How does urinary pH relate to renal stones?
Because uric acid has a pKa of 5.5, acid urine shifts the equilibrium so that the concentration of uric acid is higher than the concentration of sodium urate. At urine pH 6.5, only 10% is in the form of uric acid, and approximately 90% in the form of sodium urate. Because uric acid is 100 times less soluble than urate, uric acid stones are more likely to form in acid urine. This equilibrium is so important that uric acid stones virtually never develop unless the urinary pH is < 5.5. Because of low urinary pH, uric acid stones occur more frequently in obesity and diabetes. Obesity and type 2 diabetes are associated with insulin resistance, renal steatosis, and renal lipotoxicity. This association results in decreased insulin-dependent renal production of ammonia, decreased urinary ammonium excretion, a lower urinary pH, and a propensity for uric acid stones. Additionally, obesity and type 2 diabetes are associated with hyperinsulinemia, which decreases distal nephron calcium reabsorption and increases net calcium excretion and the risk for calcium stones. Cystine stones are also more likely in acid urine, whereas calcium phosphate (brushite) stones usually form primarily in alkaline urine (pH > 7.0). Calcium oxalate stones may develop in either acid or alkaline urine.