Magnesium oxide (MgO) is a common compound used in dietary supplements and medications, serving as a concentrated source of the mineral magnesium. When evaluating any magnesium product, it is important to distinguish between the total weight of the compound and the actual amount of the pure mineral, known as elemental magnesium. This elemental content determines how much of the mineral you are truly consuming, and magnesium oxide is one of the most concentrated forms available, consisting of approximately \(60\%\) elemental magnesium by weight.
Calculating Elemental Magnesium Content
The high percentage of elemental magnesium in magnesium oxide results from its simple molecular structure. Magnesium oxide is formed by one atom of magnesium (Mg) chemically bonded to one atom of oxygen (O). The elemental percentage is determined by calculating the atomic weights of the constituent elements.
The atomic weight of magnesium is approximately \(24.3\) grams per mole, and oxygen is about \(16.0\) grams per mole. Adding these figures yields the total molar mass of the magnesium oxide compound (MgO), which is approximately \(40.3\) grams per mole. This total mass represents the entire compound.
To find the percentage of magnesium, the atomic weight of magnesium (\(24.3\)) is divided by the total molar mass (\(40.3\)). The simple mathematical ratio of \(24.3\) divided by \(40.3\) yields a value of approximately \(0.603\). This means magnesium accounts for roughly \(60.3\%\) of the total mass of the compound. This characteristic makes magnesium oxide a dense source of the mineral compared to other forms.
Interpreting Supplement Labels
Understanding the difference between compound weight and elemental weight is necessary for correct supplement dosing. Supplement labels must legally list the amount of elemental magnesium present in each dose, which is the figure consumers should rely on. For example, a tablet containing \(500\text{ mg}\) of the magnesium oxide compound does not provide \(500\text{ mg}\) of the pure mineral.
Since magnesium oxide is about \(60\%\) elemental magnesium, a \(500\text{ mg}\) tablet of the compound provides approximately \(300\text{ mg}\) of elemental magnesium. This elemental weight should be compared against the recommended daily intake. Other magnesium compounds, such as magnesium glycinate or citrate, often have a lower elemental percentage. This means a larger pill weight is needed to deliver the same amount of the pure mineral.
Bioavailability and Absorption Rates
Despite its \(60\%\) elemental content, magnesium oxide has low bioavailability compared to other magnesium forms. Bioavailability refers to the proportion of the ingested substance absorbed and utilized by the body for systemic functions. The primary reason for its poor absorption is its low solubility in water, which hinders dissolution in the gastrointestinal tract.
Studies show the fractional absorption rate of magnesium oxide can be as low as \(4\%\). This is significantly lower than more soluble organic salts like magnesium citrate or glycinate. Since a large portion of the compound remains undissolved, it cannot be easily absorbed through the intestinal walls into the bloodstream. Highly soluble forms dissolve readily, allowing magnesium ions to be absorbed more efficiently.
This low absorption rate impacts individuals seeking to correct a systemic magnesium deficiency. Although a high dose of the elemental mineral is present, only a small fraction enters the bloodstream to support functions like nerve function or muscle relaxation. Forms with higher solubility and absorption rates are recommended for raising overall magnesium levels. The unabsorbed magnesium remains within the gut, leading to localized effects.
Primary Uses of Magnesium Oxide
The low systemic absorption of magnesium oxide dictates the compound’s primary therapeutic uses. Since most of the ingested magnesium oxide remains unabsorbed, it exerts its effects locally within the digestive system. This localized action makes it highly effective as both a laxative and an antacid.
Magnesium Oxide as a Laxative
As a laxative, magnesium oxide works as an osmotic agent, drawing water from surrounding tissues into the intestine. This increase in water volume softens the stool and promotes a bowel movement, providing relief for constipation. Low absorption into the bloodstream is beneficial here, as the magnesium remains in the colon to perform its osmotic function.
Magnesium Oxide as an Antacid
Magnesium oxide is also widely used as an antacid to treat heartburn, acid indigestion, and sour stomach. When it reaches the stomach, it acts as an alkali, neutralizing excess gastric acid and raising the stomach’s pH. These localized digestive applications are where magnesium oxide excels, contrasting with more bioavailable forms preferred for addressing nutritional deficiencies.