Magnesium hydroxide: New insights into the mechanism of its laxative effect and the potential involvement of prostaglandin E₂

The mechanism by which Mg(OH)₂ acts as a laxative is unknown. To explore the mechanism, six volunteers more than 55 years old, with normal bowel habits, were enrolled in a dose-response, randomized, placebo-controlled, double-blind, crossover design study. Each subject was studied for four inpatient periods of 5 days each on a metabolic ward with 9 days off of all medication between studies. In the hospital, all patients were on a diet fixed in calories, fluid volume, Na⁺, fiber, and Ca²⁺. At 8 p.m. on each study day, each subject took 45 ml containing either placebo or 1,200, 2,400, or 3,600 mg of Mg(OH)₂ plus 240 ml of water. On the fourth and fifth hospital days of each period, 24-h stool output was quantified and analyses performed. Compared to placebo, Mg(OH)₂ caused the following dose-dependent results: (a) increased number of bowel movements; (b) increased percentage of stool Mg²⁺; and (e) increased total stool 24-h prostaglandin E₂ (PGE₂), with mean 24-h excretions as follow: placebo, 95 ± 18 pg/24 h; 1,200 mg Mg(OH)₂, 260 ± 100; 2,400 mg Mg(OH)₂, 357 ± 117; and 3,600 mg Mg(OH)₂, 525 ± 196. There was a significant correlation between stool PGE₂ excretion and stool water consistent with a causative relationship. However, the concentration of stool prostaglandin was lower than the concentration found to alter intestinal electrolyte transport in vitro. In summary, the laxative effect of Mg(OH)₂ is associated with increased output of stool PGE₂. The contribution of the stool PGE₂ to the laxative effect of Mg(OH)₂ is unknown.