Negative balance of calcium and magnesium under relatively low sodium intake in humans

The balance of minerals (sodium [Na], potassium [K], calcium [Ca], and magnesium [Mg]) was measured in six female students for 10 d while under a relatively low Na intake (100 mmol/d or 2.2 g/d) with receiving adequate Ca (20 mmol/d or 800 mg/d) and Mg (12 mmol/d or 280 mg/d). Both the plasma renin activity (PRA) and aldosterone level were above the reference ranges throughout the experiment, which implied that the subjects were Na deficient. However, the urine Na excretion was about the same as that ingested, while there was no substantial reduction of sweat Na concentration observed during moderate physical exercise (13.2+/-2.6 mmol/L) (mean+/-SD). On the other hand, the urine Ca and Mg levels were high, but the apparent absorption of Ca and Mg was moderate (21 +/- 5%, 34 +/- 4%, respectively), which resulted in a negative balance of these two elements. It seems that the stored Na in the bone is eluted so as to compensate for the low dietary Na intake, while any excess Ca and Mg also inevitably flows into the blood stream with Na, which inhibited the intestinal absorption of both Ca and Mg and accelerates their excretion in urine.