Small increases in dietary calcium above normal requirements exacerbate magnesium deficiency in rats fed a low magnesium diet.

In North America, the calcium (Ca):magnesium (Mg) intake ratio has increased over the last several decades raising concerns about possible adverse effects of Ca intakes on Mg status. The primary objective of this study was to investigate whether small decreases or increases in dietary Ca from normal requirements worsen Mg status in rats fed a low Mg diet. Weanling male Sprague-Dawley rats were fed 1 of 8 diets for 6 weeks. The 7 test diets were supplemented with low Mg (0.18 g/kg diet) and either 1 (1Ca), 3 (3Ca), 5 (5Ca), 7.5 (7.5Ca), 10 (10Ca), 15 (15Ca) or 20 (20Ca) g Ca/kg diet. The control diet was supplemented with normal Mg (0.5 g/kg) and Ca (5 g/kg). Rats fed higher Ca gained less weight and had lower fat mass and energy efficiency. Compared to rats fed normal Ca (5Ca), Mg concentrations in serum and femur were lower in rats fed the higher Ca diets. Haemoglobin and haematocrit were also lower in rats fed the 15Ca and 20Ca diets. Rats fed the 10Ca, 15Ca and 20Ca diets had higher urine Ca compared to rats fed the 5Ca diet. Increase in urine Ca was associated with a rise in urine Mg. The higher Ca diets increased the Ca:Mg molar ratio in serum, femur, heart and kidney. These results suggest that small increases in dietary Ca exacerbate Mg deficiency in rats fed an inadequate Mg diet by reducing intestinal Mg absorption and also by impairing renal Mg reabsorption at higher Ca intakes.