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Abstract
To maintain nitrogen equilibrium when prescribed a low protein diet (LPD), metabolic
adaptations occur involving a reduction protein turnover, principally decreased muscle
protein degradation. Studies suggest that in patients with chronic renal failure (CRF)
uncomplicated by metabolic acidosis (MA), these adaptive responses are intact. Because
MA stimulates muscle proteolysis, this study examined the hypothesis that in CRF complicated
by MA, the adaptation to LPD may be impaired, inducing a nitrogen wasting state. Six
adults with CRF (mean GFR: 12.8 +/- 1.5 ml/min) and MA (mean serum bicarbonate: 17.0
+/- 1.0 mM/liter) receiving an unrestricted diet (protein intake: 1.2 g/kg body wt/day)
were converted to an isocaloric LPD (protein: 0.6 g/kg body wt/day). Two weeks later
total urinary nitrogen losses decreased, but skeletal muscle protein catabolism (SMPC),
assessed from the urinary 3-methyl histidine:creatinine ratio, increased, demonstrating
impairment in the adaptive down-regulation of SMPC. The LPD was continued for a further
two weeks and MA was corrected with oral sodium bicarbonate (mean serum bicarbonate:
24.3 +/- 1.2 mM/liter). Correcting MA decreased SMPC to a level below that measured
prior to protein restriction. The decreased SMPC was paralleled by further decreases
in urinary nitrogen losses, confirming that MA impaired nitrogen utilization. It is
concluded that MA can override the expected metabolic adaptive response to a LPD.
The associated impairment of nitrogen utilization not only diminishes the efficacy
of the diet, but also accelerates the loss of lean body mass.