bacteria, promoting health benefits. Comparably little research exists on the impact of 'unconscious' dietary modulation on the resident commensal community mediated by increased fructose and sugar substitute consumption. This review highlights mechanisms of potential host and gut microbial fructose and sugar substitute metabolism. Evidence is presented suggesting these sugar compounds, particularly fructose, condition the microbiota, resulting in acquisition of a westernized microbiome with altered metabolic capacity. Disturbances in host-microbe interactions resulting from fructose consumption are also explored.

Production, health aspects and potential food uses of dairy prebiotic galactooligosaccharides

Galactooligosaccharides are sugars composed of 3-10 molecules of galactose and glucose via a transgalactosylation reaction mediated by the enzyme β-galactosidase. Prebiotics are non-digestible food ingredients that pass through the upper digestive system relatively intact and ferment in the lower colon, producing short-chain fatty acids that support the growth of supplemented or indigenous colonic microbiota. Galactooligosaccharides and other prebiotic ingredients are increasingly being recognized as useful dietary tools for the modulation of the colonic microflora toward a healthy balance. Galactooligosaccharides compare well to other oligosaccharides in terms of their prebiotic, immunomodulation, and functional properties in foods. This review elucidates the galactooligosaccharide production process from refined lactose and/or cheese whey permeates, galactooligosaccharide market share and economic value, their health properties, and potential food applications.

Fiber intake and total and cause-specific mortality in the European Prospective Investigation into Cancer and Nutrition cohort

BACKGROUND

Previous studies have shown that high fiber intake is associated with lower mortality. However, little is known about the association of dietary fiber with specific causes of death other than cardiovascular disease (CVD).

OBJECTIVE

The aim of this study was to assess the relation between fiber intake, mortality, and cause-specific mortality in a large European prospective study of 452,717 men and women.

DESIGN

HRs and 95% CIs were estimated by using Cox proportional hazards models, stratified by age, sex, and center and adjusted for education, smoking, alcohol consumption, BMI, physical activity, total energy intake, and, in women, ever use of menopausal hormone therapy.

RESULTS

During a mean follow-up of 12.7 y, a total of 23,582 deaths were recorded. Fiber intake was inversely associated with total mortality (HR(per 10-g/d increase): 0.90; 95% CI: 0.88, 0.92); with mortality from circulatory (HR(per 10-g/d increase): 0.90 and 0.88 for men and women, respectively), digestive (HR: 0.61 and 0.64), respiratory (HR: 0.77 and 0.62), and non-CVD noncancer inflammatory (HR: 0.85 and 0.80) diseases; and with smoking-related cancers (HR: 0.86 and 0.89) but not with non-smoking-related cancers (HR: 1.05 and 0.97). The associations were more evident for fiber from cereals and vegetables than from fruit. The associations were similar across BMI and physical activity categories but were stronger in smokers and participants who consumed >18 g alcohol/d.

CONCLUSIONS

Higher fiber intake is associated with lower mortality, particularly from circulatory, digestive, and non-CVD noncancer inflammatory diseases. Our results support current recommendations of high dietary fiber intake for health maintenance.

Surfactant protein d deficiency in mice is associated with hyperphagia, altered fat deposition, insulin resistance, and increased basal endotoxemia

Pulmonary surfactant protein D (SP-D) is a host defence lectin of the innate immune system that enhances clearance of pathogens and modulates inflammatory responses. Recently it has been found that systemic SP-D is associated with metabolic disturbances and that SP-D deficient mice are mildly obese. However, the mechanism behind SP-D's role in energy metabolism is not known.

Here we report that SP-D deficient mice had significantly higher ad libitum energy intake compared to wild-type mice and unchanged energy expenditure. This resulted in accumulation but also redistribution of fat tissue. Blood pressure was unchanged. The change in energy intake was unrelated to the basal levels of hypothalamic Pro-opiomelanocortin (POMC) and Agouti-related peptide (AgRP) gene expression. Neither short time systemic, nor intracereberoventricular SP-D treatment altered the hypothalamic signalling or body weight accumulation.

In ad libitum fed animals, serum leptin, insulin, and glucose were significantly increased in mice deficient in SP-D, and indicative of insulin resistance. However, restricted diets eliminated all metabolic differences except the distribution of body fat. SP-D deficiency was further associated with elevated levels of systemic bacterial lipopolysaccharide.

In conclusion, our findings suggest that lack of SP-D mediates modulation of food intake not directly involving hypothalamic regulatory pathways. The resulting accumulation of adipose tissue was associated with insulin resistance. The data suggest SP-D as a regulator of energy intake and body composition and an inhibitor of metabolic endotoxemia. SP-D may play a causal role at the crossroads of inflammation, obesity, and insulin resistance.

