Phosphorus. Many foods high in fiber contain phytate, which may decrease intestinal magnesium absorption, probably by binding magnesium to phosphate groups on phytic acid (Brink and Beynen, 1992; Franz, 1989; Wisker et al., 1991). The ability of phosphate to bind magnesium may explain decreases in intestinal magnesium absorption seen in subjects on high phosphate diets (Franz, 1989; Hardwick et al., 1991; Reinhold et al., 1991).
Calcium. Most human studies of effects of dietary calcium on magnesium absorption have shown no effect (Fine et al., 1991a; Hardwick et al., 1991; Spencer et al., 1978b), but one has reported decreased magnesium absorption rates (Greger et al., 1981). Perfusion of the jejunum of normal subjects with 0 to 800 mg (0 to 20 mmol) calcium had no effect on magnesium absorption (Brannan et al., 1976). Increased calcium intake did not affect magnesium balance when as much as 2,000 mg (50 mmol)/day of calcium was given to adult men (Spencer et al., 1978b, 1994), or when an additional 1,000 mg (25 mmol)/day of calcium was given to adolescents (Andon et al., 1996). Magnesium intake ranging from 241 to 826 mg (10 to 34.4 mmol)/day did not alter calcium balance at either 241 mg (10 mmol) or 812 mg (20.3 mmol)/day of calcium (Spencer et al., 1994). However, intakes of calcium in excess of 2,600 mg (65 mmol)/day have been reported to decrease magnesium balance (Greger et al., 1981; Seelig, 1993). Several studies have found that high sodium and calcium intake may result in increased renal magnesium excretion (Kesteloot and Joossens, 1990; Martinez et al., 1985; Quamme and Dirks, 1986), which may be secondary to the interrelationship of the proximal tubular reabsorption of filtered sodium, calcium, and magnesium (Quamme and Dirks, 1986). Overall, at the dietary levels recommended in this report, the interaction of magnesium with calcium is not of concern.
Protein. Dietary protein may also influence intestinal magnesium absorption; magnesium absorption is lower when protein intake is less than 30 g/day (Hunt and Schofield, 1969). A higher protein intake (94 g/day) may increase renal magnesium excretion (Mahalko et al., 1983), presumably because an increased acid load increases urinary magnesium excretion (Wong et al., 1986). However, the increased urinary magnesium excretion did not change overall magnesium retention, which indicates an ability of subjects to adapt to this level of