Over a 5-year follow-up period, we aim to conduct a study among twins aged 15-44 years from the Polish population with the following objectives: 1. To determine the incidence and risk factors for the development of de novo metabolic dysfunction-associated steatotic liver disease (MASLD), liver fibrosis, or cirrhosis. We also intend to evaluate the progression of hepatic steatosis from early to advanced stages or toward fibrosis/cirrhosis. 2. To determine the prevalence and identify predictive factors for the onset and progression of features, diseases, or complications of metabolic syndrome (MS) other than liver disease, particularly within the spectrum of metabolic, nutritional, cardiovascular, endocrine, oncological, psychological, and other disorders typically associated with MS. 3. To assess the association between previous COVID-19 infection or long-COVID features and the occurrence of MASLD, liver fibrosis, or cirrhosis due to MASLD, as well as other features, diseases, or complications of MS. 4. To evaluate the prevalence of malignancies in a young twin population (aged 15-44 years) with MS or with risk factors for MS. 5. To investigate the contribution of genetic and environmental factors and gut microbiota composition to the development and progression of structural liver changes (steatosis, fibrosis, cirrhosis) in MASLD, as well as other features and complications of MS in twins discordant for the MS phenotype. 6. To assess the role of genetic and environmental components in the occurrence and severity of MS phenotype discordance in monozygotic twins.