The human gut microbiota

The human gut microbiota is a diverse collection of microbes inhabiting the gastrointestinal tract, encompassing up to 1000 different species and with a gradient of concentration going from about 10^2–3 bacteria/gram of content in the stomach to 10⁵ bacteria/gram in the duodenum and jejunum, 10⁸ bacteria/gram in the ileum and 10^11–12 bacteria/gram in the large intestine.

DNA-based tools for measuring the composition and function of gut microbiota (i.e. next-generation sequencing, quantitative polymerase chain reaction, fluorescent in situ hybridisation) have largely allowed us to overcome the technical limitations of direct cultivation of microbes and provided large amounts of information on gut microbial profiles and activities⁽Reference Lepage, Leclerc and Joossens¹⁾. The main resident bacterial populations in the human gut belong to the phyla Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria and Verrucomicrobia. Gut microbiota composition has also been described in terms of a limited number of ‘enterotypes’, according to which the human gastrointestinal tract is colonised by loose groups of co-occuring microbes centred around the genera Prevotella, Bacteroides and/or Ruminococcus ⁽Reference Arumugam, Raes and Pelletier²⁾. Despite the relatively few bacterial phyla, the gut microbiota is a highly complex ecosystem, with each bacterial species and even strain being capable of carrying out specific functions within the diverse microbial community. Resident bacteria colonise the human body in a highly host-specific manner, meaning that there is elevated inter-individual variation in terms of species and strain composition, abundance and metabolic output⁽Reference Arumugam, Raes and Pelletier²⁾. This specificity is also determined by host genotype, age, sex and health state⁽Reference Bolnick, Snowberg and Hirsch³⁾. Many disease states are characterised by alterations in microbiota composition and metabolic function⁽Reference Ottman, Smidt and de Vos⁴⁾. Nevertheless, gut microbes are deeply affected by the host's lifestyle. The environmental input represented by physical activity⁽Reference O'Sullivan, Cronin and Clarke⁵⁾, diet⁽Reference Rowland, Gibson and Heinken⁶⁾ and circadian rhythm⁽Reference Marcinkevicius and Shirasu-Hiza^7, Reference Deaver, Eum and Toborek⁸⁾ has been shown to drive changes in microbial profiles and activities, such as production of fermentation end-products such as SCFA, biotransformation of dietary compounds (i.e. plant phenolics) and xenobiotics, conversion of bile acids (BA) and modulation of circulating BA pool⁽Reference Long, Gahan and Joyce⁹⁾ (Fig. 1).

Fig. 1. SCFA regulation of metabolic health through diet:gut microbiota interaction. Healthy habits, including diet (i.e. Diet high in fibre and plant bioactives), physical exercise and appropriate sleep/wake alternation directly influence microbial fermentation and other microbial metabolic activities. Production of SCFA, biotransformation of plant phenolics, conversion of bile acids and metabolism of xenobiotics directly influence systemic metabolism, including central appetite regulation (i.e. through secretion of intestinal hormones GLP-1 and PYY), lipogenesis and adipogenesis, and modulate systemic and hepatic inflammation.

The gut microbiota contributes to human physiology mainly through immune and metabolic functions. Resident bacteria constitute an effective barrier from pathogen invasion through direct competition for nutritional substrate and ecological niches as well as the production of antimicrobial substances. Gastrointestinal microbes contribute to immune system maturation, development of immune tolerance mechanism and maintenance of epithelial barrier integrity⁽Reference Rizzetto, Fava and Tuohy¹⁰⁾. Gut bacteria provide the human organism with additional metabolic enzymes, which allow fermentation and biotransformation of dietary compounds, which would otherwise remain undigested and generate metabolites that exert their physiological role systemically⁽Reference Marchesi, Adams and Fava^11, Reference Flint, Scott and Louis¹²⁾. Saccharolytic fermentation takes place in the proximal part of the colon, while proteolytic fermentation appears to be located in the distal region of the large intestine. The result of carbohydrate fermentation is the production of beneficial SCFA, while, conversely, proteolytic fermentation generates amines, ammonia, N-nitroso compounds, sulphides, indoles and other toxic or potentially carcinogenic compounds⁽Reference Rowland, Gibson and Heinken⁶⁾.

