Metabolic syndrome (MetS) is a constellation of overweight/obesity, hypertension, and disturbances of lipid and carbohydrate metabolism. Each component of MetS is a known risk factor for the development of type 2 diabetes, atherosclerosis, and coronary artery disease (CAD). This chronic disorder with serious health and social implications is one of the major contributors of disease prevalence due to its pathophysiological link to other cardiovascular risks. It is estimated that 750 million people worldwide are overweight, out of which 300 million are obese and accounts for 325,000 deaths each year (http://www.foodcrisis2010.com/world-obesity-statistics). Since, obesity is a precursor for MetS, treating obesity with physical activities (exercises), behavioral modifications (counseling), calorie-restricted diets, weight-losing drugs, and finally with weight losing surgery will be the crucial factors in the management and control of MetS. However, strategies like exercise, behavior modifications need strong mind control and are difficult to adopt. Calorie-restricted diets are also found to be less effective in case of obese children. Similarly the current pharmacological therapies suffer from drawbacks of adverse side-effects and high cost of treatment.[¹] Hence, in the present scenario, the development of dietary strategies, i.e., designing natural food products with probiotics and prebiotics that modulate MetS will be a cost-effective approach without the fear of adverse side effects on health. Probiotics are defined as live micro-organisms with Gebnerally regarded as safe (GRAS) status, which when administered in adequate amounts confer a health benefit on the host.[²] The two key members of this group include lactobacilli and bifidobacteria. Prebiotics, on the other hand, are defined as specific indigestible substances such as inulin, oligofructose/galactose complex which selectively support the growth of probiotic bacteria and possibly other microorganisms in the intestine. Since, probiotics are now well recognized as powerful functional food and dietary ingredients with multiple health promoting functions along with their ability to fight specific diseases, they are currently the major focus of attention all over the world to be explored as potential biotherapeutics in the management of several inflammatory metabolic disorders. However, these specific physiological functions attributed to probiotics are highly strain specific and hence, selection of strain could be very crucial to demonstrate their functional efficacy.

Various dietary strategies based on probiotic and prebiotic interventions have been proposed by several investigators after establishing strong relationship between diet, gut microbiota, and pathophysiology of MetS. The scientific data reveal that the gut microbiota is one of the important environmental factors co-evolved with the host since birth and maintains dynamic interactions with host throughout the life. The metabolic role of the gut microbiota is also essential for the biochemical activities of the human body, resulting in salvage of energy, generation of absorbable compounds, and production of vitamins and other essential nutrients.[³]

The gut microbiota also regulates many aspects of innate and acquired immunity, protecting the host from pathogen invasion and chronic inflammation.[^{4, 5}] Recently, investigators related the imbalances in gut microbiota with susceptibility to infections, immune-based disorders and more importantly with obesity and insulin resistance.[^{6, 7}] These studies provided strong scientific evidence for using probiotics and prebiotics in formulation of dietary strategies in the management of MetS.[⁸–¹³]

The present review focuses on various risk factors involved in pathophysiology of MetS and metagenomic changes of gut microbiota in MetS to gain knowledge on relationship between gut microbiota and MetS. The dietary strategies based on probiotic and prebiotic formulations have also been discussed with unique mechanisms of improved gut microbial balance with conjugated linoleic acid (CLA) production, decreased food intake, decreased abdominal adiposity and total cholesterol, decreased inflammatory tone with improved mucosal integrity in the management of MetS like obesity and type 2 diabetes.

Metabolic Syndromes have a multi-factorial etiology, comprising complex interactions among factors such as genetic predisposition, life style, diet, and environmental including epigenetic changes during development.[¹⁴] The physiological risk factors like adipose tissue saturation, dyslipidaemia, lipotoxicity, insulin resistance, chronic inflammation, oxidative stress are inter-related and associated with pathogenesis and progression of metabolic abnormalities like obesity, type 2 diabetes, non-alcoholic fatty liver disease (NAFLD), hypertension, etc. These conditions often lead to pathophysiology of MetS, which increases the risk of cardiovascular diseases as shown in the following [Figure 1].

