Metabolic acidosis, nitrogen balance and weight gain in preterm infants.

Nitrogen balance, urinary NAE and the acid base parameters in the blood of 15 male preterm infants with birth weights of 1000--2370 g (mean 1715 g) and gestational age of 29--37 weeks (mean 33.3 weeks) were determined weekly, in the first six weeks of life. The sum of NAE plus acid retention, as "total acid", was used to investigate the relationship between nitrogen balance, weight gain and acid-base homeostasis. During the first three postnatal weeks, nitrogen intake, urinary and faecal loss of nitrogen as well as the nitrogen retention were rapidly increasing. Later, urinary excretion continued to rise, the intake remained unchanged and as a result, the amount of retained nitrogen decreased slightly. Urinary NAE was steadily increasing to reach the maximum of 2.8 mEq/kg by the fourth week. Acid retention was the most pronounced in the second and third week, thereafter it fell until the end of the study. The importance of NAE relative to acid retention was continuously increasing throughout the observation period. The "total acid" increased gradually and reached its peak value in the third week of life. Subsequently a continuous fall was seen. During the first three weeks of life there was a significant positive correlation between "total acid" and nitrogen intake and urinary nitrogen extraction. Since the increase in "total acid" went parallel with the increasing nitrogen retention, the latter may be assumed to be an additional factor in producing acidosis. Calculated per 100 mg nitrogen ingested, retained or excreted with urine, "total acid" was decreasing with the increasing rate of weight gain. This indicates that the growth process -- irrespective of the postnatal development of renal H+ handling -- is also involved in the elimination of acids.