Introduction:
This study is aimed to evaluate the effects of total daily energy expenditure (TDEE) on fat mass (FM) reduction in children with obesity and to develop nomograms to support the planning of weight reduction programs in children.

Material and methods:
A total of 219 children with obesity aged 12–15 years participated in a 4-week summer camp weight reduction program. The intervention combined dietary energy restriction with various activities to increase exercise levels. Total daily energy expenditure was measured using continuous 24/7 HR monitoring. Changes in body weight and body composition – including fat mass, fat-free mass, and total body water – were measured using the bioelectrical impedance analysis.

Results:
On average, TDEE exceeded dietary energy intake by approximately 2590 kcal per 24 h, resulting in significant body weight and FM reductions. Fat mass loss was greater in boys (3.3 kg) than in girls (1.8 kg). The prescribed physical exercise lasted 3–4 h per day and accounted for an average of 32% of total daily expenditure. Boys’ total energy expenditure was markedly higher than that of girls, which corresponded to their significantly greater FM losses.

Conclusions:
The weight reduction program proved effective for reducing FM in obese children. Fat mass loss was significantly affected not only by TDEE but also by the proportion of energy expended during therapeutic exercise, the energy cost of physical activity, and the overall daily energy balance of the participants. Individual engagement in physical activity was dependent on the degree of obesity at the start of the program. The results and the prepared nomogram can be used to develop recommendations for weight loss.