In a prospective randomized study, the urine pH of 170 premature and small-for-gestational-age (SGA) newborns was routinely screened to detect patients with spontaneously developing maximum renal acid stimulation, an obligatory early stage in the development of late metabolic acidosis. Nitrogen assimilation was evaluated from the ratio of urinary nitrogen excretion and intake. Forty-two premature infants and 10 SGA prematures and newborns after intensive care therapy with body weights greater than 1.5 kg and 25 prematures (including 7 SGA infants) with body weights less than 1.5 kg, spontaneously showed urine pH values below 5.4 on two consecutive days, suggesting maximum renal acid stimulation. These patients were randomly given either oral alkali therapy with sodium bicarbonate 2 mmol/kg/day or no therapy for a period of seven days. In both groups, urine pH was controlled daily. Patients in the control group without alkali therapy and with urine pH values less than 5.4 for seven days showed a significant decrease in weight gain and a tendency to decreased nitrogen assimilation. We assume that a regular check of urine pH in low-birth-weight infants is a useful non-invasive method of detecting patients in the early stages of development of late metabolic acidosis, i.e. in the stage of "incipient late metabolic acidosis". This would provide the possibility of starting early effective therapy and thereby reduce the mean duration of admission to neonatal wards.(ABSTRACT TRUNCATED AT 250 WORDS)

Two hundred and eighty-two patients with birthweights below 2.0 kg were routinely screened for spontaneous development of maximum renal acid stimulation (urine-pH < 5.4). Sixty episodes in 53 patients of incipient late metabolic acidosis (urine pH < 5.4 on 2 consecutive days) were randomly allocated to oral therapy with 2 mmol/kg/day of either NaHCO3 or NaCl for 7 days. All 27 patients on NaHCO3 therapy, but only 15 from 26 patients on NaCl therapy, showed an increase in urine pH values, combined with a relatively high gain in body weight and a tendency to increased N-assimilation. Eleven patients on NaCl therapy showed persistent maximal renal acid stimulation on all 7 days with possibly lower weight gain and no clear change in N-assimilation. Thus, in patients with incipient late metabolic acidosis, NaCl therapy is not as beneficial as NaHCO3 therapy.

In 19 preterm infants fed a standard formula for prematures (calcium (Ca) 13.5 mmol/l; phosphorus (P) 12.9 mmol/l), biochemical parameters of blood, serum and urine were determined before and during supplementation with Ca-L-lactate (final Ca concentration 20 mmol/l). In 8 preterm boys Ca and P balance were evaluated in addition. During Ca supplementation, the serum Ca levels, urine pH (without supplement 6.31, with supplement 6.73), and calciuria (46 mumol/kg/d vs. 98 mumol/kg/d) were increased, and urinary P (1.05 mmol/kg/d vs. 0.65 mmol/kg/d) and net acid excretion (1.70 mEq/kg/d vs. 0.89 mEq/kg/d) were decreased. Balance studies showed increased net intestinal Ca absorption during supplementation (37% vs. 56%) as well as improved Ca (0.8 mmol/kg/d vs. 1.85 mmol/kg/d) and P retention (0.97 mmol/kg/d vs. 1.45 mmol/kg/d). These data show that increased Ca intake given to optimize the Ca:P ratio improves mineral retention in preterm infants fed a standard formula. Ca and P intake should be thoroughly balanced to avoid side-effects like hypercalciuria or high renal net acid excretion.

Late metabolic acidosis was observed in a term baby boy with renal tubular acidosis type 4 who received two cow's milk formulas in succession. Suboptimal mineral composition of the formulas turned out to be an important risk factor for the development of late metabolic acidosis.

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 1,600 g demonstrated a higher daily weight gain (22 ± 3 vs. 14 ± 5 g/kg/day), lower blood pCO₂ (35.4 ± 5.0 vs. 38.9 ± 3.8 mm Hg), lower 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 above 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 chloride, 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 therefore 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 metabolism 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.