Sequential analysis of variation in glomerular filtration rate to calculate the haemodynamic response to a meat meal

The renal haemodynamic response to a meat meal is usually measured as either filtration capacity (maximal achieved GFR), or renal reserve (maximal GFR increase over baseline), or percent renal reserve (maximal GFR increase as a percentage of baseline). The time-course of GFR response to a meat meal varies in different individuals as the peak GFR tends to occur late in renal disease. This study proposes a new method to measure the GFR response independently of differences in peaking time.

The study is based on measurement of GFR (inulin clearance, ml/min x 1.73 m2 BSA) in three 30-min pre-meal clearance periods (baseline) followed by analysis of the GFR changes for up to 180 min (four 30-min and one 60-min clearance periods) after a meat meal (2 g of protein/kg of BW as red cooked meat). Data were analysed from 85 healthy people (GFR > or = 100) and 273 individuals with renal disease (RD) who were divided into three groups based on their baseline GFR (RD1, n = 115, GFR 99-66; RD2, n = 85, GFR 65-33; RD3, n = 73, GFR < 33).

In healthy people after the meat meal GFR peaked between 30 and 60 min and returned to baseline by 120 min. In the three RD groups GFR peaked later than in healthy people (P < 0.001) and remained higher than baseline for up to 180 min (P < 0.001). Cumulative post-meal GFR changes, calculated as cumulative GFR increase over baseline up to 120 min after meal (ml/120 min x 1.73 m2 BSA), were significantly different (P < 0.01) in the four groups (healthy people, 937 +/- 141; RD1, 1222 +/- 141; RD2, 587 +/- 104; RD3, 361 +/- 89). Interindividual variability in cumulative GFR increase was only partially explained by the value of filtration capacity (r2 = 0.285), renal reserve (r2 = 0.640), and percent renal reserve (r2 = 0.175).

The data indicate that commonly used parameters are poor indices of the actual total time-course of the renal response to a protein load.