Renal handling of hydrogen ion excretion in relation to maturity indices in premature infants fed human milk.

In 40 premature infants fed human milk with an actual gestational age of 261 +/- 16 days and an actual body weight of 1.06-2.75 kg, 44 urine samples were collected, and blood acid-base status was measured on day 32 (+/- 16) of life. In the urine, the following results (mean +/- SD) were obtained: urine pH 6.05 +/- 0.65, titratable acidity 0.24 +/- 0.14 mmol/kg/day, ammonium 0.78 +/- 0.25 mmol/kg/day, net acid excretion 0.83 +/- 0.47 mmol/kg/day. There was no significant correlation between renal net acid or ammonium excretion and actual body weight. However, urine pH was positively correlated with body weight. Obviously, premature infants with an actual body weight below 1.5 kg need a higher stimulation of renal hydrogen ion secretion to excrete the same amount of ammonium than those with an actual body weight of about 2.5 kg. The limited renal acidification capacity of very low birth weight infants is a risk factor for the development of late metabolic acidosis.