Potassium bicarbonate attenuates the urinary nitrogen excretion that accompanies an increase in dietary protein and may promote calcium absorption.

Context: Protein is an essential component of muscle and bone. However, the acidic byproducts of protein metabolism may have a negative impact on the musculoskeletal system, particularly in older individuals with declining renal function.

Objective: We sought to determine whether adding an alkaline salt, potassium bicarbonate (KHCO3), allows protein to have a more favorable net impact on intermediary indices of muscle and bone conservation than it does in the usual acidic environment.

Design: We conducted a 41-d randomized, placebo-controlled, double-blind study of KHCO3 or placebo with a 16-d phase-in and two successive 10-d metabolic diets containing low (0.5 g/kg) or high (1.5 g/kg) protein in random order with a 5-d washout between diets.

Setting: The study was conducted in a metabolic research unit.

Participants: Nineteen healthy subjects ages 54-82 yr participated.

Intervention: KHCO3 (up to 90 mmol/d) or placebo was administered for 41 d.

Main Outcome Measures: We measured 24-h urinary nitrogen excretion, IGF-I, 24-h urinary calcium excretion, and fractional calcium absorption.

Results: KHCO3 reduced the rise in urinary nitrogen excretion that accompanied an increase in protein intake (P = 0.015) and was associated with higher IGF-I levels on the low-protein diet (P = 0.027) with a similar trend on the high-protein diet (P = 0.050). KHCO3 was also associated with higher fractional calcium absorption on the low-protein diet (P = 0.041) with a similar trend on the high-protein diet (P = 0.064).

Conclusions: In older adults, KHCO3 attenuates the protein-induced rise in urinary nitrogen excretion, and this may be mediated by IGF-I. KHCO3 may also promote calcium absorption independent of the dietary protein content.