Magnesium Oxide Absorption
Yes, magnesium oxide is absorbed, but only minimally—approximately 4% of the administered dose is bioavailable, making it one of the poorest absorbed magnesium preparations available. 1
Mechanism and Absorption Profile
Magnesium oxide is insoluble in water and alcohol but dissolves in dilute gastric acid, converting to magnesium chloride in the stomach. 2 This conversion is necessary for any absorption to occur.
Despite containing the highest amount of elemental magnesium per dose compared to other magnesium salts, magnesium oxide demonstrates fractional absorption of only 4%, which is significantly lower than organic salts like citrate (which shows superior bioavailability). 1, 3
In controlled bioavailability studies, magnesium oxide supplementation resulted in no differences compared to placebo in terms of serum magnesium levels or urinary excretion. 4, 3
Primary Mechanism of Action
Magnesium oxide works primarily through osmotic effects in the gastrointestinal tract rather than through systemic absorption. 4 This is why it's effective as a laxative at doses of 500 mg-1.5 g/day for chronic constipation, despite poor absorption. 4
Most magnesium salts are poorly absorbed and may worsen diarrhea or stomal output, which is why magnesium oxide is specifically chosen in certain clinical contexts where the osmotic effect is desired. 2
Clinical Context for Use
Magnesium oxide is commonly prescribed at 12-24 mmol daily (approximately 480-960 mg elemental magnesium) in patients with short bowel syndrome or jejunostomy, given as gelatin capsules of 4 mmol (160 mg) each. 2
Administration at night when intestinal transit is slowest maximizes the limited absorption that does occur. 2, 4
The small amount that is absorbed is sufficient for some clinical benefit in hypomagnesemia when combined with correction of water and sodium depletion (which addresses secondary hyperaldosteronism). 2
Important Caveats
Normal intestinal magnesium absorption ranges from 30-50% for most magnesium compounds, but magnesium oxide falls far below this range. 5, 2 Under basal conditions, the small intestine typically absorbs 30-50% of magnesium intake, but this does not apply to magnesium oxide. 5
If oral magnesium oxide fails to normalize magnesium levels, alternative strategies include switching to more bioavailable organic salts (citrate, lactate, aspartate, chloride) or adding 1-alpha hydroxycholecalciferol, or providing intravenous/subcutaneous magnesium. 2
Avoid all magnesium supplementation in patients with creatinine clearance <20 mL/min due to hypermagnesemia risk. 4