###### Abstract  
(1) Background: Overweight children usually have lower levels of physical activity (PA) than their normal-weight peers. Low PA predisposes to an increase in body fat mass. The aim of this study was to evaluate the effectiveness of school-based PA intervention on the anthropometric indicators and body composition of overweight and obese children during a two-year observation period, from the age of 10 to 12. (2) Materials and methods: The research included 148 children with overweight or obesity who were born in 2007. There were two groups of children with standard and elevated PA at school (SPA and EPA; 4 and 10 h of physical education lessons a week, respectively). Anthropometric and body composition measurements were carried out. Body mass index (BMI), fat mass (FM), fat-free mass (FFM), skeletal muscle mass (SMM), and total body water (TBW) were estimated. In total, five measurements were conducted at intervals of 5-6 months. (3) Results: In EPA group, the percentage of obese children decreased from 46% to 35.25%, and in SPA group, this percentage increased from 42.75% to 46%. Only 3.33% of children in the standard PA group (one child) and 17.2% in the elevated PA group (six children) changed their BMI and FM categories from overweight to normal body mass. In EPA children, FM decreased from 28.65% to 26.41%, and in SPA children, it increased from 30.31% to 31.75%. The highest increase in FFM was in EPA boys, while SPA boys recorded the highest decrease. In SPA children, SMM increased by 0.43% from 38.85% to 39.28%, and in EPA children, it increased by 2.51% from 40.27% to 42.78%. In EPA children, TBW slightly increased from 53.26% to 53.59%, and in SPA children, it decreased from 50.70% to 49.93%. (4) Conclusions: Elevated PA at school has a positive effect on the BMI and body composition of children who are overweight or obese. Increasing the mandatory number of physical education lessons in school curricula is an efficient strategy for reducing obesity in school-aged children. Keywords:children; obesity; BMI; body composition; adipose tissue; physical activity +
Footnote †: journal: Medical  
is particularly worrying, with research showing that Polish children are gaining weight at the fastest rate in Europe [6; 7]. Currently, every third eight-year-old is overweight [8; 9]. It is estimated that at least 80% of obese school-age children will grow up to be obese adults [10]. One of the dominant factors in the etiology of many metabolic diseases is the excess of adipose tissue in the body [1; 2; 3]. Adipose tissue performs immunological, metabolic, and endocrine functions associated with the secretion of adipokines, enzymes, growth factors, and cytokines [11]. An increase in the amount of adipose tissue causes hormonal imbalance, leading to an increase in the release of leptin and pro-inflammatory cytokines into the circulatory system, and a decrease in the release of anti-inflammatory adiponectin [4; 5]. In connection with the above, obesity is associated with the occurrence of a state of chronic inflammation (low-grade inflammation). In the state of inflammation of adipose tissue, the activity of lipoprotein lipase (LPL) also increases, which leads to an increase in the concentration of free fatty acids in the blood and dyslipidemia [3; 10; 11]. The inflammatory process associated with the increase in the metabolic activity of adipose cells also contributes to the development of insulin resistance [11]. The above is one of the main pathomechanisms of the development of metabolic syndrome. In addition, excess adipose tissue is associated with the activation of orthorhombic and pro-inflammatory processes, and, therefore, with atherogenic factors [11; 12; 13]. There is a correlation between the excessive accumulation of adipose tissue and chronic non-communicable diseases such as diabetes, hypertension, and dyslipidemia and a steady increase in their incidence [1; 2; 3; 4]. Therefore, actions to counteract childhood obesity should be a major public health priority.