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].

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