The main SCFA produced in the human colon are acetate, propionate and butyrate, in a molar ratio about 3:2:2. Acetate is the main SCFA produced by gut microbes and is an essential nutrient for the growth of many bacteria. Once it is produced acetate is absorbed and enters systemic circulation to the peripheral tissues, where it serves as a substrate for cholesterol metabolism and lipogenesis, as well as satiety regulation. Recently, acetate has been shown to play an important role in the browning of white adipose tissue, thermogenesis and protection from obesity⁽Reference Weitkunat, Stuhlmann and Postel^13, Reference Jocken, González Hernández and Hoebers¹⁴⁾. Propionate is absorbed through the portal vein to the liver and is converted into glucose through hepatic gluconeogenesis. It also plays an important role in regulating the inflammatory and endocrine output of adipose tissue, again protecting from obesity and metabolic disease⁽Reference Al-Lahham, Roelofsen and Priebe¹⁵⁾. Butyrate is the main source of energy for enterocytes and it has an anti-cancer activity through the promotion of apoptosis and repression of proliferation by inhibition of histone de-acetylation. Butyrate is also involved in maintaining the integrity of the gut wall, protecting against ‘leaky gut’ and associated inflammation and regulation of T-cell function and inflammatory response⁽Reference Kim, Park and Kim¹⁶⁾.

Biotransformation of dietary plant polyphenols by the gut microbiota through hydrolysis, dehydroxilation, demethylation and other enzymatic activities generates polyphenols catabolites with increased bioavailability, which are then absorbed through the intestinal mucosa and carry out biological effects in several districts of the organism⁽Reference Ozdal, Sela and Xiao¹⁷⁾. Small phenolic compounds originated from polyphenol metabolism by gut bacteria were shown to have an anti-inflammatory, detoxifying, anti-cancer and anti-protein glycation activity in vitro and in laboratory animals⁽Reference Del Rio, Rodriguez-Mateos and Spencer¹⁸⁾.

The gut microbiota explicates its metabolic function also through conversion of primary BA to secondary and tertiary BA, as a result of enzymatic activities such as bile salt hydrolase and 7-α-dehydroxilase⁽Reference Ridlon, Harris and Bhowmik¹⁹⁾. Circulating BA mediate their systemic effect through stimulation of BA receptors (i.e. nuclear receptors, such as farnesoid X receptor (FXR), vitamin D receptor, pregnane X receptor and G-protein coupled membrane receptors, such as the G protein-coupled BA receptor GP-BAR1/TGR5, muscarinic receptor, Formyl-peptide receptors receptor) in several tissues of the body, including smooth and striated muscle, heart, macrophages, neurons and adipose tissue. BA stimulated cell signalling involves not only the entherohepatic axis (i.e. regulation of BA synthesis and turnover) but also lipid metabolism, energy expenditure and glucose homeostasis. Different BA were previously shown to differentially impact on the earlier mentioned physiological effects⁽Reference Joyce and Gahan²⁰⁾. The pool of circulating BA strongly depends on gut microbiota activity and dietary input can apparently influence both hepatic BA neosynthesis and intestinal production of secondary and tertiary BA⁽Reference O'Keefe, Li and Lahti²¹⁾.

Dietary compounds deeply affect the growth and metabolism of gut bacteria, since fermentation of nutrients is one core function of the human gut microbiota. The study of the contribution of microbes to systemic metabolic pathways has been recently made possible thanks to the employment of integrating ‘-omics’ technologies, such as metabolomics, in addition to microbiota metagenomics analysis⁽Reference Nicholson, Holmes and Kinross²²⁾. Particularly through measuring the gut microbiota response to a definite dietary component or food in terms of generated microbially-derived metabolites, as well as concomitant changes in bacterial composition and systemic clinical parameters, it is now possible to gain information regarding bacterial functions in the human organism.

