Underlying metabolic processes behind non-alcoholic fatty liver disease in population with metabolic syndrome features: nutritional status and non-invasive liver tools

Non-alcoholic fatty liver disease (NAFLD) is currently the most common cause of liver disease in the Western world, affecting up to 20-30% of the general adult population. It is a growing public health problem, because of the increasing prevalence of the pathologies that contribute to its development such as obesity and inflammation status. The diagnostic method of choice for NAFLD is liver biopsy, but since it is a long-standing disease, it is difficult to follow large population groups through serial biopsies. Therefore, it is necessary to identify noninvasive liver damage tools that facilitate the diagnosis and prognosis of the disease and allow for the establishment of a follow-up protocol that prevents progression to severe stages of NAFLD. Moreover, the main therapeutic basis is the modification of lifestyle through diet and exercise In this context, this research work aimed: 1) to assess the possible association between a validated Dietary Inflammatory Index (DII) and specific dietary components with suitable non-invasive markers of liver status in overweight/obese subjects with cardiovascular risk; 2) to evaluate the influence of two energy-restricted diets on non-invasive markers and scores of liver damage in overweight/obese individuals with features of metabolic syndrome after six months of follow-up and to assess the role of fiber content in metabolic outcomes; 3) to assess the influence of weight loss on lysophospholipid metabolism and liver status in overweight/obese subjects as well as to provide new evidence regarding the interaction of lysophospholipids metabolism as a key factor in the onset and management of obesity-related diseases such as liver damage; 4) to analyze the response of FGF-21 after a weight loss intervention and the relationships with other putative inflammatory liver biomarkers; 5) to evaluate the fatty liver status by several validated approaches and to compare imaging techniques, lipidomic and routine blood markers with magnetic resonance imaging (MRI) in adults subjects with NAFLD. In relation with the first objective, the results of this chapter evidended that the consumption of an anti-inflammatory dietary pattern might contribute to the reduction of obesity and related comorbidities, especially NAFLD through precision nutrition guidelines. Concerning the second objective, we concluded that the design of dietary patterns based on the consumption of insoluble fiber and fiber from fruits in the context of energy restriction is a good choice the management of obese patients suffering from NAFLD. The third chapter suggested a generalized decrease in circulating lysophospholipids (LP), particularly lysophosphatidylcholine, after a weight loss intervention in a population with metabolic syndrome features. Changes in fatty liver index, waist circumference and BAAT score revealed positive association with lysophosphatidylcholine score. The involvement of particular LP in liver metabolism and obesity merits further attention. The fourth chapter revealed that FGF-21 changes exhibit a great association with non-alcoholic fatty liver inflammation, M30 fragment and PAI-I, independently of weight loss. These findings suggest that FGF-21 is involved in the obesity-inflammation-liver process. Finally, in relation to the fifth objective we concluded that ultrasonography, a metabolomic test and a panel combination including routine blood markers linked to insulin resistance showed the highest associations with MRI, which is considered the gold standard for non-invasive liver fat content assessment, suggesting these tools could contribute to the diagnosis and prognosis of NAFLD