PP094. A comparison of gestational changes in urinary excretion of magnesium with those of calcium

Magnesium is one of the essential minerals required in various cellular functions. Some investigators have postulated that some aspects of pathophysiology in preeclampsia could be associated with alteration in regulatory mechanisms of the mineral. However, gestational changes in absorption, excretion and blood concentration of magnesium have remained less elucidated compared with those of calcium, which is another important mineral forming a divalent cation.

The purpose of this study was to clarify if urinary excretion of magnesium during pregnancy might be altered compared with those of calcium.

Sixty specimens of 24h urine and 586 samples of spot urine were collected from healthy pregnant women who gave consent. Determination of the minerals were according to Orthocresol-phthalein complexone method for calcium, and xylidyl blue method for magnesium. Urinary creatinine was determined by Jaffe Method.

Daily excretions of magnesium determined with 24h samples were 60, 70, 81, 65, and 102mg in 1st, 2nd, and early 3rd trimesters, term, and postpartum 4weeks, respectively. There were no statistical differences among the values. Those of calcium were 174, 186, 139, 52, and 40mg, respectively. The values in term and in postpartum were significantly lower than those in 1st through early 3rd trimesters. The Mg/Cr ratios (mg/mg Cr) determined with spot urine samples were 0.064, 0.071, 0.066, 0.067, and 0.086, in 1st, 2nd, and early 3rd trimesters, term, and postpartum 4weeks, respectively. The value of Mg/Cr in postpartum was significantly higher than the values in pregnant period. The Ca/Cr ratios (mg/mg Cr) were 0.164, 0.163, 0.135, 0.118, and 0.090, respectively. There was a trend of decreasing pattern in changes of Ca/Cr from 2nd trimester to postpartum.

It is suggested that there is a mechanism of preservation of calcium in the late phase of pregnancy or puerperal period by reducing urinary excretion of the mineral. However, human pregnancy does not seem to show such a function controlling magnesium metabolism.