Gut microbiota and obesity

The concept of microbial dysbiosis, as firstly introduced to explain the shifts in gastrointestinal bacterial makeup and in relation to intestinal dysfunction characteristic of inflammatory bowel disease, has more recently been extended to describe the differences in gut microbiota observed between healthy individuals and individuals affected by different chronic extra-intestinal diseases, including CVD, obesity, metabolic syndrome, autoimmune pathologies (i.e. psoriasis, rheumatoid arthritis, asthma), allergies, neurodegenerative disease and some cancers⁽Reference Carding, Verbeke and Vipond²³⁾. Research done on the gut microbiota over the last decade made progress in understanding the role of resident bacteria in relation to Western metabolic diseases linked to overweight and associated pathologies. Studies comparing gut microbial profiles of obese and lean mice described an ‘obese microbiota phenotype’, which was shown to differ from the ‘lean microbiota phenotype’ for reduced prevalence of Bacteroides, one of the main bacterial phylum in the mouse and in the human gut and for concomitant increased prevalence of the phylum Firmicutes⁽Reference Ley, Turnbaugh and Klein^24–Reference Turnbaugh, Ley and Mahowald²⁶⁾. Further analysis identified specific bacterial candidates that differentiate obese and lean individuals at a higher taxonomic level within the Firmicutes phylum. The Mollicutes class was found higher in Western diet-induced obese mice compared with controls, and its abundance was reduced especially after dietary intervention with carbohydrate restriction and weight loss⁽Reference Turnbaugh, Bäckhed and Fulton²⁷⁾. Authors suggested that the observed microbiota profile in mice consuming a Western-style diet was due to the functional advantage of Mollicutes to process high-sugar Western foods⁽Reference Turnbaugh, Bäckhed and Fulton²⁷⁾. These studies also reveal the contribution of diet in shaping gut microbiota in relation to obesity and obesity-correlated diseases. The initial animal studies showing altered Firmicutes:Bacteroidetes ratio in genetically obese leptin deficient ob/ob mice were confirmed in obese human subjects and raised attention towards gut microbiota monitoring in association with metabolic health and obesity. Also, the role of diet in inducing obesity-associated gut microbiota changes was highlighted by showing how the ‘obese microbiota phenotype’ reverted to ‘lean microbiota phenotype’ upon dietary restriction (i.e. carbohydrate- or fat- restricted diet). Work by Cani and Delzenne focused on the importance of low-grade chronic inflammation linked to obesity. Cani et al.⁽Reference Cani, Amar and Iglesias²⁸⁾ showed that in high-fat-fed obese mice had circulating levels of bacterial lipopolysaccharide, defined as ‘metabolic endotoxemia’, as well as metabolic syndrome symptoms (i.e. glucose homeostasis impairment, insulin resistance, inflammation) and dysbiotic microbiota (particularly decreased Bifidobacterium spp). The authors hypothesised increased intestinal permeability and consequent ‘leaky gut’ as the cause of metabolic endotoxemia. In further studies, Cani and coworkers demonstrated that restoration of gut barrier integrity and lowering of circulating lipopolysaccharide concentration could be achieved by dietary supplementation of dietary fibre (i.e. oligofructose) to a high-fat diet⁽Reference Cani, Neyrinck and Fava²⁹⁾. High-fat + oligofructose restored the levels of Bifidobacterium spp and promoted the growth of Akkermansia muciniphila, increased faecal SCFA, promoted epithelial barrier integrity through tight junction proteins expression, decreased plasma lipopolysaccharide, increased satiety through gut hormones expression (i.e. glucagon-like peptide-1, glucagon-like peptide-2, Peptide YY (PYY)), stimulated endocannabinoid system, and improved glucose homeostasis⁽Reference Cani, Neyrinck and Fava^29–Reference Muccioli, Naslain and Bäckhed³²⁾. Several human studies confirmed the increased intestinal permeability in association with obesity⁽Reference Amar, Burcelin and Ruidavets^33, Reference Rainone, Schneider and Saulle³⁴⁾.

Obesity-associated dysbiosis was also seen in correlation with the altered bacterial metabolic activity of gut bacteria, represented by changes in the production of fermentation end-products SCFA. The main SCFA, acetate, propionate and butyrate, were seen to be raised in faecal samples of obese animals and human subjects⁽Reference Turnbaugh, Ley and Mahowald^26, Reference Fava, Gitau and Griffin³⁵⁾, although the same trend was not observed in plasma. In obese mice, with spontaneous metabolic syndrome, the concentration of plasma SCFA was observed to be significantly lower compared with the lean counterparts⁽Reference Nishitsuji, Xiao and Nagatomo³⁶⁾. Higher plasma SCFA were seen as positively affecting gut hormone secretion and satiety regulation. Inulin consumption was shown to promote SCFA production and fat oxidation in overweight men⁽Reference van der Beek, Canfora and Kip³⁷⁾. Raised plasma acetate, propionate and butyrate concentration, following intraperitoneal infusion of SCFA in similar concentrations to the ones reached after inulin consumption, was also shown to improve fat oxidation, increase energy expenditure, induce gut satiety hormone PYY and induce lipolysis⁽Reference Canfora, van der Beek and Jocken³⁸⁾. These studies highlight the importance of fermentation to mediate diet:gut microbiota interaction in obesity and associated conditions. They also provide new mechanistic insight to the now recognised inverse relationship between dietary fibre intake and obesity/metabolic disease and give further confirmation on the beneficial role of foods which stimulate SCFA production. However, they also raise questions about the validity of faecal concentration of SCFA as a biomarker of intestinal fermentation, as discussed by Verbeke et al.⁽Reference Verbeke, Boobis and Chiodini³⁹⁾. SCFA concentration measurement in faecal samples is not representative of SCFA colonic production and absorption. Therefore, considering the effect of intestinal fermentation end-products on systemic metabolism, appetite regulation, lipid and glucose homeostasis, immunity and inflammation, measurement of SCFA production should be carried out by integrating information coming from metabolite analysis in different compartments of the organisms, in order to gain knowledge on the nutrikinetic of foods involving gut microbial metabolism.

