Is Nutritional–Behavioral Intervention Effective in the Fight Against Obesity?

Background/Objectives: Obesity is a complex systemic condition driven by interactions between biological mechanisms and environmental factors. This retrospective, uncontrolled, completer study aimed to evaluate changes in body composition and long-term weight maintenance, as well as the impact of comorbidities, in adult patients undergoing personalised dietary and behavioural interventions. Methods: A retrospective analysis of outpatient medical records (2018–2024) was conducted. Of the 1416 patients analysed, 267 overweight or obese adults completed the active treatment phase and a one-year post-intervention follow-up period. The primary endpoints were absolute changes in body weight, BMI, and body fat mass (BFM), as measured by bioelectrical impedance analysis (BIA), as well as the maintenance of ≥10% weight loss after one year. The intervention consisted of a personalised diet based on a food-exchange system, combined with psychodietetic support. Results: Participants achieved a mean weight loss of 10.45 ± 7.56 kg, which was associated with significant decreases in BMI and BFM. The average loss of fat-free mass and skeletal muscle mass was 1.72 ± 2.20 kg and 1.11 ± 1.36 kg, respectively. Weight maintenance, defined as maintaining a weight loss of at least 10% of baseline body weight after a one-year follow-up period, was achieved by 23.2% of participants who completed the study. Conclusions: Participants experienced significant reductions in body weight and body fat while preserving lean body mass to a moderate extent. However, long-term maintenance of weight loss was achieved by less than a quarter of participants after one year, highlighting the ongoing challenge of weight regain.