Gastrointestinal Absorption of Magnesium

Gastrointestinal absorption of dietary magnesium (Mg)

Site

Mg absorption

% of intake absorption

mmol/day

mg/day

1 Stomach

0

0

0

1 Duodenum

0.63

15

5

1 1 Jejunum

1.25

30

10

1 ■ Proximal

1.BB

45

15

Ileum

1 1 Distal

1.25

30

10

Ileum

I Colon

0.63

15

5

Total*

5.6

135

45

*Normal dietary Mg intake = 300 mg (12.5 mmol) per day

*Normal dietary Mg intake = 300 mg (12.5 mmol) per day

FIGURE 4-5

Gastrointestinal absorption of dietary intake of magnesium (Mg). The normal adult dietary intake of Mg is 300 to 360 mg/d (12.5-15 mmol/d). A Mg intake of about 3.6 mg/kg/d is necessary to maintain Mg balance. Foods high in Mg content include green leafy vegetables (rich in Mg-containing chlorophyll), legumes, nuts, seafoods, and meats. Hard water contains about 30 mg/L of Mg. Dietary intake is the only source by which the body can replete Mg stores. Net intestinal Mg absorption is affected by the fractional Mg absorption within a specific segment of intestine, the length of that intestinal segment, and transit time of the food bolus. Approximately 40% to 50% of dietary Mg is absorbed. Both the duodenum and jejunum have a high fractional absorption of Mg. These segments of intestine are relatively short, however, and the transit time is rapid. Therefore, their relative contribution to total Mg absorption is less than that of the ileum. In the intact animal, most of the Mg absorption occurs in the ileum and colon. 1,25-dihydroxy-vitamin D3 may mildly increase the intestinal absorption of Mg; however, this effect may be an indirect result of increased calcium absorption induced by the vitamin. Secretions of the upper intestinal tract contain approximately 1 mEq/L of Mg, whereas secretions from the lower intestinal tract contain 15 mEq/L of Mg. In states of nausea, vomiting, or nasogastric suction, mild to moderate losses of Mg occur. In diarrheal states, Mg depletion can occur rapidly owing to both high intestinal secretion and lack of Mg absorption [2,6,8-13].

A [Mg] in bicarbonate saline, mEq/L

FIGURE 4-6

C Mg intake, mEq/meal

FIGURE 4-6

Intestinal magnesium (Mg) absorption. In rats, the intestinal Mg absorption is related to the luminal Mg concentration in a curvilinear fashion (A). This same phenomenon has been observed in humans (B and C). The hyperbolic curve (dotted ¡ine in B and C) seen at low doses and concentrations may reflect a saturable tran-scellular process; whereas the linear function (dashed ¡ine in B and C) at higher Mg intake may be a concentration-dependent passive intercellular Mg absorption. Alternatively, an intercellular process that can vary its permeability to Mg, depending on the luminal Mg concentration, could explain these findings (see Fig. 4-7) [13-15]. (A, From Kayne and Lee [13]; B, from Roth and Wermer [14]; C, from Fine and coworkers [15]; with permission.)

Mechanism of intestinal magnesium absorption

Nucleus

Mechanism of intestinal magnesium absorption

Nucleus

FIGURE 4-7

Proposed pathways for movement of magnesium (Mg) across the intestinal epithelium. Two possible routes exist for the absorption of Mg across intestinal epithelial cells: the transcel-lular route and the intercellular pathway. Although a transcellular route has not yet been demonstrated, its existence is inferred from several observations. No large chemical gradient exists for Mg movement across the cell membrane; however, a significant uphill electrical gradient exists for the exit of Mg from cells. This finding suggests the existence and participation of an energy-dependent mechanism for extrusion of Mg from intestinal cells. If such a system exists, it is believed it would consist of two stages. 1) Mg would enter the apical membrane of intestinal cells by way of a passive carrier or facilitated diffusion. 2) An active Mg pump in the basolateral section of the cell would extrude Mg. The intercellular movement of Mg has been demonstrated to occur by both gradient-driven and solvent-drag mechanisms. This intercellular path may be the only means by which Mg moves across the intestinal epithelium. The change in transport rates at low Mg concentrations would reflect changes in the "openness" of this pathway. High concentrations of luminal Mg (eg, after a meal) are capable of altering the morphology of the tight junction complex. High local Mg concentrations near the intercellular junction also can affect the activities of local membrane-associated proteins (eg, sodium-potassium adenosine triphosphate [Na-K ATPase]) near the tight junction and affect its permeability (see Fig. 4-6) [13-15].

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  • roxanne
    Where is magnuesium absorbed from the gi tract?
    2 years ago
  • OTHO
    Where is magnesium absorbed in gi tract?
    8 months ago
  • hildigard
    Is magnesium absorb in the gi tract?
    7 months ago
  • Mariam Muhammed
    What part of the colon magnesium is absorbed?
    6 months ago
  • sebastian r
    Where is magnesium absorbed in intestine?
    6 months ago
  • awate
    Is magnesium absorbed in ileum?
    6 months ago
  • rose
    Where in intestines is magnesium absorbed?
    3 months ago
  • Melissa
    Where is magnesium absorted?
    3 months ago

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