Gut microbiota: healthy diet interaction and metabolic health

Healthy dietary habits, specifically diets with low energy coming from fat and protein and high energy coming from carbohydrates, specifically complex carbohydrates and dietary fibre, are widely recognised for their protective effect against CVD risk and there is mounting evidence that many of the effects of these diets include a contribution from gut microbial metabolic activities. Epidemiological and dietary interventions have extensively demonstrated that a Mediterranean-style diet lowers CVD, improves mental health and reduces inflammation and cancer⁽Reference Bonaccio, Di Castelnuovo and De Curtis^40–Reference Schwingshackl and Hoffmann⁴³⁾. Mediterranean dietary patterns are characterised by low energy intake, energy intake largely derived from carbohydrates and less from fats and proteins, the prevalence of complex carbohydrates over simple carbohydrates/sugars and of monounsaturated and polyunsaturated fats over saturated fats, as well as fish, pulses and dairy over red meat. Dietary fibre and polyphenols are both pillars of the Mediterranean diet and have both been shown to independently contribute to lower disease risk and mortality⁽Reference Bonaccio, Di Castelnuovo and Costanzo^44, Reference Pounis, Costanzo and Bonaccio⁴⁵⁾. Such dietary traits are not unique to the Mediterranean diet but are also found in some traditional oriental diets, such as the Okinawan diet in Japan, characterised by life-long energy restriction, low meat intake, high vegetable-based foods and high fish consumption. Adherence to Okinawan diet was observed to be protective against all-cause mortality and age-related diseases⁽Reference Willcox, Willcox and Todoriki⁴⁶⁾. This is logical from a molecular and biochemical point of view, as these diverse diets share common characteristics now known to regulate body weight and metabolic disease risk, such as low energy intake throughout life and high intake of dietary fibre, polyphenols and other plant bioactives.

When trying to find a link between healthy high-fruit and vegetables diets and gut microbiota modulation of systemic health, we need to consider that whole vegetable foods are made up of a complex mixture of bioactive compounds such as dietary fibres, plant sterols and polyphenols, as well as vitamins and minerals, which render the response of each individual and individual's gut microbiota to each food highly specific. The effects of plant-derived foods on the gut microbiota are multiple and intertwined. Beneficial effects are likely to be mediated by more than one plant bioactive compounds, maybe even acting synergistically, and by microbial consortia, rather than individual species and strains. Epidemiological and intervention studies succeeded to consistently prove the connection between high fruit and vegetables consumption, beneficial gut microbial profiles and health⁽Reference Tuohy, Conterno and Gasperotti⁴⁷⁾.

Recent dietary interventions in subjects at high risk of developing CVD have shown that diets high in fruit and vegetables improve cardiovascular health and impact on the gut microbiota⁽Reference Guasch-Ferré, Hu and Ruiz-Canela^48, Reference Klinder, Shen and Heppel⁴⁹⁾. In particular the FLAVURS study showed that increasing the consumption of fruit and vegetables up to six 80 g portions daily caused a significant increase in total urinary flavonoids, vitamin C and other phytochemicals, as well a significant increase in total dietary fibre intake, calculated as NSP food diary estimate⁽Reference Chong, George and Alimbetov^50, Reference Macready, George and Chong⁵¹⁾. Gut microbial C. leptum-R. bromii/flavefaciens, Bifidobacterium and Bacteroides/Prevotella all increased in people consuming high fruit and vegetables⁽Reference Klinder, Shen and Heppel⁴⁹⁾.

