DIETARY-PROTEIN AS A MODULATOR OF THE RENAL NET ACID EXCRETION CAPACITY - EVIDENCE THAT AN INCREASED PROTEIN-INTAKE IMPROVES THE CAPABILITYOF THE KIDNEY TO EXCRETE AMMONIUM

To test the hypothesis that increments in protein intake increase the capacity of the kidney to excrete ammonium (NH4+), two strictly contro lled diet studies (both repeated-measures design with n = 6 healthy ad ult subjects) were performed. In study 1, comprising two 5-day diet pe riods (A and B, protein intake: 120 and 95 g/day, respectively), the u rine pH was adjusted to a low pH level by administration of L-methioni ne (20 mmol/day) along with diet B. In study II, comprising three I-da y diet periods (C1-C3: C1, basic diet containing 50 g protein/day; C2, basic diet + 32 g of egg white protein/day; C3, basic diet + 10 mmol L-methionine/day), the urine pH was adjusted to a high pH level by sod ium citrate (6.67 mmol/day) administered with diets C2 and C3. Urine p H constancy, which guarantees a constant tubular NH4+ transfer, was ac hieved for either study (diets A, B:5.5 +/- 0.2, 5.4 +/- 0.2; diets C1 , C2, C3:6.7 +/- 0.1, 6.8 +/- 0.1, 6.7 +/- 0.1), thus allowing assessm ent of urinary NH4+ excretion rates as an index of the renal NH4+ prod uction capacity. With the higher protein intake on diet A compared wit h diet B, a significantly elevated NH4+ excretion (82.8 +/- 11.4 vs. 7 3.6 +/- 7.2 mmol/day; P < 0.01) was observed. NH4+ excretion also incr eased from 29.4 +/- 5.5 mmol/day (C1) to 34.8 +/- 5.1 mmol/day (C2) in study II (P < 0.05) after the protein intake was raised with egg prot ein. When methionine was administered instead of egg protein as a diff erent source of sulfur (C3), no effect on renal NH4+ output occurred, thus demonstrating that protein as a whole has a specific impact. It i s concluded that the renal capacity to excrete excess acidity as NH4is modulated (independently of the underlying level of the respective renal acid load) by the amount of protein ingested.