Define absorption, transport and excretion of magnesium?
Magnesium absorption to some extent is similar to that of Ca. Absorption of Mg occurs throughout the small intestine, although jejunum and ileum are important sites. It crosses the intestinal membrane by both active transport and passive diffusion. Colon may also play a role in its absorption. About 30-65% of dietary Mg is absorbed in healthy adults. Like Ca, absorption of Mg is also more efficient when its status is marginal or intake is low. Regulation of intestinal absorption is generally thought to occur only for active component of absorption, although mechanism is unclear. Because of chemical similarity of Mg to Ca, it is postulated that vitamin D could regulate its absorption. However, it appears that only large changes in vitamin D intakes could lead to alternations in Mg absorption. As observed for calcium, some dietary factors influence absorption of Mg, although data supporting this is limited.
High intakes of dietary fibre (40-50 g/day) lower magnesium absorption. This is probably attributable to the magnesium-binding action of phytate phosphorus associated with the fibre. However, consumption of phytate- and cellulose-rich products increases magnesium intake (as they usually contain high concentrations of magnesium) which often compensates for the decrease in absorption. The effects of dietary components such as phytate on magilesiurn absorption are probably critically important only when magnesium intake is low. There is no consistent evidence that modest increases in the intake of calcium, iron, or manganese affect magnesium balance. In contrast, high intakes of zinc (142 mg/day) decrease magnesium absorption and contribute to a shift towards negative balance in adult males.
Unabsorbed fatty acids present in 'high quantities (Steatorrhoea) may bind to Mg to form soaps, Lactose, fructose and protein appear to increase its absorption. In the plasma, most magnesium is found free (55%), some is bound to protein (32%) while small amounts (13%) is complexed with citrate, phosphate or other ions. Magnesium homeostasis is maintained chiefly by controlling its excretion through urine. The kidney has a very significant role in magnesium homeostasis. Active reabsorption of magnesium takes place in the loop of Henle in the proximal convoluted tubule and is influenced by both the urinary concentration of sodium and probably by acid - base balance. Contrary to calcium homeostasis which is under tight hormonal control, regulation of Mg homeostasis occurs chiefly through renal excretion. About 70% of serum Mg is filtered by kidney, but 95% of this is reabsorbed by a healthy kidney. When the dietary Mg intake is low, renal output of Mg is further reduced. When Mg intake is severely restricted in humans with normal renal functions, Mg output reaches lowest levels of 6 mg/ day (< 0.25 millimoles/day) within 5-7 days. Intake of diuretics increases Mg excretion. Similarly thyroid and aldosterone stimulate excretion while PTH inhibits excretion.