Phosphate Excretion Differentiates the Amount of Nephroprotective Effect of Amino Acid Ketoanalogues Treatment with Low Protein Diet in Chronic Kidney Disease—A Retrospective Single-Center Cohort Study

Chronic kidney disease (CKD) affects more than 10% of the world’s population, increasing the risk of cardiovascular disease and mortality. Background: Nephroprotective interventions can reduce the risk of end-stage renal disease, delay the time to dialysis, and prolong life. However, there is ongoing debate about the effectiveness of combining amino acid ketoanalogues (KAA) with a low-protein diet (LPD) to slow CKD progression. This study aimed to retrospectively analyze kidney function outcomes after a 6-month KAA+LPD regimen in patients with CKD. Methods: The analysis included results from 38 non-dialyzed patients (12 F, 26 M; age 64.0 ± 13.6 years) with stable CKD in stages G4 to G5, who followed LPD with KAA (Ketosteril, Fresenius Kabi) treatment as part of the Polish National Health Fund Ketosteril Drug Program. Results: No significant change in estimated glomerular filtration rate (eGFR) was observed during 6 months of KAA+LPD therapy. However, eGFR increased or decreased in half of the patients (p < 0.001), and this change was associated only with initial protein intake and urinary phosphate excretion. Initial high phosphate excretion was independently associated with an increase in eGFR, and initial phosphaturia > 0.5 g/24 h identified eGFR improvement (sensitivity 84.2%; specificity 57.9%; AUC 0.712; p = 0.018) in CKD patients who started KAA+LPD treatment. Conclusions: Six-month treatment with KAA+LPD may be associated with stabilization of kidney function in patients with CKD stages G4-G5. The individual effect of KAA+LPD on renal function may be related to the initial protein intake level and urinary phosphate excretion. Further studies are needed to validate these findings across larger patient populations with a broader spectrum of symptoms.