Calming supplements are one of the top categories of a popular equine supplement company, and magnesium is one of the most common ingredients in these products. Magnesium does appear to play a role in normal brain and motor neuron (the nerves that send signals to muscle) function, as evidenced by several conditions seen with magnesium deficiency. The question then is does magnesium supplementation increase brain function, muscle activity or affect equine behaviour?

In a thorough search of the scientific literature, I was only able to find one abstract that examined the role of magnesium supplementation on equine behaviour, and specifically reaction speed was quantified with horses supplemented with either 2.5 or 10 grams of magnesium aspartate.

The study found only a trend of an effect for overall supplementation, but did report a statistical difference in reaction time comparing the control vs. the 10-gram dose (the high dose slowed reaction time – is this a good thing?).

Magnesium has also been recommended for head shaking, but the only published study I could find that included a magnesium supplement found no benefit.

Magnesium is also found in most “metabolic” support supplements, as some research in humans have found that magnesium can help improve glucose and insulin dynamics. The only published study I could find investigating the effect of magnesium (and chromium) supplementation on horses did not improve insulin sensitivity or body fat measurements.

Some studies report that as intake of magnesium increases so does fecal excretion, suggesting that excessive dietary magnesium isn’t even absorbed.

Others report that supplemented magnesium is absorbed, and results in increases in both blood magnesium and urinary magnesium excretion. However, supplementation of magnesium also appears to decrease blood sodium and increase calcium excretion. It is also reported that the only form of magnesium that is absorbed is magnesium-aspartate, while most supplements contain magnesium-oxide.

So, do you even need to supplement magnesium? The National Research Council’s Nutrient Requirements of Horses (2007) recommends mature idle horses (eg. 500 kg body weight horse) need 7.5 grams of magnesium per day, while horses at heavy work need 15 grams. The average amount of magnesium in grass hay is 0.18%, so an idle horse could meet his magnesium requirements with only 4 kilograms of hay (4000 g of hay x 0.0018 = 7.2g), and a horse in heavy work could meet it with 8.3 kilograms of grass hay – both amounts of hay that would be easily consumed by these types of horses. Magnesium is also found in many cereal grains and is found in many commercial feeds at amounts of 0.2 – 0.4%. Therefore, to meet magnesium requirements, good quality feed in a balanced diet is sufficient.

Because there is only limited evidence that magnesium supplementation will in fact calm horses (and how calm would you want your horse to be if you are riding them?) and no evidence that magnesium improves insulin sensitivity, there is little justification for its supplementation, particularly when most commercial products do not use a form that is bioavailable. Also, because of the potential for interaction with other, perhaps more “important” minerals (calcium and sodium), magnesium supplementation should be cautioned. This becomes critical when horses are already fed a good diet (already meeting magnesium requirements), and are fed multiple supplements (for example both a calming supplement AND a metabolic supplement, and perhaps also general vitamin-mineral supplement) thereby increasing magnesium intake to potentially dangerous levels.