Increased urinary excretion of inorganic sulfate in premature infants fed bovine milk protein.

We report measurements of urinary inorganic sulfate (iSO4) in 38 very low birth weight (VLBW) premature infants receiving various protein intakes in the first 2 months of life. The primary source of urinary iSO4 is the metabolism of amino acids containing sulfur (methionine, cysteine, taurine). It was hypothesized that urinary iSO4 excretion would be increased in VLBW infants fed the relatively high concentrations of protein in mother's own milk (HM), mother's own milk fortified with 0.85 gm/dl bovine whey (fortified HM), and a special formula for premature infants (Similac Special Care, 20 cal/oz), and that urinary iSO4 excretion would correlate with calcium excretion. VLBW premature infants fed HM (protein intake 3.3 gm/kg day) excreted very small amounts of urinary iSO4 compared with infants fed fortified HM (4.5 gm/kg/day protein), Similac SC (2.9 gm/kg/day protein), or Similac (2.7 gm/kg/day protein), all three of which contain bovine whey. Unlike the case in adults, there was no correlation between either total protein intake and urinary calcium excretion or urinary iSO4 excretion. There was, however, a significant correlation between methionine intake and urinary iSO4 excretion (r = 0.48). We speculate that increased urinary iSO4 excretion is indicative of an overload of sulfur-containing amino acids, namely methionine, present in bovine whey protein. The data also support the ability of premature infants to catabolize relatively large quantities of sulfur-containing amino acids after 2 weeks of age.