Objective: Relatively little is known about the influence of chronic oral protein intake on the kidney function. In most studies only the effect of a short-term change in protein intake [6-28 days] or the effect of an acute protein load on the glomerular filtration rate was studied. The purpose of this study was to investigate the effect of chronic oral protein intake on endogenous creatinine clearance and on the albumin excretion rate.
Design and subjects: In a prospective study 88 healthy volunteers with normal renal function (32 vegetarians, 12 body-builders with no supplementary protein concentrates, 28 body-builders with supplementary protein concentrates and 16 subjects with no special diet) were examined. In order to investigate the effect of chronic oral protein intake, the participants were on their diet for at least 4 months.
Results: Endogenous creatinine clearance as a measure for glomerular filtration rate varied between 32 ml/min and 197 ml/min or 34 and 186 ml/min x 1.73 m2, respectively. Nitrogen excretion rate was used as a measure for the daily protein intake, since it is known to correlate linearly with the daily protein intake. Nitrogen excretion rates ranged between 2.66 g/d and 33.93 g/d reflecting a daily protein consumption between 17 and 212 g/d or 0.29 g/kg bw/d and 2.6 g/kg bw/day, respectively. Between nitrogen excretion rate and endogenous creatinine clearance a non linear, highly significant correlation was found showing a saturation with a maximum endogenous creatinine clearance of 181.7 ml/min (dose response curve). A similar correlation was observed between urea excretion rate and endogenous creatinine clearance. Using a model for multiple regression analysis the dependence of the albumin excretion rate on nitrogen excretion rate and endogenous creatinine clearance was examined. Only a significant correlation was found between albumin excretion rate and endogenous creatinine clearance, while the correlation between albumin excretion rate and nitrogen excretion rate was not significant.
Conclusion: This investigation shows that chronic oral protein intake of widely varying amounts of protein is a crucial control variable for the glomerular filtration rate in subjects with healthy kidneys. It is suggested that these changes reflect in part structural changes of the glomerulus and tubules due to chronic protein intake.