The obesity rate and the related complications, including non-alcoholic fatty liver disease, dramatically increase in all age groups all over the world. Given the health consequences and the following economic burden on the healthcare systems, their prevention and treatment have become a main priority. It turns out that the standard dietary measures and changes in the lifestyle, as well as the medications (i.e. anti-oxidants, oral hypoglycemic agents and lipid-lowering agents), often fail due to poor compliance and/or lack of efficacy. Here I present you systematically information about the latest pathophysiological mechanisms associated with the cascade of events:

intestinal permeability, oxidative stress, insulin resistance, hepatic inflammation and fibrosis.

The main role of the intestinal epithelium is to separate the content in the intestinal lumen from the interstitium. The integrity of the epithelial cells and their specific way of connection (tight junction) are defining the separate function of the mucosa.

tight-junction-intestinal-permeability

The tight intercellular intestinal connections are selectively permeable and this permeability may be increased physiologically in response to the nutrients in the lumen or pathologically in cases of presence of mucosal immune cells and cytokines, as well as under the influence of the enteric nervous system. The compromised function of the intestinal barrier is associated with numerous clinical conditions, both intestinal and systemic.

The protective role of the intestinal epithelium finds expression in prevention of the access of the potentially harmful intestinal content to the systemic circulation, including the microflora. The impairment of this protective function is associated with diseases such as inflammatory bowel diseases (IBD), celiac disease and irritable bowel syndrome, type I diabetes, rejection of transplanted organ or graft versus host disease (GVHD), HIV, multiple sclerosis, rheumatoid arthritis and autism.

The disease is known among the general public as “leaky gut syndrome” or syndrome of the leaking intestines and there are plenty of unsupported statements claiming that exactly this condition underlies incredible number of disorders, including chronic fatigue syndrome, fibromyalgia, allergies, depression and some skin diseases. This article will address the main determinants of the intestinal barrier function as well as some experimental data that could throw light on the pathogenic effect of the defective intestinal permeability and on the potential therapeutic measures for restoration of the selective intestinal permeability.

Increased_intestinal_permeability

Intestinal permeability is a term describing the absorption and the exchange of substances and fluids from the intestinal lumen into the systemic circulation. In contrast, the function of the intestinal barrier refers to the ability of mucosa and the extracellular barrier components, i.e. mucus, to prevent this exchange. Neither the intestinal permeability, nor the barrier function are absolute values. However, the term intestinal barrier is used quite often, especially in the context of its immunomodulatory properties and antibacterial activity.

The intestinal mucus layers are composed of the secretion of the goblet cells and form the first barrier level, the apical mucus is the one that lies directly on the epithelial cells. Mucous layers do not allow the access of microorganisms and large molecules like food particles to the epithelium, but do not contribute much in respect to the flow of small molecules, ions and water. Animal studies show that the discontinuation of mucus production may lead to intestinal damage and inflammatory bowel disease. The membranes of the epithelial cells provide effective barrier against most of the hydrophilic dissolved substances. This barrier is sufficiently reliable also because of the specific tight junction between the cells.

There is increasing evidence that the leaky gut syndrome is the main factor for the development and progression of nonalcoholic fatty liver disease in the sequence:

Bacterial overgrowth – intestinal dysbiosis – increased intestinal permeability

Steatosis (fatty liver) is one of the manifestations of metabolic syndrome and a leading cause of chronic liver disease in children and adults living in industrialized countries. Non-alcoholic fatty liver disease (NAFLD) includes steatosis and non-alcoholic steatohepatitis (NASH) which is characterized with steatosis and periportal and lobular inflammation. The progression to fibrosis and cirrhosis is one of the main complications. The pathogenic mechanisms that lead to non-alcoholic steatosis seem to be strongly associated with peripheral insulin resistance and the permanent oxidative stress in hepatocytes. The reduced cellular response to insulin leads to its increase in the serum. As a result, the overactivated by insulin lipase increases the lipolysis in the adipose tissue, which in turn elevates the level of free fatty acids. Insulin resistance activates gluconeogenesis and reduces glycogen synthesis in the liver, which increases the speed of production of free fatty acids and inhibits beta-oxidation. The consequences can be compensated with antioxidant hepatic mechanisms in the cell, but the excess of free fatty acids leads to overloading of the mitochondria with free radicals which in turn causes additional lipid peroxidation. Many inflammatory pathways are activated and as a result of the necro-inflammatory events fibrosis and cirrhosis of the liver occur.

leaky-gut-syndrome

The layer of columnar cells (epitheliocyti columnares) of the intestinal epithelium, whose main function is absorption, constitutes 80%. The rest 20% are presented by the Paneth cells, goblet cells and enteroendocrine cells. The intestinal epithelial homeostasis requires constant coordination between the specific intercellular junctions, endocytosis and intracellular signaling pathways. The structure of tight junction is constantly remodeling as a result of the interaction with external stimuli such as food residues and pathogenic or intestinal bacteria. Except for the regulation of the peri- and intracellular permeability, the intestinal barrier may activate immune cells of the innate immunity as well (e.g. dendritic cells), thus preventing systemic infections caused by intestinal microorganisms. The normal intestinal content and microflora also can induce immune response by activation of specific B-cells (“acquired immunity”). The health of the gastrointestinal tract is maintained also by the Paneth cells, specialized in secretion of antimicrobial and antibiotic substances on the mucosal surface of the small intestine.