The type and quantity of dietary fat and carbohydrate alter faecal microbiome and short-chain fatty acid excretion in a metabolic syndrome 'at-risk' population

INTRODUCTION AND OBJECTIVES

An obese-type human microbiota with an increased Firmicutes:Bacteroidetes ratio has been described that may link the gut microbiome with obesity and metabolic syndrome (MetS) development. Dietary fat and carbohydrate are modifiable risk factors that may impact on MetS by altering the human microbiome composition. We determined the effect of the amount and type of dietary fat and carbohydrate on faecal bacteria and short chain fatty acid (SCFA) concentrations in people 'at risk' of MetS.

DESIGN

A total of 88 subjects at increased MetS risk were fed a high saturated fat diet (HS) for 4 weeks (baseline), then randomised onto one of the five experimental diets for 24 weeks: HS; high monounsaturated fat (MUFA)/high glycemic index (GI) (HM/HGI); high MUFA/low GI (HM/LGI); high carbohydrate (CHO)/high GI (HC/HGI); and high CHO/low GI (HC/LGI). Dietary intakes, MetS biomarkers, faecal bacteriology and SCFA concentrations were monitored.

RESULTS

High MUFA diets did not affect individual bacterial population numbers but reduced total bacteria and plasma total and LDL-cholesterol. The low fat, HC diets increased faecal Bifidobacterium (P=0.005, for HC/HGI; P=0.052, for HC/LGI) and reduced fasting glucose and cholesterol compared to baseline. HC/HGI also increased faecal Bacteroides (P=0.038), whereas HC/LGI and HS increased Faecalibacterium prausnitzii (P=0.022 for HC/HGI and P=0.018, for HS). Importantly, changes in faecal Bacteroides numbers correlated inversely with body weight (r=-0.64). A total bacteria reduction was observed for high fat diets HM/HGI and HM/LGI (P=0.023 and P=0.005, respectively) and HS increased faecal SCFA concentrations (P<0.01).

CONCLUSION

This study provides new evidence from a large-scale dietary intervention study that HC diets, irrespective of GI, can modulate human faecal saccharolytic bacteria, including bacteroides and bifidobacteria. Conversely, high fat diets reduced bacterial numbers, and in the HS diet, increased excretion of SCFA, which may suggest a compensatory mechanism to eliminate excess dietary energy.

Butyrate activates the cAMP-protein kinase A-cAMP response element-binding protein signaling pathway in Caco-2 cells

Butyrate is a major SCFA produced by microbial fermentation of dietary fiber in the gastrointestinal tract. Butyrate is widely thought to mediate the benefits of fiber and resistant starch consumption to colon health in humans. Besides serving as a substrate for energy production, butyrate has many regulatory effects in animals. Little is known about the signaling mechanisms underlying the regulatory effects of butyrate and other SCFA. In this study, we determined whether butyrate can activate cAMP-protein kinase A (PKA)- cAMP response element (CRE)-binding protein (CREB) signaling in Caco-2 cells, a model of intestinal epithelial cells. Butyrate promoted luciferase expression from a CRE-reporter construct, induced phosphorylation of CREB, increased the activity of PKA, and elevated the levels of cAMP in Caco-2 cells. These data suggest that butyrate activates cAMP-PKA-CREB signaling in Caco-2 cells. Butyrate, however, had no effect on the activities of adenylyl cyclase (AC) and phosphodiesterase (PDE), two enzymes that determine the production and degradation of intracellular cAMP, respectively. Because the activities of AC and PDE are primarily regulated by G protein-coupled receptor (GPR)-mediated intracellular signaling, lack of an effect of butyrate on these two enzymes suggests that butyrate does not activate cAMP-PKA-CREB signaling through GPR. Butyrate-treated Caco-2 cells had greater concentrations of ATP than untreated cells. Because ATP is the substrate for cAMP production, this difference suggests that butyrate may activate cAMP-PKA-CREB signaling in Caco-2 cells through increased ATP production. Overall, this study raises the possibility that some of the regulatory effects of butyrate in animals, including those on the colonocytes, may be mediated by the cAMP-PKA-CREB signaling pathway at the cellular level.

Gut microbiota and sirtuins in obesity-related inflammation and bowel dysfunction

Obesity is a chronic disease characterized by persistent low-grade inflammation with alterations in gut motility. Motor abnormalities suggest that obesity has effects on the enteric nervous system (ENS), which controls virtually all gut functions. Recent studies have revealed that the gut microbiota can affect obesity and increase inflammatory tone by modulating mucosal barrier function. Furthermore, the observation that inflammatory conditions influence the excitability of enteric neurons may add to the gut dysfunction in obesity. In this article, we discuss recent advances in understanding the role of gut microbiota and inflammation in the pathogenesis of obesity and obesity-related gastrointestinal dysfunction. The potential contribution of sirtuins in protecting or regulating the circuitry of the ENS under inflamed states is also considered.