A healthy diet, specifically diet with a low energy coming from fat and protein and a high energy coming from carbohydrates, specifically complex carbohydrates and dietary fibre, was previously associated with a specific gut microbiota composition. De Filippo et al.⁽Reference Filippo, Cavalieri and Paola⁵²⁾ showed that Burkina Faso children, who consumed a traditional high-fibre, high-vegetables/pulses/tuberous and low-protein, low-fat diet, harboured a gut microbiota with significantly higher Bifidobacterium and Prevotella spp, and significantly lower prevalence of potentially pathogenic Enterobacteriaceae (Klebsiella, Salmonella, Shigella and Escherichia coli spp) compared with Italian children consuming a high-fat high-simple sugar diet⁽Reference Filippo, Cavalieri and Paola⁵²⁾. Interestingly, Burkina Faso individuals were also colonised by the cellulose-degrader Xylanibacter, thus reflecting the role of diet in favouring colonisation of the human gastrointestinal tract by bacteria with a specific metabolically advantageous function. High prevalence of Prevotella spp was described in a further study as one of the main microbial features that differentiated individuals following a long-term low-fat/high-carbohydrates diet from individuals consuming a high-fat/high-protein/low-fibre diet⁽Reference Wu, Chen and Hoffmann⁵³⁾. Differences in gut microbiota composition were observed in another study that compared geographical and cultural setups in South America and in the USA. The study showed that the differences could be explained by the populations’ dietary habits. Particularly high intake of carbohydrate-based traditional foods was thought to be responsible for the distinction of Amerindians and Malawians from US Americans in terms of microbiota profile, consisting in higher abundance of Prevotella spp, as well as Ruminococcus spp, Lactobacillus spp and some Actinobacteria in non-US Americans compared with US Americans⁽Reference Yatsunenko, Rey and Manary⁵⁴⁾. One long-term human dietary intervention study on the effect of different types of fat and carbohydrates on the gut microbiota showed that high-carbohydrate/low-fat diets increased faecal Bifidobacterium and Bacteroides spp compared with high-fat/low-carbohydrate diet⁽Reference Fava, Gitau and Griffin³⁵⁾.

In summary diet low in complex carbohydrates and plant-derived compounds, and high in simple sugars, fat and proteins modulate the gut microbiota by promoting the growth of detrimental facultative anaerobic bacteria, such as members of the Enterobacteriaceae family, and of potentially opportunistic pathogens, such as Bacteroides spp. Conversely diets high in complex carbohydrates and fibre favours the growth of polysaccharide degraders and related bacteria, such as Prevotella, Bifidobacterium, Lactobacillus Ruminococcus spp. and some Lachnospiraceae.

Bile acids

Primary (1°) BA are synthesised in the liver from cholesterol and released into the small intestine in response to food intake to aid lipid absorption and cholesterol catabolism. Within the gut, they are converted into secondary (2°) BA by the microbiota via deconjugation, dehydrogenation and dehydroxylation. Importantly, this process is regulated by negative feedback through ileal and hepatic nuclear FXR activation by BA. The gut microbiota, therefore, regulates the production of 2° BA via transformation and synthesis of 1° BA via the enterohepatic FXR-fibroblast growth factor 15 axis 1. Recent studies in animal models have shown that probiotic bile salt hydrolase activities⁽Reference Joyce, MacSharry and Casey⁷⁵⁾, polyphenols⁽Reference Chen, Yi and Zhang⁷⁶⁾ and prebiotic fibres such as oat β-glucans⁽Reference Drzikova, Dongowski and Gebhardt^77, Reference Arora, Loo and Anastasovska⁷⁸⁾ can modulate the gut microbiota and the profile of BA returning through the enterohepatic circulation (Fig. 2). Indeed, resveratrol, a well-studied polyphenol characteristic of the Mediterranean diet, has recently been shown to reduce production of the cardiotoxicant trimethylamine-N-oxide and associated vascular disease by increasing hepatic BA neosynthesis via gut microbiota modulation of the enterohepatic FXR-fibroblast growth factor 15 axis in mouse models of CVD⁽Reference Chen, Yi and Zhang⁷⁶⁾. Studies by Joyce and coworkers have shown the potential of probiotics possessing different bile salt hydrolysing enzymes to impact on the enterohepatic circulation of BAs and regulate metabolic pathways responsible for glucose and lipid metabolism, intestinal integrity, inflammation and circadian rhythm⁽Reference Joyce, MacSharry and Casey^75, Reference Govindarajan, MacSharry and Casey⁷⁹⁾. In particular, in germ-free laboratory animals, colonisation with one individual bile salt hydrolysing positive bacteria can have a profound impact on the expression of genes related to circadian rhythm, lipid metabolism, intestinal homeostasis and immune function⁽Reference Joyce, MacSharry and Casey⁷⁵⁾.

