INCREASED RENAL NET ACID EXCRETION IN PREMATURES BELOW 1,600-G BODY-WEIGHT COMPARED WITH PREMATURES AND SMALL-FOR-DATE NEWBORNS ABOVE 2,100-G ON ALIMENTATION WITH A COMMERCIAL PRETERM FORMULA

In 76 low birth weight infants with an actual body weight ranging from 1,210 to 2,540 g and fed a commercial preterm formula, urine samples were collected and blood acid base status was measured on day 38 (+/- 17, mean +/- SD) of life. Infants with an actual body weight below 1,6 00 g demonstrated a higher daily weight gain (22 +/- 3 vs. 14 +/- 5 g/ kg/day), lower blood pCO(2) (35.4 +/- 5.0 vs. 38.9 +/- 3.8 mm Hg),lowe r urine pH(5.8 +/- 0.5 vs. 6.5 +/- 0.3), higher renal net acid(1.86 +/ - 0.38 vs. 1.28 +/- 0.55 mmol/ kg/day) and higher phosphorus excretion (0.67 vs. 0.52 mmol/kg/day) than infants with an actual body weight a bove 2,100 g. Urinary ionogram data of these 2 groups of infants show that the increased renal net acid excretion of the smaller prematures is the result of a lower urinary excretion of sodium, potassium and ch loride, due to a higher daily weight gain, probably a higher retention of these minerals, and a higher urinary phosphorus excretion probably due to an age-specific lower intestinal calcium absorption, and there fore a lower rate of calcium and phosphorus retention. Considering the low renal capacity for hydrogen ion excretion, very low birth weight infants still run a considerable risk for disturbances of acid base me tabolism due to the high mean level of net acid excretion in nutrition with preterm formulas and an additional age-specific augmentation of renal acid load.