Magnesium is a mineral involved in over 300 biochemical reactions, supporting functions from nerve and muscle regulation to energy production. Supplementation is common, but a frequent side effect is gastrointestinal distress, often diarrhea. This discomfort is largely dependent on the specific chemical form of magnesium consumed. Understanding how different magnesium compounds interact with the digestive system is key to finding a well-tolerated supplement.
Understanding the Laxative Effect
The digestive side effects of certain magnesium supplements stem from how much of the mineral is absorbed in the small intestine. Unabsorbed magnesium ions remain in the intestinal lumen, creating a concentration gradient. This is known as the osmotic effect, where the high concentration of magnesium draws water from the body’s tissues into the intestines.
The influx of water increases the volume and fluidity of the intestinal contents. This distension stimulates peristalsis, the muscle contractions that propel matter through the digestive tract. The combination of softer stool and increased motility results in a laxative effect. The severity of this response depends on the compound’s bioavailability, or the percentage of the mineral absorbed before reaching the colon.
Magnesium Forms That Are Gentle on the Stomach
Forms of magnesium gentle on the stomach are designed for high bioavailability. They are efficiently absorbed in the small intestine, leaving little unabsorbed mineral to create the osmotic effect. These are often chelated compounds, where the magnesium ion is bonded to an amino acid or an organic acid to facilitate transport across the intestinal wall.
Magnesium Glycinate
Magnesium Glycinate (sometimes labeled bisglycinate) is highly recommended for sensitive digestive systems. Magnesium is chelated to the amino acid glycine, which is transported via pathways that result in better uptake and fewer free magnesium ions in the gut. The glycine component may also contribute to a calming effect, making this a popular choice for nighttime supplementation.
Other Highly Bioavailable Forms
Another well-tolerated option is Magnesium L-Threonate. While often marketed for its cognitive benefits due to its ability to cross the blood-brain barrier, its high absorption rate makes it less likely to trigger the laxative effect. Magnesium Malate, a compound where the mineral is bound to malic acid, is also considered highly bioavailable and well-tolerated. The malic acid helps keep the magnesium soluble in the gut, supporting absorption and reducing digestive upset.
Magnesium Forms Used Primarily as Laxatives
Conversely, the forms most likely to cause diarrhea are those with inherently poor absorption rates, ensuring a large portion of the magnesium reaches the colon to trigger the osmotic effect. These forms are typically inexpensive and are often intentionally used as over-the-counter laxatives or antacids.
Magnesium Oxide
Magnesium Oxide is a prime example, possessing a very low fractional absorption rate, sometimes estimated to be as low as 4%. This poor uptake means the majority of the ingested magnesium remains in the digestive tract, making it an effective and common agent for treating constipation. This same poor absorption is precisely why it causes significant gastrointestinal side effects when taken for daily supplementation.
Magnesium Citrate and Sulfate
Magnesium Citrate is another widely available form that acts as a saline laxative, actively drawing water into the intestines. While its bioavailability is higher than that of magnesium oxide, it is frequently used for its reliable laxative action, including for cleansing the bowel before medical procedures. Magnesium Sulfate, commonly known as Epsom salt, is the most potent of the group, used specifically to induce a strong, watery bowel movement due to its rapid osmotic action. When seeking a daily supplement, these forms should be avoided if digestive sensitivity is a concern.
Maximizing Absorption and Minimizing Side Effects
Even when choosing a highly absorbable form of magnesium, certain strategies can further reduce the chance of experiencing digestive side effects. The most important factor in avoiding diarrhea is managing the total dose of elemental magnesium taken at any one time. Since the body’s capacity to absorb the mineral is dose-dependent, a higher single dose results in a lower percentage of absorption and more unabsorbed ions in the gut.
To counter this, splitting the daily intake into two or three smaller doses throughout the day can significantly improve overall absorption and tolerability. Taking the supplement with food also helps by slowing down the digestive transit time and diluting the magnesium compound, which minimizes the potential for stomach irritation. For most adults, the Tolerable Upper Intake Level for magnesium from supplements is set at 350 milligrams of elemental magnesium per day, a level designed to prevent the laxative and other adverse effects.