Circadian variations of urinary electrolyte concentrations in preterm and term infants

Sodium Intake Requirements for Preterm Neonates: Review and Recommendations

It is widely accepted that sodium is an essential nutritional electrolyte and its deficiency is associated with neurological sequelae and poor growth. The provision of an adequate sodium intake to preterm neonates is hampered by the technical difficulty in clinically assessing total body sodium content. As addressed in this review, there is a lack of consensus on the definition of hyponatremia early in life, but there is no evidence that it should deviate from the widely accepted normative data for adult subjects. A low urinary sodium content is accepted by many as reflecting total body sodium deficiency, yet spot urinary sodium measurements are of questionable clinical value. The hormonal regulation of sodium homeostasis is here reviewed and the mechanism accounting for sodium deficiency-induced growth impairment in preterm infants addressed. Lastly, we provide evidence-based gestational and postnatal age-dependent recommendations for the provision of adequate sodium intake to preterm neonates.

Changes in Biochemical Parameters of the Calcium-Phosphorus Homeostasis in Relation to Nutritional Intake in Very-Low-Birth-Weight Infants

Preterm infants are at significant risk to develop reduced bone mineralization based on inadequate supply of calcium and phosphorus (Ca-P). Biochemical parameters can be used to evaluate the nutritional intake. The direct effect of nutritional intake on changes in biochemical parameters has not been studied. Our objective was to evaluate the effect of Ca-P supplementation on biochemical markers as serum (s)/urinary (u) Ca and P; alkaline phosphatase (ALP); tubular reabsorption of P (TrP); and urinary ratios for Ca/creatinin (creat) and P/creatinin in Very-Low-Birth-Weight infants on Postnatal Days 1, 3, 5, 7, 10, and 14. This observational study compared two groups with High (n = 30) and Low (n = 40) intake of Ca-P. Birth weight: median (IRQ) 948 (772–1225) vs. 939 (776–1163) grams; and gestational age: 28.2 (26.5–29.6) vs. 27.8 (26.1–29.4) weeks. Daily median concentrations of biochemical parameter were not different between the groups but linear regression mixed model analyses showed that Ca intake increased the uCa and TrP (p = 0.04) and decreased ALP (p = 0.00). Phosphorus intake increased sP, uP and uP/creat ratio and ALP (p ≤ 0.02) and caused decrease in TrP (p = 0.00). Protein intake decreased sP (p = 0.000), while low gestational age and male gender increased renal excretion of P (p < 0.03). Standardized repeated measurements showed that biochemical parameters were affected by nutritional intake, gestational age and gender.

Monitoring of urinary calcium and phosphorus excretion in preterm infants: comparison of 2 methods

OBJECTIVES

Premature babies require supplementation with calcium (Ca) and phosphorus (P) to prevent metabolic bone disease of prematurity. To guide mineral supplementation, 2 methods of monitoring urinary excretion of Ca and P are used: urinary Ca or P concentration and Ca/creatinine (Crea) or P/Crea ratios. We compare these 2 methods in regards to their agreement on the need for mineral supplementation.

METHODS

Retrospective chart review of 230 premature babies with birth weight <1500 g, undergoing screening of urinary spot samples from day 21 of life and fortnightly thereafter. Hypothetical cutoff values for urine Ca or P concentration (1 mmol/L) and urine Ca/Crea ratio (0.5 mol/mol) or P/Crea ratio (4 mol/mol) were applied to the sample results. The agreement on whether to supplement the respective minerals based on the results with the 2 methods was compared. Multivariate general linear models sought to identify patient characteristics to predict discordant results.

RESULTS

A total of 24.8% of cases did not agree on the indication for Ca supplementation, and 8.8% for P. Total daily Ca intake was the only patient characteristic associated with discordant results.

CONCLUSIONS

With the intention to supplement the respective mineral, comparison of urinary mineral concentration with mineral/Crea ratio is moderate for Ca and good for P. The results do not allow identifying superiority of either method on the decision as to which babies require Ca and/or P supplements.

