Why Feed Magnesium to Stressed Horses?

The link between stress and magnesium is very well established. Inside the body, high calcium-magnesium ratios intensify the release of stress hormones (catecholamines and corticosteroids) which also further magnesium loss. Magnesium depletion is both a cause and result of stress, creating a reinforcing loop.Focuz

Many factors will affect your horses’ magnesium status, some are dietary. For example, feed manufacturers tend to use magnesium oxide to boost content even though its bioavailability is questionable2. Diets high in calcium, phosphorous or fat (often used to add ‘non-heating’ calories) have also been shown to hinder magnesium absorption.

More individual factors include conditions known to waste magnesium such as insulin resistance. Certain drugs, particularly corticosteroids, have the same wasting effect, while omeprazole (used to treat gastric ulcers) inhibits magnesium absorption.

Probably far more significant however, is the fact that stress depletes magnesium. Anxiety, excitement, depression, pain, isolation, noise, physical exertion, heat, cold, physical trauma and hunger are all recognised causes of stress which deplete magnesium.

Our focus is not deficiency, which is very rare, but factors that may cause a transient shortfall in magnesium and whether supplementation could have a protective role in relation to stress.

Classen et al3 discuss a number of studies in animal models of stress. Magnesium enriched diets reduced stress induced gastric lesions in mice and reduced sudden death of pigs during transport stress. Mice stressed by immobilisation showed greatly reduced secretion of stress hormones when supplemented with magnesium aspartate hydrochloride (MAH®). Similarly, heat stressed laying hens showed less weight loss when injected with MAH® compared to controls4.

A review of literature by M.S. Seelig1 discusses a number of other examples. Workers in a high noise environment and students preparing for final examinations experienced a rise in stress (blood pressure) on diets providing about 5mg/Kg/day – well above RDA – but there was no rise in blood pressure when magnesium was supplemented to increase daily intake to 6-7mg/kg. A study in mice showed that only slight magnesium deficit resulted in reduced swimming endurance compared to the supplemented group. What is startling is that the amount of magnesium needed to improve endurance was ten times greater in mice swimming under severe stress. In a study of healthy, trained subjects who had undertaken a 120km march, 89% showed magnesium depletion 72hours after the march and lowered levels were seen to persist for three months, despite a diet which provided over 700mg/day (nearly double RDA).
All of these studies are examples of a normal diet, providing plentiful levels of magnesium, not being enough to meet the additional demands of stress.

While there are numerous factors which contribute toward a horse’s behaviour, the huge amount of stress we subject them to should certainly be a primary concern, especially as it also affects their overall wellbeing.

A study at the Department of Veterinary Medicine, Justus Liebig University (Germany) using 63 mares found that those supplemented with 20mg/Kg magnesium (magnesium aspartate) for 8 weeks showed lower blood lactate levels after treadmill exercise compared to controls. The author concluded that ‘the normal diet was evidently poor in magnesium’. In a further study of 31 mares, 16 were supplemented with magnesium (MAH®) at 20mg/Kg for four weeks prior to transport and training. The magnesium supplemented group showed reduced heart rate in response to mental stress compared to controls. These studies were carried out in association with a manufacturer but in an independent study at the Royal Agricultural College, magnesium (again as MAH®) was found to reduce blink rates in horses stressed by triggers such as noise and isolation. More research is needed to confirm these results with larger sampling groups and greater control over variables; unfortunately horses make for incredibly difficult test animals!

There is a lot more we need to understand about the role of magnesium in relation to physical and mental stress, especially among performance horses. In the meantime, magnesium provides a safe calming option for owners that should not be viewed as a shortcut (in the way that more drowsy options might). Assessing the results you get remains the best way to tell if your horse would benefit from supplementation.

1. Seelig, M. Consequences of Magnesium deficiency on the Enhancement of Stress Reactions; Preventative and Therapeutic Implications (A review). Journal of American College of Nutrition (1994) 13:5 429-446
2. Ellen Kienzle and Natalie Zorn. Bioavailability of Minerals in the Horse. Proceeding of the 3rd European Equine Nutrition & Health Congress, Mar 2006 – Ghent University, Merelbeke, Belgium
3. Classen, H.G. Fisher, G. Marx, J. Schimatschek, H. Schmid, C. Stein, C. Prevention of Stress-Induced Damage in Experimental Animals and Livestock by Momomagnesium-L-Aspartate Hydrochloride. Magnesium (1987) 6: 34-39
4. Donoghue, D.J. Krueger, W.F. Conoghue, A.M. and Bryd, J.A. Magnesium-Aspartate-Hydrochloride Reduces Weight Loss in Heat Stressed Laying Hens. Poultry Science (1990) 69:1862-1868

Author: The Editor

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