The mucosal barrier interacts directly with the above components and therefore may affect the microbial balance. The release of mucus, containing antimicrobial molecules, prevents the bacterial colonization and further provides transduction signals that modulate pro-inflammatory and apoptotic pathways. However, the intestinal microorganisms have developed several intelligent strategies to overcome these mechanisms, namely release of mucolytic enzymes, inhibition of mucin synthesis and damages to the tight intercellular junctions. An interesting fact is that the low levels of Akkermansia Muciniphila for example, are associated with obesity. Inhabiting the mucus of the intestinal epithelium, they reduce the absorption of nutrients by increasing the thickness of the mucus layer.

The disturbances of the intestinal barrier function cause dislocation of dangerous intestinal bacteria and their products into the mesenteric portal blood and in this way they play an important role in the pathogenesis and development of non-alcoholic fatty liver disease.

The intestinal microflora is composed of trillions of microorganisms (i.e. about 1014 bacteria of more than 1000 species with a total weight of about 2.1kg). The dysbiosis of intestinal microflora is an imbalance of the microbial community in term of quantitative and qualitative changes, metabolic activity and topographic distribution. Human intestinal microflora is composed mainly of Bacteroidetes and Firmicutes. Proteobacteria, Verrucomicrobia, Actinomycetes, Fusobacteria and cyanobacteria are found in minor amounts. They contribute to the delivery of about 5% – 15% of our energy intake by absorption of the fermented food residues. The microflora of people with overweight and obesity possesses a higher capacity for energy usage from the food, thus providing substrates that can activate lipogenic pathways.

microbiome

The intestinal microbiome is influenced by both exogenous (dietary habits, vegetable fibers, unabsorbable carbohydrates that play a “prebiotic” role, lifestyle, medications, and even the way of birth delivery) and endogenous factors (bacterial mucosal receptors, intestinal pH, immune system).

The assessment of the profile of the intestinal microflora becomes more precise due the entering new techniques for molecular microbiology (next generation sequencing). Researches show that the intestinal microbiome may influence conditions which include not only the gastrointestinal tract (celiac disease, inflammatory bowel disease and irritable bowel syndrome), but also a large number of additional extra-intestinal pathologies, including obesity, insulin resistance, diabetes, cardiovascular diseases, allergic diseases and autism.

Obesity affects the gastrointestinal health in different ways:

  • Interference in the composition of the intestinal microbiome
  • reduced peristaltic movements
  • increased intestinal permeability
  • bacterial translocation

The composition of the intestinal microflora in obese individuals has been a subject of several controversial studies. With few exceptions (probably due to ethnic and/or nutritional differences), most of the studies show a proportional increase of Firmucutes over Bacteroidetes. Some medications (e.g. proton pump inhibitors) also may influence the microfloral profile as the growth of Firmicutes is encouraged. Recently, the studies are focused on the genes of the intestinal microbiome whose number seems to be reduced in persons with obesity that is related to metabolic syndrome.

The studies dealing with the leaky gut syndrome and the related disorders prove the obvious efficacy of probiotic supplements combined with changes in the lifestyle compared to only changes in the lifestyle. The primary endpoint (reduction of ALT) is achieved after just 14 weeks of treatment and is maintained until the end of the study. Inflammatory mediators (CRP, TNF-? and NF-?B p65) are also significantly decreased. Comparative study of the effect of therapy with metformin (Met) +/- probiotics (MetPr) in 32 adult patients with non-alcoholic fatty liver disease (NAFLD) shows a more significant reduction of the aminotransferases, cholesterol, triglycerides and BMI levels in the group MetPr compared to the placebo group Met. As a whole, it seems that probiotics act upon different targets: they change the composition of the intestinal microflora and reduce the intestinal permeability and translocation of bacterial products into the portal circulation, and thus modulate the inflammatory response of liver. However, the different aspects remain unclear; it is not yet known which probiotics act specifically, and only in several studies the effects of one probiotic are compared to those of another one.

Leakygutcartoon2Other_03

In conclusion, it becomes clear that the intestinal microbiome has a direct influence on intestinal permeability, and hence on systemic inflammation and metabolism. Probiotics – alone or in combination with other therapy, modulate the deposition of visceral and liver fat tissue. Therefore, they can be offered as a treatment of non-alcoholic fatty liver disease, in addition to the standard dietary and behavioral strategies.

The different probiotic supplements may possess different effects on insulin resistance and metabolic syndrome. And finally, studies show the close link between fructose and obesity, and particularly liver accumulation of fat tissue, insulin resistance and intestinal permeability, and the use of pro- and prebiotics limits these unfavorable consequences by reduction of TLR4 receptor activation – another attractive strategy that deserves more attention.

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