Objective: Given the complicated relationship between body composition and mortality, changes in fat mass (FM) and fat-free mass (FFM) would be important parameters. Body compositions measurement may provide prognostic information. The primary aim was to observe the nature of body composition changes with weight reduction by lifestyle therapy.

Material and methods: Obese patients who visited in Obesity Clinic, Srinagarind hospital were enrolled in this retrospective observational study. Body composition changes during lifestyle therapy was measured by bioelectrical impedance analysis (BIA). Fat mass index (FMI) and fat-free mass index (FFMI) were calculated and used as normalization for FM and FMI, respectively.

Results: Mean body weight of 121 patients was 113.8 kg and mean body mass index was 42.3 kg/m². At 3-month, 49.6% of patients achieved 3% weight loss. At 6-month, 54.1% of the patients could achieve 5% weight reduction. There was a significant reduction in FMI afterward weight reduction at 6-month (mean difference 4.03 kg/m²; 95%CI 1.39 to 6.68) while FFMI had non-significant change (mean difference 0.50 kg/m²; 95%CI -1.25 to 2.25). At 6-month, patients who achieved 5% weight reduction had significant loss of FMI compared with who did not achieve (4.12 kg/m² + 0.25 VS 0.42 kg/m² + 0.22; P-value < 0.001).

Conclusion: Lifestyle therapy could maintain FFM and lose FM in those who achieved weight reduction. Application of body composition analysis for daily practice will provide information of changes in the two distinct major body compartments. Maintenance of FFM with persistent FM reduction will be beneficial for long-term clinical outcomes.

Keywords: body composition, weight reduction, obesity, fat mass, fat-free mass, lifestyle therapy