Magnesium oxide (MgO) is a common form of magnesium found in dietary supplements. The amount of elemental magnesium within the compound is key to understanding a supplement's potential effectiveness, but bioavailability is equally important.
The Calculation: Determining the Elemental Magnesium Percentage
The elemental magnesium content in magnesium oxide can be calculated using the atomic weights of magnesium (Mg) and oxygen (O). Magnesium has a molar mass of about 24.305 g/mol, and oxygen is 15.999 g/mol. The molar mass of MgO is the sum, 40.304 g/mol. Dividing the mass of magnesium by the total molar mass (24.305 / 40.304) and multiplying by 100% yields approximately 60.30% elemental magnesium. This high percentage makes magnesium oxide one of the most magnesium-dense forms available.
Bioavailability: Why High Elemental Content Is Misleading
Despite its high elemental content, magnesium oxide has low bioavailability, with absorption rates around 4%. This is due to its poor water solubility, limiting its absorption in the digestive system. Unabsorbed magnesium oxide draws water into the colon, resulting in a laxative effect. Therefore, it is mainly used as a laxative and antacid, not for effectively raising systemic magnesium levels. More bioavailable forms are better suited for addressing deficiencies.
Common Uses and Considerations
Magnesium oxide is useful for occasional constipation relief due to its laxative effect and as an antacid to neutralize stomach acid. It has also been studied for migraine prevention. Its high concentration can be beneficial in tablet formulations. However, consulting a healthcare professional is important, especially for long-term use, as it can cause hypermagnesemia, particularly in those with kidney issues.
Choosing the Right Magnesium Supplement
Choosing a magnesium supplement depends on your health goals. Consider this comparison of common forms:
| Feature | Magnesium Oxide | Magnesium Citrate | Magnesium Glycinate |
|---|---|---|---|
| Elemental Content (Approx.) | 60% | 16% | 14% |
| Bioavailability | Very low (~4%) | Moderate-High | High |
| Primary Use | Laxative, Antacid | Constipation, Raising Levels | Increasing Levels, Relaxation |
| Digestive Impact | Can cause diarrhea | Mildly laxative | Easy on the stomach |
| Cost | Low | Moderate | Moderate to High |
For constipation or heartburn, magnesium oxide is a cost-effective, short-term option. For increasing systemic magnesium levels or seeking benefits like relaxation, more bioavailable forms like magnesium glycinate or citrate are preferable.
Which form is right for you?
- For high absorption: Choose magnesium citrate or glycinate.
- For constipation relief: Magnesium oxide is an effective, short-term option.
- For gentle supplementation: Magnesium glycinate is known for being easy on the digestive system.
- For cost-effectiveness: Magnesium oxide is typically a more affordable option per milligram of elemental magnesium, though much of it is not absorbed.
Conclusion
Magnesium oxide contains approximately 60% elemental magnesium by weight, but its low bioavailability (around 4% absorption) means it is not effective for increasing the body's overall magnesium levels. Its primary uses are for its laxative and antacid effects. For systemic magnesium supplementation, forms with higher absorption rates, such as magnesium citrate or glycinate, are better choices. Understanding the difference between elemental content and bioavailability is crucial for selecting the appropriate magnesium supplement. For further information, see research on magnesium oxide from the National Institutes of Health.
