Why do low-dose magnesium (Mg) supplements cause loose stool?

Why Low-Dose Magnesium Supplements Cause Loose Stool

Magnesium supplements cause loose stool even at low doses because magnesium acts as an osmotic laxative by drawing water into the intestinal lumen, and this mechanism occurs regardless of dose—it's simply more pronounced at higher doses. 1, 2

The Osmotic Mechanism

• Magnesium ions are poorly absorbed in the intestinal tract and create an osmotic gradient that pulls water into the bowel lumen, increasing stool fluidity. 2
• For every 1 mmol increase in fecal magnesium output, fecal weight increases by approximately 7.3 grams due to water retention. 3
• This osmotic effect is the fundamental mechanism by which magnesium works as a laxative, and it occurs even with supplemental doses intended for nutritional purposes rather than constipation treatment. 1, 2

Why Individual Sensitivity Varies

• Different magnesium formulations have varying osmotic potencies—magnesium citrate creates a stronger osmotic gradient than other forms, making it more likely to cause loose stool even at lower doses. 1
• Organic magnesium salts (citrate, glycinate, aspartate, lactate) have higher bioavailability than inorganic forms (oxide, hydroxide), but the citrate form specifically has stronger osmotic effects. 1
• Even magnesium glycinate, which is often marketed as "gentle," can cause gastrointestinal effects like bloating and diarrhea due to its osmotic properties, though these are typically milder than with citrate. 1

Additional Contributing Factors

• Beyond the osmotic effect, magnesium may stimulate release of digestive polypeptides like cholecystokinin and activate nitric oxide synthase, which can enhance intestinal motility and contribute to the laxative effect. 2
• Individual absorption capacity varies—some people absorb less magnesium from the gut, leaving more in the intestinal lumen to exert osmotic effects. 4
• The rate of magnesium release matters: immediate-release formulations dump magnesium into the gut quickly, overwhelming local absorption capacity and maximizing the osmotic effect. 4

Practical Solutions to Minimize Loose Stool

• Divide the daily dose into smaller amounts taken throughout the day rather than a single large dose, as this allows better absorption and reduces the osmotic load at any given time. 1
• Consider switching to a slow-release magnesium formulation, which provides continuous low-dose release over 6 hours and improves absorption while reducing gastrointestinal side effects. 4
• Use liquid or dissolvable formulations rather than pills, as these are better tolerated with fewer gastrointestinal side effects. 1
• Start at the recommended daily allowance (320 mg for women, 420 mg for men) and increase gradually according to tolerance. 1

Critical Caveat

• Patients with significant renal impairment (creatinine clearance <20 mL/min) should avoid all magnesium supplements due to risk of life-threatening hypermagnesemia, as magnesium is excreted renally. 1, 5, 6, 7
• Even in patients with normal renal function, excessive magnesium intake can cause chronic diarrhea—studies show that 4.2% of patients with chronic diarrhea have excessive magnesium ingestion as an important contributing cause. 3