Abstract
We examined whether escalating doses of potassium bicarbonate (KHCO(3)) supplements alter urinary nitrogen excretion expressed as a ratio to same day nitrogen intake (measure of muscle-protein breakdown). The ratio declined significantly from placebo to low to high dose of KHCO(3) supplementation in older adults over 3 months, suggesting muscle-sparing. INTRODUCTION: Neutralization of dietary acid load with alkali supplementation (i.e., KHCO(3)) has been hypothesized to have muscle protein-sparing effects. In controlled feeding studies with fixed nitrogen (N) intake/day, 24-h urinary N excretion is a good marker of muscle breakdown. However, in studies with self-selected diets, changes in 24-h urinary N excretion can be influenced by shifts in N intake. METHODS: We evaluated changes in 24-h total urinary N excretion as a ratio of N excretion to concurrent N intake in 233 older men and women who participated in an 84-day KHCO(3) supplementation randomized placebo-controlled trial. RESULTS: After adjustment for relevant cofactors, escalating doses of KHCO(3) (1 mmol/kg/day [low] or 1.5 mmol/kg/day [high]) resulted in a progressive decline in urinary N excretion/N intake compared to placebo (overall P for trend = 0.042). The 84-day change in urinary N excretion/N intake in the high-dose KHCO(3) group was statistically significantly lower compared to placebo (P = 0.012) but not compared to the low-dose KHCO(3) group (P = 0.276). The 84-day change in urinary N excretion/N intake in the low-dose KHCO(3) group did not differ significantly from placebo (P = 0.145). CONCLUSIONS: Urinary N excretion expressed as ratio to same day N intake declined steadily with increasing doses of KHCO(3) supplementation from low 1 mmol/kg/day to high 1.5 mmol/kg/day, suggesting a nitrogen-sparing effect. Compared to urinary N excretion alone, this ratio could be a more reasonable measure of muscle protein metabolism in large-scale long-term human studies. TRIAL REGISTRATION: Clinicaltrials.gov NCT1475214.