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Dietary protein has long been thought to play a role in the progression of chronic renal disease, but clinical trials to date have not consistently shown that dietary protein restriction is beneficial.To use meta-analysis to assess the efficacy of dietary protein restriction in previously published studies of diabetic and nondiabetic renal diseases, including the recently completed Modification of Diet in Renal Disease Study.The English-language medical literature published from January 1966 through December 1994 was searched for studies examining the effect of low-protein diets in humans with chronic renal disease. A total of 1413 patients in five studies of nondiabetic renal disease (mean length of follow-up, 18 to 36 months) and 108 patients in five studies of type I diabetes mellitus (mean length of follow-up, 9 to 35 months) were included.Randomized, controlled studies were selected for nondiabetic renal disease; randomized, controlled studies or time-controlled studies with nonrandomized crossover design were selected for diabetic nephropathy.Data in tables, figures, or text were independently extracted by two of the authors.The relative risk for progression of renal disease in patients receiving a low-protein diet compared with patients receiving a usual-protein diet was calculated by using a random-effects model. In five studies of nondiabetic renal disease, a low-protein diet significantly reduced the risk for renal failure or death (relative risk, 0.67 [95% CI, 0.50 to 0.89]). In five studies of insulin-dependent diabetes mellitus, a low-protein diet significantly slowed the increase in urinary albumin level or the decline in glomerular filtration rate or creatinine clearance (relative risk, 0.56 [CI, 0.40 to 0.77]). Tests for heterogeneity showed no significant differences in relative risk among studies of either diabetic or nondiabetic renal disease. No significant differences were seen between diet groups in pooled mean arterial blood pressure (diabetic and nondiabetic patients) or glycosylated hemoglobin level (diabetic patients only).Dietary protein restriction effectively slows the progression of both diabetic and nondiabetic renal diseases.Ann Intern Med.1996;124:627–632.