bly reflects a composite of these tubular reabsorptive processes (Quamme and Dirks, 1986).

During experimental magnesium depletion in humans, magnesium decreases in the urine to very low levels (< 20 mg [1 mmol]/day) within 3 to 4 days (Fitzgerald and Fourman, 1956; Heaton, 1969; Shils, 1969). Despite the close regulation of magnesium by the kidney, no one has described a hormone or factor that is responsible for renal magnesium homeostasis. Because patients with either primary hyper- or hypoparathyroidism usually have normal serum magnesium concentrations and a normal tubular maximum for magnesium, it is probable that parathyroid hormone (PTH) is not an important regulator of magnesium homeostasis (Rude et al., 1980). Glucagon, calcitonin, and ADH affect magnesium transport in the loop of Henle in a manner similar to PTH, but the physiological relevance of these actions is unknown (Quamme and Dirks, 1986). Little is known about the effect of vitamin D on renal magnesium handling.

Excessive alcohol intake has been shown to cause renal magnesium wasting, which, if a diet is marginal in magnesium content, could place an individual at risk for magnesium depletion. Indeed, nearly all chronic alcoholics have symptoms of magnesium depletion (Abbott et al., 1994). However, the evidence does not substantiate the suggestion that alcoholism is due to magnesium deficiency.

A growing list of medications has been found to result in increased renal magnesium excretion. Diuretics commonly used in the treatment of hypertension, heart failure, and other edematous states may cause hypermagnesuria (Ryan, 1987).

Factors Affecting the Magnesium Requirement
Bioavailability

As mentioned previously, net absorption of dietary magnesium in a typical diet is approximately 50 percent. High levels of dietary fiber from fruits, vegetables, and grains decrease magnesium absorption and/or retention (Siener and Hesse, 1995; Wisker et al., 1991). Men consuming 355 mg (14.8 mmol)/day of magnesium were in positive magnesium balance on a low-fiber (9 g/day) diet but in negative balance on a high-fiber (59 g/day) diet (Kelsay et al., 1979). Similar trends were observed in young women consuming 243 to 252 mg (10.0 to 10.5 mmol)/day of magnesium and receiving a lower fiber (23 g/day) versus higher fiber (39 g/day) diet (Wisker et al., 1991).



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