Assessment of the pathogenetic role of physical exercise in renal stone formation

The effects of moderate physical exercise (performed on a bicycle ergometer to 70-75% of maximum oxygen consumption) without fluid replenishment on urinary chemistries and crystallization of kidney stone-forming substances were compared to those of rest in six normal subjects. Moderate physical exercise significantly decreased urinary pH [from 6.35 ± 0.32 (±sd) to 5.79 ± 0.33; P< 0.05] and citrate [from 121.1 ± 63.5 to 88.2 ± 44.4 mg/6-h period from initiation of physical exercise; P< 0.05 (630 ± 331 to 459 ± 231 μ mol/6 h)], owing to induced metabolic acidosis. The total renal excretion of stoneforming constituents decreased [for example, calcium from 31.2 ± 15.8 to 21.4 ± 6.5 mg/6 h (0.8 ± 0.4 to 0.5 ± 0.2 mmol/6 h), phosphorus from 155 ± 42 to 127 ± 27 mg/6 h (5.01 ± 1.4 to 4.1 ± 0.9 mmol/6 h), and uric acid from 172 ± 60 to 117 ± 13 mg/6 h (1.0 ± 0.4 to 0.7 ± 0.1 mmol/6 h), each P< 0.05], probably due to extracellular volume contraction (from sweating) and enhanced renal tubular reabsorption. However, the urinary concentration of stone-forming constituents significantly increased during and after moderate exercise because of the fall in urinary volume from 847 ± 312 to 290 ± 36 ml/6 h (P< 0.01). Thus, urinary calcium oxalate saturation increased significantly from 2.62- to 6.68-fold saturation (P< 0.01). The urinary undissociated uric acid concentration significantly rose [from 31.6 ± 24.8 to 125.7 ± 60.3 mg/L (0.19 ± 0.15 to 0.76 ± 0.36 mmol/L; P < 0.01)], due to higher total uric acid concentration and reduced urinary pH. The saturation of calcium phosphate (brushite) did not change significantly, because the rise in urinary calcium concentration was compensated for by reduced phosphate dissociation (from lower urinary pH). The propensity for spontaneous precipitation of calcium oxalate was greater after exercise, as less soluble oxalate was required to elicit nucleation of calcium oxalate [58.0 ± 21.2 to 49.0 ± 16.4 mg/L (644 ± 236 to 544 ± 182 Mmol/L); P < 0.05]. The results suggest that moderate physical exercise, without increased fluid intake to compensate for excessive sweating, may cause the crystallization of uric acid and calcium oxalate in urine and may enhance the risk of the formation of renal stones composed of these salts.