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The childhood obesity epidemic, that is rapidly increased in most high-income, low- and middle-income countries, is considered as one of the most serious global public health challenges for the 21st century. It may be associated with adverse health effects during childhood and an increased risk of premature morbidity and mortality later in life. Investigating possible therapeutic strategies able to counteract negative effects on child health and the risk of more severe comorbidities during adulthood is considered as a major priority. Intensive lifestyle modifications, involving diet, physical activity and behaviors are fundamental to achieve this goal. However, the characteristics of all intervention components as well as the length, the intensity, and the effectiveness of lifestyle interventions may vary largely among studies. Additionally, recently a great deal of attention has been focused on the gut microbiota as ?environmental factor? playing an important role in the development of obesity and its complications and several mechanisms able to explain this association have been proposed. This evidence needs to be further elucidated because it may have a relevant role in prevention and treatment of childhood obesity. Lastly, diets high in fruits and vegetables are widely recommended for their health-promoting properties, as they are important sources of dietary fiber, vitamins, especially vitamins C and A, minerals and phytochemicals, especially antioxidants and polyphenols. It has been suggested that, among phytochemicals, salicylic acid may have an important role, being involved in the regulation of inflammation, oxidative stress and glucose metabolism. The present PhD thesis tried to further elucidate these topics through three different tasks. The primary aim of the present PhD thesis was to evaluate whether a 1-year lifestyle intervention, based on normocaloric diet, promotion of physical activity and behavior changes, may improve obesity, metabolic profile and obesity-related comorbidities, as glucose metabolism alterations, hyperlipidemia, prehypertension/hypertension, increased liver echogenicity and metabolic syndrome, in a cohort of obese children. Secondary aims were to evaluate qualitatively and quantitatively gut microbiota biodiversity in obese and normal-weight children and to compare gut microbiota profiles with SCFAs and BMI z-scores to gain insights into the structure and activity of the microbiota in pediatric obesity. The tertiary aim was to determine the concentrations of serum salicylic acid in a group of obese children, compared to normal-weight children, and to evaluate if an association may exist between serum salicylic acid and fruit and vegetable consumption. Our results confirmed that obesity is associated with detrimental effects on health already during pediatric age, thus children may show prehypertension/hypertension, insulin resistance, pre-diabetes, hyperlipidemia, liver steatosis and metabolic syndrome. Moreover, childhood obesity may be associated with changes of some core microbial species, preexisting or diet-induced, and these changes may be involved in the etiology of obesity. Among these, an alteration of the gut microbiota composition of obese children, characterized by an increased abundance of Firmicutes and a decreased abundance of Bacteroidetes, was observed. Although beneficial effects of fruit and vegetable consumption are well-known, results from our study showed that fruit and vegetable consumption in children was very low, about 50% lower of the minimum recommended value by WHO (400 g daily). Furthermore, obese children had lower levels of serum salicylic acid than normal-weight children. These results suggest that nutrition education towards an adequate fruit and vegetable consumption should be stressed in children. Moreover, although the relationship of serum salicylic acid with fruit and vegetables consumption did not reach statistical significance both in obese and normal-weight children, promotion of fruit and vegetables with higher content of salicylic acid might be considered as part of the nutrition counseling for obese children. Finally, findings from our longitudinal study clearly highlighted the importance of a 1-year lifestyle intervention, based on a normocaloric Mediterranean balanced diet for pediatric age, promotion of physical activity and behavior changes, in the improvement of cardio-metabolic risk factors and in the reduction of the prevalence of some obesity-related comorbidities, as insulin resistance, pre-diabetes, prehypertension/hypertension, hypertriglyceridemia, higher liver echogenicity and metabolic syndrome.