Human gastrointestinal (GI) tract contains a complex consortia of trillions of microorganisms (approximately 1 × 10¹³ to 1 × 10¹⁴), which includes thousands of bacterial phylotypes, methanogenic archaea with a collective genome (also termed microbiome), encoding a consortium of genes exceeding the human genome by a magnitude of 150.[^{15, 16}] Although, the exact composition of the gut microbiota is not known, advances in metagenomic technologies have recently begun to unravel the diversity of our microbial partners (human microbiome). It is estimated that each individual houses at least 160 such species from a consortium of 1000 to 1150 prevalent bacterial species.[¹⁷] Amongst these bacteria, 90% of the bacterial phylotypes are members of two phyla viz. Bacteroidetes and Firmicutes followed by Actinobacteria and Proteobacteria.[¹⁷–¹⁹] Importantly, due to the persistence of difficulties in collecting samples from the different regions of the intestine, most of the studies for investigating the ecology and activities of microbiota within the intestinal tract have been carried out using fecal matter of the host.[²⁰]

Recent studies based on large-scale 16S rRNA gene sequencing, quantitative real time PCR (qRT-PCR), fluorescent in situ hybridization (FISH), high-throughput technology of pyrosequencing and DNA barcodes have shown a relationship between the composition of the intestinal microbiota and metabolic diseases like obesity and diabetes. Researchers have demonstrated that obesity may lead to the composition shift of gut microbiota in both mice and humans. Lean experimental animals (mice, rats, and pigs) have a greater abundance of bacteroidetes compared with their obese counterparts where firmicutes predominate.[^{19, 21}–²³] However, it should be noted that many of the above models involve high-fat feeding and thus the diet itself may affect the microbial composition.[^{23, 24}] Metagenomic analysis of the gut microbiota in obese mice supports the hypothesis that shifts in microbial ecology affect functional shifts in the microbiota that could contribute to the obese phenotype. Compared with lean wild type littermates, the metagenomes of obese mice are enriched in genes that encode the catabolism of complex polysaccharides, including glycoside hydrolases, which results in increased energy absorption from the gut.[²⁵] Transplantation of the microbiota from obese mice to Germ Free (GF) wild-type C57Bl/6 recipient mice caused a greater increase in adiposity than that caused by transplantation of a microbiota from lean counterparts, which directly demonstrated that the gut microbiota could modulate obesity.[^{23, 26}] In humans, the picture is somewhat more unclear where bacteroidetes-related taxa have been reported to increase, remain neutral, or decrease after weight loss.[²⁷–³⁰] It should be noted that rather than using 16S rRNA for enumeration of metagenomic techniques, these studies assessed specific taxa using probes, which raises the question of how much impact the differences in methodology/probe can have on the patterns observed. In another study, using full cloning and full length sequencing of 18,348 16S rRNA clones obtained from 12 obese subjects who were randomly assigned to either carbohydrate restricted or fat-restricted diets, it was observed that bacteroidetes bacteria positively correlated with reductions in host weight.[³¹] A subsequent larger human study examining the microbiome (the complement of genes encoded by the microbiota) associated with obesity in twins concordant for either leanness or obesity confirmed that obesity was associated with reduced levels of bacteroidetes, reduced bacterial density with enrichment in carbohydrate and lipid utilizing genes in the microbiome.[²⁵] Thus, this study supports that similar mechanisms affect obesity in both mice and humans and may involve increased microbial metabolism of carbohydrates and lipids. The outcome of few of the metagenomic studies with regard to MetS particularly obesity have been given in Table 1.

