Renal magnesium handling in infants and children

Renal handling of magnesium (Mg) has been incompletely studied during infancy and childhood due to the difficulty, until recently, of measuring the diffusible fraction of plasma Mg. In the present investigation this methodology has been used to assess Mg homeostasis in 45 healthy infants, aged 1 to 12 months, and in 63 healthy children, aged 1 to 15 years. When compared to children, infants had significantly higher plasma values (mean +/- SD) for both total (0.76 +/- 0.08 versus 0.70 +/- 0.06 mmol l(-1); p < 0.001) and ultrafilterable Mg (0.51 +/- 0.07 versus 0.49 +/- 0.04 mmol l(-1) p < 0.05). No significant correlations were present between values of plasma Mg and plasma concentrations of calcium, creatinine, total protein or albumin. The ratio U-Mg/U-Cr, calculated in the second morning urine (median, 3rd-97th centiles), was also significantly higher during infancy (0.023, 0.009-0.07 versus 0.015, 0.006-0.04; p < 0.001). On the contrary, fractional excretion of Mg (median, 3rd-97th centiles) was identical in both age groups and did not correlate significantly with age (infants: 3.2, 1.0-7.8%, children 3.4, 1.6-8.1%;p = NS). During a Mg infusion, carried out in six children, we could establish an approximative value for renal Mg threshold (plasma ultrafilterable Mg = 0.50 mmol l(-1)) close to that found in adults. These results indicate that no functional immaturity is present during infancy for renal tubular reabsorption of Mg and that the high U-Mg/U-Cr ratio observed in this age group is a phenomenon not dependent on a higher urinary Mg excretion but probably related to a lower urinary creatinine excretion per unit of lean body mass.