Urinary mineral excretion in preterm neonates during the first month of life

BACKGROUND

Estimation of urinary parameters in preterm infants is a useful method for identifying metabolic derangements.

OBJECTIVES

A prospective, longitudinal, hospital-based study was designed to examine the variability and the associations in renal excretion of calcium (Ca), magnesium (Mg), phosphate (P) and sodium (Na) in formula-fed preterm infants during the first month of life.

PATIENTS

Thirty-four infants <32 weeks gestational age, clinically stable, not receiving nephrotoxic drugs.

METHODS

Measurements of serum and 8-hour urinary mineral and creatinine (Cr) concentrations were made in all infants during three periods (at 7-10, 14-17 and 21-26 days postnatally). The urinary parameters, FENa, FEP, UCa/UCr, UMg/UCr were calculated. 24- hour urinary excretion was estimated by extrapolation of the 8-hour values.

RESULTS

The 24-hour excretion values (median and range) (mmol/kg) during the three study periods were, respectively, for Ca: 0.027 (0.015-0.15), 0.030 (0.007-0.12), 0.031 (0.008-0.12), for P: 0.26 (0.07-0.83), 0.29 (0.06-0.67), 0.41 (0.22-0.70), for Mg: 0.025 (0.007-0.14), 0.025 (0.008-0.16), 0.027 (0.010-0.10) and for Na: 2.85 (0.8-15), 1.45 (0.3-17), 1.56 (0.6-4.3). Na excretion declined while the excretion of the other minerals remained stable. A positive correlation was observed between excretion of Ca, Mg and Na (Ca vs. Na r = 0.63, p < 0.0001; Ca vs. Mg r = 0.65, p < 0.0001; Na vs. Mg r = 0.38, p = 0.012) as well as between the renal parameters FENa, FEP, UCa/UCr, UMg/UCr and the respective 24-hour excretion values (r = 0.80, 0.86, 0.84, 0.81) CONCLUSIONS: Urinary excretion of Ca, P, and Mg in preterm formula-fed infants is stable during the first month of life while urinary Ca, Na and Mg are closely correlated during the same period.

Reference values for urinary calcium and phosphorus to prevent osteopenia of prematurity

The prevention of osteopenia of prematurity is an important issue in the care of preterm infants. Fetal bone mineral accretion has been achieved in preterm infants by establishing and maintaining a simultaneous slight excretion of calcium (Ca) and phosphorus (P) (urine concentrations of 1-2 mmol/l) by means of an individual supplementation with Ca and/or P, resulting in a slight surplus supply (SSS). In this issue, Aladangady et al. present associations between urinary Ca/Cr and PO(4)/Cr ratios of preterm infants and biochemical variables of bone mineral metabolism. However, to date it has not been proven that these variables are a reliable substitute for direct measurement of bone mineral content (BMC). Before Ca/Cr and PO(4)/Cr ratios can be recommended as a new reference for improving BMC, the following steps are required: (1) direct measurement of BMC, (2) a prospective interventional trial to test and compare this new reference with the existing one (SSS, urinary Ca and P of 1-2 mmol/l) investigating BMC as primary outcome, and (3) adequate proof that Ca and P/Cr ratios are superior to simple urinary Ca and P concentrations.

[Endocrine theory of idiopathic nocturnal enuresis]

Body fluid homeostasis is maintained by the kidney. Such an accurate control in achieved via the secretion of antidiuretic hormone (ADH), the secretion of which is regulated by hypothalamic osmoreceptors. Both urine flow rate and the excretion of most electrolytes have a diurnal rhythm; they increase during daytime and decrease during nighttime. Such a rhythm seems to be absent in some subjects who suffer from bedwetting because of relative polyuria. In these cases, the polyuria is associated with a decreased nocturnal secretion of ADH and the subsequent excretion of dilated urine. A deficit in the nocturnal secretion of ADH thus appears to explain the response to desmopressin of children with a polyuric form of enuresis.