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Background. Since it has been demonstrated that soy diet can improve endothelial function, in the present study we evaluated the effect of dietary substitution of 25 g of animal proteins with soy proteins on endothelial dysfunction in renal transplant patients.Methods. In 20 renal transplant patients (55 ± 11 years, serum creatinine 1.7 ± 0.6 mg/dl), brachial artery flow mediated dilation (FMD) and endothelium-independent vasodilation (sublingual nitroglycerine, 25 µg) were measured at baseline, after 5 weeks of a soy diet and finally after 5 weeks of soy wash-out. Changes in plasma lipids, markers of oxidative stress (lipid peroxides, LOOH) and inflammation (C-reactive protein), isoflavones (genistein and daidzein), asymmetric dimethyl arginine (ADMA) and l-arginine were also evaluated.Results. At baseline, patients showed a significantly lower FMD as compared with age-matched healthy subjects (3.2 ± 1.8 vs 6.3 ± 1.9, respectively; P < 0.001), while response to nitroglycerine was similar. After soy diet, actual protein intake was not changed, cholesterol and lipid peroxides were significantly reduced, and isoflavones were detectable in plasma. Soy diet was associated with a significant improvement in FMD (4.4 ± 2.0; P = 0.003 vs baseline), while response to nitroglycerine was unchanged. Improvement in FMD was related to l-arginine/ADMA ratio changes, but no significant relation was found to changes in cholesterol, lipid peroxides or genistein and daidzein plasma concentrations. After 5 weeks of soy diet discontinuation, FMD (3.3 ± 1.7%) returned to baseline values and isoflavones were no longer detectable in plasma.Conclusions. A soy protein diet for 5 weeks improves endothelial function in renal transplant patients. This effect seems to be strictly dependent on soy intake as it disappears after soy withdrawal and is mediated by an increase in the l-arginine/ADMA ratio, independently of change in lipid profile, oxidative stress or isoflavones. [ABSTRACT FROM AUTHOR]