The article analyses scientific literature on the actual problem of medical science of the present concerning study and analysis of structural and functional changes in the liver during obesity. One of the most common chronic liver diseases in the modern world is non-alcoholic fatty liver disease which becomes a pandemic on the background of obesity. Data of researches of domestic and foreign scientists about non-alcoholic fatty liver disease represent important evidence of significant structural changes in the liver at different levels of organ organization (organ, tissue, cell, and molecule). The results of scientists` researches point at the fact that 75% of patients with obesity have non-alcoholic fatty liver disease. Almost all the patients have non-alcoholic fatty liver disease during the morbid obesity which is one of the most common liver diseases all over the world and gets the status of the pandemic one nowadays. Pathogenetic mechanisms of non-alcoholic fatty liver disease development are complicate enough and still are the question for scientists to be discussed. Non-alcoholic fatty liver disease is the main hepatic demonstration of the metabolic syndrome and has close connection to the obesity and to the insulin resistance. Obesity leads to changes in the structural organization of the liver which affect on its functional state and metabolic processes. Non-alcoholic fatty liver disease combines several pathological statuses, fatty steatosis or fatty liver dystrophy which has mostly benign character, non-alcoholic steatohepatitis, which is characterized with the progress to the liver fibrosis, and then to the liver cirrhosis, moreover cirrhosis passes into the hepatocellular carcinoma. Many researchers are trying to find the pathological, genetic and morphological features of non-alcoholic liver steatosis and non-alcoholic steatohepatitis during obesity. They are two morphological features or stages of disease: steatosis and steatohepatitis are mentioned in the International Statistical Classification of Diseases and Related Health Problems. Morphologists are still using such terms as "fatty liver dystrophy", "fatty liver", "fatty liver degeneration" and other ones in order to determine two possible forms as stages of disease: steatosis and steatohepatitis. Fatty liver disease is characterized with the fatty scurf in the liver, which leads to the failure of the normal development of hepatocytes. Healthy cells are replaced by the products of the fat exchange (metabolism) and the liver stops its biological functioning. This stage is called liver steatosis and is characterized by the fatty dystrophy of the liver cells. Liver steatosis is the histological indication of non-alcoholic fatty liver disease, development of which one is characterized with the insulin resistance, hyperinsulinism and increased quantity of the liberal fatty acids. The high concentrations of the liberal fatty acids in conditions of the insulin resistance are the main mechanism of forming the steatosis among people with non-alcoholic fatty liver disease. This regularity is confirmed by the results of the clinic and laboratory researches. Accumulation of the certain percent of lipids in the liver cells leads to the appearance of the second stage of the fatty disease – steatohepatitis, which is characterized with the inflammatory processes in the liver and with the necrosis or hepatocytes. The professional literature mentions that investigating the liver biopsy in the conditions of experimental model of non-alcoholic fatty liver disease we can indicate the apoptosis of hepatocytes and expressive changes of the structure and form of hepatocytes (phenomenon of polymorphism). The following factors can cause appearance of the steatohepatitis and non-alcoholic fatty liver disease: obesity, endogenous disorders of the lipid and carbohydrate metabolism, metabolic failures, the syndrome of malabsorption, medication use etc. The third (the last) stage is characterized with the replacing of the liver cells by cells of the connective tissue. Respectively, we can watch the development of fibrosis and later - cirrhosis, so the liver loses its main functions. In the professional editions researchers point at the fact that non-alcoholic fatty liver disease appeared due to the bacterial growth in the small intestine. Oxidative liver stress could depend not only on the high concentration of circulating liberal fatty acids during obesity, but also on the high concentration of endotoxins on the background of disbiotic violations being characteristic features of the syndrome of enormous bacterial growth. Increasing the amount of circulating endotoxins in the portal blood activates the liver Kupfer's cells which lead to the appearance of anti-inflammatory cytokines and chemokines which are the substrate of the inflammatory component of steatosis transformation during non-alcoholic steatohepatitis and induction of fibrogenesis. The study of pathogenetic mechanisms for the development of non-alcoholic fatty liver disease, the classification of risk facts, criteria of diagnosis, estimation of the prognosis of the course of the disease are important for the search and substantiation of effective methods of prevention and treatment.
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