Fig. 2. Bile acid (BA) regulation of metabolic health through diet:gut microbiota interaction. BAR, BA receptor; CA, cholic acid, CDCA, chenodeoxycholic acid; LCA, litocholic acid; DCA, deoxycholic acid; UDCA, ursodeoxycholic acid; CHOL, cholesterol regulation within the liver; PXR, pregnane X receptor; LXR, liver X receptor; CAR,Constitutive Androstane receptor; VDR, Vitamin D receptor.

Similarly, other studies have shown certain fibres (e.g. β-glucans/oats) and complex plant food polyphenols such as the proanthocyanidins or condensed tannins, may sequester BAs, tightly binding them in the small intestine and driving them distally into the colon where they may be excreted or further transformed by the resident microbiota into 2° BA or modified chemical structures. Indeed, BA sequestering agents are recognised pharmaceutical agents of considerable clinical and market importance in treating the metabolic disease as they lower cholesterol, reduce obesity, improve insulin sensitivity and induce thermogenesis⁽Reference Watanabe, Morimoto and Houten⁸⁰⁾. Similarly, antibiotic attenuation of the gut microbiota alters GATA4-controlled expression of apical sodium-dependent BA transporter, increasing absorption and decreasing hepatic synthesis of BA⁽Reference Out, Patankar and Doktorova⁸¹⁾. In animal models of obesity, exercise has been confirmed to alter the composition of the gut microbiota and BA metabolism⁽Reference Meissner, Lombardo and Havinga^82, Reference Petriz, Castro and Almeida⁸³⁾. This observation if confirmed in human subjects, would provide powerful support for current healthy diet/healthy life guidelines. Furthermore, BA themselves also have a direct modulatory affect upon the gut microbiota⁽Reference Devkota, Wang and Musch⁸⁴⁾, with taurocholate, for example, increasing the relative abundance of bacteria associated with inflammation in animal models of inflammatory bowel disease. Therefore, diet and exercise, by directly modulating the gut microbiota and their role in the enterohepatic circulation of BA have the potential to alter circulating BA profiles and their subsequent ability to regulate host lipid, glucose and xenobiotic metabolism, energy usage/storage and appropriate immune response (Fig. 2). Qualitative and quantitative changes in intestinal and plasma BA profile in response to dietary fibre-induced gut microbiota modulation was shown to positively impact on endothelial function in a murine model for CVD⁽Reference Catry, Bindels and Tailleux⁸⁵⁾. The study showed that fructan supplementation induced an increase in Bifidobacterium spp and Akkermansia muciniphila, modulation of BA composition, increase in intestinal endocrine L-cells and glucagon-like peptide-1 production, all acting on the nitric oxide synthase/nitric oxide pathway. However, we currently lack direct evidence in human subjects that foods, diets and life-style interventions which modulate the gut microbiota also modulate circulating BA profiles and influence cardio-metabolic health. Similarly, because CBA have rarely been measured in nutritional interventions designed to assess the efficacy of foods/diets to reduce the risk of chronic diet-associated disease, the mechanistic role of modified BA signalling in improving host health, despite demonstration in animal studies, has yet to be demonstrated clearly in at-risk human populations. The project entitled CABALA_DIET&HEALTH⁽Reference Cabala⁸⁶⁾ is a current research project designed to specifically address these shortcomings and aims to establish circulating BA profiles as biomarkers of metabolic health, modulated by diet and exercise.