The application of probiotics as prospective biotherapies in the management of metabolic disorders including obesity and diabetes has been explored in some studies conducted by some workers. A new study by Danisco indicates that probiotic strain Bifidobacterium (B.) animalis subsp. lactis 420 (B420) could significantly improve the MetS by counteracting the adverse effects of a high-fat diet (http://www.danisco.com/wps/wcm/connec). The outcome of this study revealed that, the probiotic treatment led to significant reduction in tissue inflammation and metabolic endotoxaemia. A different but related multicenter, double-blind, randomized placebo-controlled intervention trial was conducted on 87 subjects with high body mass index who were randomly assigned to receive Lactobacillus (L.) gasseri SBT 2055 (LG2055).[³⁶] In this study, the probiotic LG2055 was provided as an adjunct culture in yoghurt that had been fermented using conventional yoghurt cultures, Streptococcus thermophilus and L. delbrueckii ssp. bulgaricus; yoghurt without LG2055 was used as placebo. The outcome of this study concludes that the probiotic strain significantly reduced the abdominal adiposity, body weight and other measures suggesting its beneficial influence on metabolic disorders. In a subsequent study, the same research workers used visceral adiposity as a measure of obesity, and the level of soluble intercellular adhesion molecule-1 (sICAM-1) in the blood as an inflammatory marker that is elevated in obesity.[³⁷] The results of the study showed that the probiotic strain inhibited the enlargement of visceral adipocytes and prevented up regulation of sICAM-1. In another study, oral administration of L. gasseri BNR17 prevented increases in body weight and adipose tissue in diet-induced overweight rats.[³⁸] The supplementation of probiotic B. breve strain B-3 in a mouse model with obesity induced by high fat diet suppressed the accumulation of body weight and epididymal fat.[³⁹] In a previous study, the association of CLA with decreased body fat and increased lean body mass was also been established.[⁴⁰] In this context, the anti-obesity activity of human derived L. rhamnosus PL60, L. plantarum PL62 producing trans-10, cis-12-CLA was also investigated.[^{41, 42}] After 8 weeks of feeding, L. rhamnosus PL60 reduced body weight without reducing energy intake and caused a significant, specific reduction of white adipose tissue. Thus, the amount of CLA produced by L. rhamnosus PL60 was adequate to produce an anti-obesity effect.[⁴¹] Recombinant L. paracasei NFBC 338 (Lb338) with CLA expressing CLA isomerase from Propionibacterium acnes showed 4-fold increase in trans-10, cis-12 CLA in adipose tissues of the mice when compared with mice that received the isogenic non-CLA-producing strain.[⁴³] These data demonstrated that a single gene (encoding CLA isomerase) expressed in an intestinal microbe can improve the lipid profile of the host. The supplementation of polyphenols with low amounts of probiotics in diet has also been proposed recently for weight loss .[⁴⁴] These dietary strategies with probiotics, CLAs, and polyphenols could have relevant implication in planning a successful dietary regimen and/or neutraceutical/pharmaceutical preparations for achieving and maintaining a normal body weight in obese individuals [Table 2].

The putative role of prebiotics in the management of metabolic disorders has also been studied by different investigators. The scientific literature documents several favorable putative effects of prebiotics on food intake, body weight, glucose homeostasis, plasma lipid profile, and associated risk factors for cardiovascular disease.[⁴⁹] To support this hypothesis several mechanisms have been proposed: First, the modulation of gut flora microbiota supporting beneficial organisms.[^{20, 31}] Second, induce enteroendocrine L cell proliferation and modulate gut peptide production and secretion (i.e., glucagon-like peptide-1 [GLP-1], peptide-YY, and ghrelin).[^{7, 49, 50}] Third, modulate inflammation in obese individuals.[⁵]

Combined data established the interrelationship between diet, gut microbiota, and metabolic syndrome. Unraveled mechanisms of probiotic and prebiotics action provided strong scientific base for developing dietary intervention strategies. However, more in depth studies related to their efficacy and effectiveness are required to be carried in human subjects to make these therapies more competitive in global functional food market.

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Conflict of Interest: None declared.