Objectives: In adults, adaptation to changes in magnesium intake is largely due to changes in fractional magnesium absorption and urinary magnesium excretion. We sought to examine whether these homeostatic mechanism also occurred in young children.
Methods: Children, 12-48 m old were studied (n=30). They were adapted to a home diet representative of their usual magnesium intake for 7 d then admitted for a stable isotope study. Children received 5 mg Mg-25 intravenously, and 10 mg Mg-26 orally (5 mg with breakfast and 5 mg with lunch). Magnesium absorption was calculated from the relative fractional excretion of the oral and intravenous isotopes in the urine samples. Endogenous fecal magnesium absorption was calculated in a subgroup from the fecal and urinary excretion of the intravenous tracer.
Results: Magnesium intake (mean +/- SD; 106 +/- 25 mg/d) was significantly greater than the Estimated Average Requirement (EAR) described by the Institute of Medicine in the US (65 mg/d, p < 0.0001). Across the range of intake studied, fractional magnesium absorption was significantly (P = 0.0383) but weakly (r(2) = 0.144) related to magnesium intake. Absolute magnesium absorption (the product of fractional absorption and intake) significantly increased as intake increased (r(2) = 0.566, P < 0.0001). Urinary magnesium excretion was unrelated to magnesium intake (r(2) = 0.036, P = 0.31). Endogenous fecal magnesium excretion tended to increase as magnesium intake increased (r(2) = 0.312, P = 0.12). Magnesium retention (absolute absorption minus urinary and fecal losses) was positive in 26 of the 30 subjects studied, and was linearly related to magnesium intake (r(2) = 0.157, P = 0.0304). A magnesium intake of 52-78 mg/d would appear to be required to meet the needs for absorbed magnesium for half the children at this age range, suggesting that the current EAR is broadly appropriate.
Conclusions: In young children, consuming magnesium intakes typical of the US population, fractional magnesium absorption is a major site of magnesium homeostasis, but magnesium retention increased linearly across the intake range studied. Our results support at EAR for magnesium of 55-80 mg/d and an RDA of 70-100 mg/d.