You’ve heard the benefits of adding magnesium to your horse’s diet. They include:
➡ Vital to muscle performance
➡ Important to nerve cell transmission
➡ And can have a calming effect on your horse
This article investigates some of the benefits and drawbacks to the various types of magnesium available.
◆︎ Overview:
Magnesium is a very reactive mineral. It binds with other ions or proteins to be stable. Because of this, there are many choices of compounds available. When contemplating what to feed, there are a number of things to consider: cost of the supplement, the amount to feed, and the bioavailability of that supplement.
You can determine how much magnesium is needed based on the formulation that you purchase. The amount of elemental magnesium in a compound is rarely mentioned. For instance, 10 grams is a common amount of magnesium that many people supplement. But what does that mean for some of the supplement formulations? Because it is bound to something else – a molecule, a protein, an ion – the percentage of magnesium may vary.
For instance:
➡ The percentage of elements in magnesium oxide (magox) consists of magnesium at 60% plus oxygen at 40%. So, to get 10 grams of elemental magnesium, you would need to feed approximately 17 grams of magox.
➡ The percentages in My Best Horse’s magnesium glycinate are magnesium 22% plus glycine 78%. To get 10 grams of elemental magnesium, you would need to feed approximately 45 grams of magnesium glycinate.
➡ If you were to look at Epsom salts, which is magnesium sulfate + water, the percentages are: magnesium at 10%, with sulfate + water being 90%. To get 10 grams of elemental magnesium, you would need to feed 100 grams of Epsom salt.
Significance: When determining the magnesium needed for a horse, it’s important to realize that 20 grams of magox will give approximately 12 grams of magnesium, but 20 grams of magnesium glycinate will give only 4 grams of magnesium.
◆︎ Bioavailability:
The other factor to consider is the amount that will be available after absorption. For the horse, we have data which shows that magnesium oxide, magnesium sulfate, and magnesium carbonate all have about 70% bioavailability.
We do not know the bioavailability numbers for magnesium glycinate in the horse, but in humans, the bioavailability is about the same as for magox, at about 22%.
There are numerous posts on the internet which indicate that magnesium glycinate has a higher bioavailability in humans than magox. The actual published study measured the bioavailability of magnesium glycinate vs magox for people who had an intestinal resection. For the group as a whole, the absorption was the same between the two supplements. However, in 4 of the 12, the magnesium glycinate was superior, with less-than-expected absorption of magnesium oxide. These individuals had short small intestines.
The problem with extrapolating this data to horses is that the horse has a long intestine, so you don’t know if that extra length changes the amount that is absorbed.
◆︎ Other considerations:
Magnesium oxide: Purity can be an issue with magnesium oxide. For magnesium oxide from mineral sources, the most common contaminant is iron, which can be up to 5%. These means that 20 grams of magnesium oxide may contain twice as much iron as 22 lbs of hay (with iron levels of 500 ppm). Giving your horse a magox supplement with a high iron content can worsen your horse’s iron overload. My Best Horse's MagOx Elite is human food grade, with low iron levels.
Magnesium glycinate: While the magnesium glycinate may not have the purity issue of magnesium oxide, we don’t know the actual bioavailability in horses.
Magnesium sulfate: In humans, magnesium sulfate is sometimes used as a laxative. In horses, low levels are well-tolerated. High levels may sometimes be associated with diarrhea.
◆︎ Final considerations:
When supplementing a horse, which form of magnesium to use is a personal choice. However, it’s important to know the amount magnesium that needs to be fed to achieve the desired results.
We do the best that we can for our horses. Understanding the amount of a mineral in our supplements is a part of that.
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Disclaimer: Statements regarding dietary supplements have not been evaluated by the FDA and are not intended to diagnose, treat, cure, or prevent any disease or health condition.
©Joan Kulifay 2025