Does Glycine Slow Gut Motility?
No, glycine itself does not slow gut motility like magnesium does—the laxative effect of magnesium glycinate comes entirely from the magnesium component, not the glycine. The glycine amino acid is simply the carrier molecule that improves magnesium absorption and tolerability.
Understanding the Mechanism
Magnesium causes the laxative effect through osmotic action, regardless of which salt form it's bound to. Magnesium citrate, magnesium oxide, and magnesium glycinate all draw water into the intestines through osmotic gradient creation, though the strength of this effect varies by formulation 1. The glycinate portion is metabolically inert and does not contribute to gastrointestinal motility changes 2.
Why Magnesium Glycinate May Seem Different
Magnesium glycinate causes less pronounced diarrhea compared to magnesium citrate or oxide because it has superior bioavailability—more magnesium gets absorbed before reaching the colon where osmotic diarrhea occurs 1, 3.
The better absorption profile means less unabsorbed magnesium remains in the intestinal lumen to exert osmotic effects 1.
If constipation is your primary concern, magnesium citrate is actually preferable due to its stronger osmotic effect 1.
What Glycine Actually Does
Glycine is a small, non-essential amino acid that serves as a carrier molecule to improve magnesium absorption 2.
Glycine has demonstrated cytoprotective and anti-inflammatory properties in the GI tract—it improves gastroduodenal tolerability of medications but does not directly affect motility 2, 4.
In one controlled study, glycine addition to aspirin eliminated dyspeptic symptoms without changing endoscopic findings, suggesting it improves mucosal tolerance rather than altering motility 4.
The Research on Glycine and Motility
One animal study found that chronic magnesium glycinate administration decreased intestinal motility through beta-adrenergic receptor pathways, but this effect was attributed to the magnesium component, not glycine 5. The study specifically examined magnesium's relaxant effects on smooth muscle, which mirrors its known effects on skeletal muscle 5.
Clinical Decision Algorithm
For patients with gastrointestinal issues:
If you need magnesium supplementation WITHOUT worsening diarrhea: Choose magnesium glycinate at 320-420 mg elemental magnesium daily, as it has the best absorption with minimal laxative effect 1, 3.
If constipation is the primary problem: Choose magnesium citrate or magnesium oxide (500-1000 mg daily) for stronger osmotic effects 1, 3.
If you cannot tolerate any oral magnesium without loose stools: Use magnesium oxide capsules taken at night when intestinal transit is slowest, or consider parenteral administration 6.
Critical Precautions
Avoid all magnesium supplementation if creatinine clearance is <20 mL/min due to life-threatening hypermagnesemia risk 1, 3.
All magnesium forms can cause loose stools through osmotic effects—the difference is only in degree, not mechanism 6.
Drug interactions occur with the magnesium component, not glycine—magnesium can interfere with tetracycline absorption and is affected by diuretics and proton pump inhibitors 7.