Background/Objectives: Obesity remains a major public health concern in Portugal. Hypocaloric diets with reduced carbohydrate content have shown promise in promoting weight loss and improving metabolic health, but long-term adherence remains a challenge. This study aimed to evaluate the effects of a hypocaloric and moderate-carbohydrate diet on body composition and follow-up retention over 6 months in overweight and obese adults attending a hospital-based nutrition clinic.

Subjects and Methods: This retrospective longitudinal observational study included adults with body mass index (BMI) ≥25 kg/m² followed at a hospital-based nutrition outpatient clinic in Portugal. Participants received an individualized hypocaloric diet with moderate carbohydrate content. Anthropometric and body composition data were collected at baseline, 3 months, and 6 months. Longitudinal changes were analysed using non-parametric methods.

Results: Body weight, body mass index (BMI), and visceral fat (VF) decreased significantly over 6 months (97.0 ± 19.7 to 93.2 ± 19.9 kg; 35.6 ± 5.3 to 34.2 ± 5.1 kg/m²; and 13.3 ± 5.5 to 12.1 ± 5.0, respectively; all p < 0.01), with the largest changes observed within the first 3 months. No significant changes were observed for fat mass (FM), fat-free mass (FFM), or total body water. The 6-month follow-up rate was 46.3%.

Conclusion: In conclusion, a hypocaloric diet with moderate carbohydrate content was associated with modest but significant reductions in body weight, BMI, and visceral fat over 6 months in a real-world clinical setting. Early response was more pronounced, highlighting the importance of initial intervention and sustained engagement to optimize outcomes.