Rethinking Onconephrology: A Nephro-Nutritional Integrated Approach in Patients with Chronic Kidney Disease and Urological Malignancies

Background: Nutritional therapy is central in the management of chronic kidney disease (CKD) and cancer, yet these conditions impose partially conflicting requirements. The 2024 KDIGO guideline recommends a controlled protein intake (~0.8 g/kg/day) to reduce metabolic burden in non-dialysis CKD patients, whereas the ESPEN (European Society for Clinical Nutrition and Metabolism) guidelines support higher protein intake (≥1.0–1.5 g/kg/day) to prevent cancer-related malnutrition. Evidence guiding patients affected by both conditions is limited. We evaluated the effects of a Mediterranean-like controlled protein diet in onconephrological patients compared with CKD controls. Methods: In this retrospective study, 358 CKD patients (183 onconephrological, 175 controls) were followed at a tertiary center (2017–2024). Patients received a protein-controlled diet (0.6–1.0 g/kg/day) tailored to comorbidities and nutritional status. Nutritional assessment included bioelectrical impedance analysis and anthropometry. Renal function was evaluated using creatinine and cystatin C, and measured GFR by iohexol clearance at baseline and 12 months. Results: Baseline body composition was comparable between groups. After intervention, serum urea significantly decreased in both groups, without a decline in measured or estimated GFR. Fat mass and central adiposity indices were reduced, while lean mass and phase angle remained stable. No evidence of protein–energy wasting or catabolic activation emerged. Longitudinal analyses showed no significant time × cancer interaction for renal function or most bioimpedance-derived body composition parameters. However, at extended follow-up, arm circumference and tricipital skinfold thickness showed significant time × cancer interactions, suggesting different longer-term peripheral anthropometric trajectories according to cancer status. Conclusions: In this retrospective real-world cohort, structured nephro-nutritional management with an individualized Mediterranean-like controlled protein prescription was associated with preserved renal function and no evidence of overt nutritional deterioration in onconephrological patients. These findings support the feasibility and apparent safety of this approach in selected patients, while highlighting the need for prospective studies with objective dietary adherence assessment and longer-term evaluation of cancer-related anthropometric trajectories.