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Background: Soy protein is used for meal replacement therapy in obesity, however the influence on renal function parameters is not adequately investigated. This study evaluates glomerular filtration rate (GFR) and renal plasma flow (RPF) in patients with the metabolic syndrome and healthy controls after ingestion of different amounts of soy protein. Methods: 10 patients with the metabolic syndrome but no signs of kidney disease and 10 healthy controls ingested 1 g protein/kg body weight of a commercial soy-yoghurt-honeypreparation. The patient group was also given a protein challenge of 0.3 g/kg body weight. Results: Baseline GFR and RPF both were significantly higher in the patient group (147±34.8 vs. 116±21.1 ml/min, p=0.01 and 848±217 vs. 637±121 ml/min, p=0.02) and were strongly correlated with body weight. Use of different algorithms to estimate GFR resulted in underestimation of GFR, particularly in the patients with the metabolic syndrome. The challenge with an acute protein load of lg protein per kilogram body weight induced a significant increase in GFR and RPF in healthy controls (GFR: +12.6±11.0 % (p=0.01), RPF: +13.6±15.6 % (p=0.04)) and even more in patients with the metabolic syndrome (GFR: +31.5±32.2 % (p=0.01); RPF: +19.4±22.7 % (p=0.02)). The ingestion of 0.3 g protein/kg body weight did not induce significant changes. Conclusions: Basic renal function is changed in patients with the metabolic syndrome, even without microalbuminuria. In addition, there is an elevated susceptibility for protein load. However, the protein amount recommended for use in soy-protein based meal replacement therapy induced no significant changes. [ABSTRACT FROM AUTHOR]