Bariatric surgery is currently the most effective therapy for morbid obesity, and indeed obesity-associated diabetes often goes into remission rapidly following gastrointestinal re-modelling⁽Reference Mingrone, Panunzi and De Gaetano^87, Reference Schauer, Mingrone and Ikramuddin⁸⁸⁾. Recent studies have suggested that early changes in BA metabolism, particularly increases in ursodeoxycholic acid (UDCA) and its glycine and taurine conjugates in the first month after surgery act as insulin-sensitising agents⁽Reference Albaugh, Flynn and Cai⁸⁹⁾. Work by others shows that deoxycholic acid (DCA) is characteristic of obesity⁽Reference Yoshimoto, Loo and Atarashi⁹⁰⁾. Transformation of conjugated cholic acid into DCA by the colonic microbiota has been shown to determine the percentage of DCA in blood⁽Reference Thomas, Veysey and French⁹¹⁾. Importantly, both UDCA and DCA, as secondary BA, are produced by the gut microbiota, respond to changes in diet in animal models and are closely linked to profiles of gut bacteria and their bile salt hydrolysing and 7α/β-hydroxylation capabilities. They also show differential abilities to activate FXR and TGR5, two key BA receptors responsible for lipid, glucose, BA and immune homeostasis. Indeed, oral feeding of UDCA to non-diabetic human subjects lowers blood postprandial glucose levels and increases secretion of glucagon-like peptide-1, an important incretin involved in regulating glucose-induced insulin secretion, in addition to controlling gastric emptying⁽Reference Murakami, Une and Nishizawa⁹²⁾. In rats, UDCA ingestion ameliorates fructose-induced metabolic syndrome⁽Reference Mahmoud and Elshazly⁹³⁾. Similarly, rectal delivery of BA increases satiety and reduces food intake by stimulating glucagon-like peptide-1 and PYY⁽Reference Wu, Li and Liu⁹⁴⁾. UDCA, therefore with low agonist potential for the G protein-coupled BA receptor and FXR, is amongst the more promising modulators of insulin/glucose homeostasis, steroid and lipid/BA metabolism, intestinal function and inflammation pathways directly regulated by these receptor molecules, suggesting competitive exclusion or synergistic signals may play an important role here.

Animal studies have shown that energy restriction diets, associated with longevity and low incidence of chronic diet associated disease, can radically modulate circulating BAs both in quantity (increasing total serum BA by 162 %) and in profile⁽Reference Fu and Klaassen⁹⁵⁾. Similarly, oral ingestion of UDCA decreases age-related adiposity and inflammation in mice⁽Reference Oh, Bae and Lee⁹⁶⁾. Cheng et al.⁽Reference Cheng, Larson and McCabe⁹⁷⁾ using LC/MS quantification of 217 plasma metabolites in 647 individuals followed up after 20 years found that the BA taurocholate was associated with lower odds of longevity (reaching the age of 80 years), although apparently not through CVD or cancer incidence. However, they noted that current lack of understanding of factors governing circulating BA concentrations limited the usefulness of taurocholic acid (TCA) as a marker of overall BA activity⁽Reference Cheng, Larson and McCabe⁹⁷⁾. Profiles or ratios of circulating BA have been suggested to reflect the risk of colorectal cancer and to be sufficiently reproducible for epidemiological studies⁽Reference Costarelli, Key and Appleby⁹⁸⁾. More recently, Ho et al.⁽Reference Ho, Larson and Ghorbani⁹⁹⁾ using targeted metabolomics of plasma from 2383 Framingham Offspring cohort participants, found that circulating BA profiles were strongly associated with insulin resistance in obese but not lean individuals⁽Reference Ho, Larson and Ghorbani⁹⁹⁾. Using 183 metabolites identified from untargeted MS-based metabolomics, Alonso et al.⁽Reference Alonso, Yu and Qureshi¹⁰⁰⁾ identified two conjugated CBA (glycolithocholate sulfate and glycocholenate sulfate) as significant biomarkers of CVD risk in 1919 African-American participants of the Atherosclerosis Risk in Communities study but concluded that further study and replication were needed⁽Reference Alonso, Yu and Qureshi¹⁰⁰⁾.

CBA profiles are therefore emerging as important signalling molecules intricately involved in mammalian cholesterol and lipid metabolism, glucose homeostasis, thermogenesis, inflammation and intestinal function. Diet:microbiota interactions in the gut are likely to play a key role in influencing both hepatic BA neosynthesis and production of 2° BA in the intestine. Dietary change has been shown to alter both faecal BA profiles and the ability of the gut microbiota to generate 2° BA⁽Reference O'Keefe, Li and Lahti²¹⁾ but we still know little about how diet:microbe interactions in humans regulates CBA profiles. Human dietary intervention studies could highlight the link between BA profiles and metabotype with gut microbiota signatures in order to highlight the molecular basis of BA regulation of immune and metabolic homeostasis.