Urinary oxalate excretion in premature infants: effect of human milk versus formula feeding.

OBJECTIVE: To study urinary oxalate excretion in infants fed human milk versus formula, and to compare urinary calcium oxalate and calcium phosphate saturation in premature infants with term infants and adults.
METHODOLOGY: We measured urinary oxalate-to-creatinine ratio and urinary oxalate concentration in 15 premature infants fed human milk compared to 16 formula-fed premature infants, and in eight human milk-fed term infants compared to 17 formula-fed term infants. We then studied urinary calcium oxalate and calcium phosphate saturations based on our observations of elevated urinary oxalate excretion in premature infants. Urinary calcium oxalate and calcium phosphate saturations were calculated from urinary concentrations of oxalate, calcium, sodium, potassium, chloride, uric acid, magnesium, phosphorus, and urinary pH. We calculated urinary calcium oxalate and calcium phosphate saturations in nine healthy adults and nine formula-fed term infants to establish control values for urinary saturation. Urinary calcium oxalate and calcium phosphate saturations were determined in nine premature infants receiving a glucose and electrolyte solution, 11 premature infants receiving parenteral nutrition, nine formula-fed premature infants, and 11 human milk-fed premature infants.
RESULTS: Urinary oxalate excretion was higher in formula-fed compared to human milk-fed premature infants whether expressed as oxalate-to-creatinine ratio (0.32 +/- 0.04 versus 0.18 +/- 0.03, P < .01) or urinary oxalate concentration (0.047 +/- 0.007 versus 0.022 +/- 0.002 mg/mL, P < .01). Urinary oxalate excretion was higher in formula-fed term infants than in human milk-fed term infants whether expressed as oxalate-to-creatinine ratio (0.14 +/- 0.01 versus 0.07 +/- 0.01, P < .01) or urinary oxalate concentration (0.022 +/- 0.002 versus 0.012 +/- 0.002 mg/mL, P < .01). The urinary calcium oxalate saturation in healthy adults was 2.84 +/- 0.79; the value in formula-fed term infants was 2.12 +/- 0.31. The urinary calcium oxalate saturation was significantly higher in premature infants receiving formula (15.68 +/- 3.15), human milk (15.02 +/- 2.27), or parenteral nutrition (11.38 +/- 2.56) compared to adults or term infants (P < .01). Urinary calcium oxalate saturation in premature infants receiving a glucose and electrolyte solution (2.45 +/- 0.36) was not significantly different from that in adults or term infants. In contrast, urinary calcium phosphate saturation in premature infants as well as term infants and adults was less than 1; precipitation of calcium phosphate is not likely to occur under these conditions.
CONCLUSION: Formula-fed infants have higher urinary oxalate excretion than human milk-fed infants. Premature infants receiving standard nutritional regimens may have urinary calcium oxalate saturation levels at which dissolved calcium oxalate may form nuclei of its solid phase.