Dangers in giving magnesium sulfate intravenously to horses highlighted in study

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Intravenous magnesium has been used in equine events as a calming and performance-enhancing agent, with the ultimate goal of giving a competitive edge to horses that receive it.
Photo by Sarah Olive

Intravenous administration of magnesium sulfate to horses to provide a competitive edge is a nefarious and dangerous practice, say researchers.

Magnesium is an essential macroelement involved in many physiological processes.

In equine medicine, intravenous magnesium sulfate is used to treat heart rhythm irregularities, neurological disorders, low magnesium levels in the blood that might arise during critical illness, and for low levels of calcium in the blood which are not responding to other treatments.

However, more recently, intravenous magnesium has been used in equine events as a calming and performance-enhancing agent, with the ultimate goal of giving a competitive edge to horses that receive it. It is a practice forbidden at FEI events under the international governing body’s anti-doping rules because of its calming and metabolic effects.

Researchers at Ohio State University set out in their study to determine the effects of intravenous magnesium sulfate on serum calcium-regulating hormones and plasma and urinary electrolytes in healthy horses.

Stephen Schumacher and his fellow researchers used 12 healthy Standardbred mares from the university’s teaching herd.

The horses had no history of illness six months before the study, were under a routine health program, and were considered healthy based on physical examination, hematology, serum chemistry, and urine analysis. They were fed the same diet of grass, meadow hay, and alfalfa hay, and received no concentrate feed.

All horses were given a single intravenous dose of magnesium sulfate over a period of five minutes at a level that would induce hypermagnesemia (excess amounts of magnesium in the bloodstream).

Baseline blood samples were taken beforehand, with a further 11 samples taken regularly over the following six hours for analysis.

The dose rate was chosen because it would increase concentrations of ionized magnesium three-fold over baseline values, which is expected to activate the calcium-sensing receptor, and is also the dose anecdotally used for its calming effects at equestrian events.

The study team, reporting in the journal PLOS ONE, found that plasma concentrations of ionized magnesium increased 3.7-fold over baseline values at five minutes and remained elevated for 120 minutes. Concentrations of ionized calcium decreased from 30 to 60 minutes, but sodium, potassium and chloride concentrations did not change.

Serum concentrations of parathyroid hormone dropped initially but rebounded and remained elevated from 30 to 60 minutes, while alcitonin concentrations increased at 5 minutes, but returned to baseline by 10 minutes.

The urinary fractional excretion of magnesium, calcium, sodium, potassium and chloride increased, while urine osmolality (a measure of how much one substance has dissolved in another substance) decreased from 30–60 minutes compared to thr baseline.

They said a single intravenous administration of magnesium sulfate to healthy horses caused prolonged hypermagnesemia, a transient decrease in ionized calcium concentrations, and rapid changes in calcium-regulating hormones, increasing the urinary excretion of electrolytes.

“This information enhances our understanding of equine magnesium and calcium biology and has clinical implications,” they said.

“For example, based on information from this study, it would be of interest to know whether prolonged hypermagnesemia from excessive magnesium supplementation leads to electrolyte depletion, water wasting, altered calcium homeostasis, with subsequent clinical consequences, including changes in neuromuscular activity, cardiovascular function, and behavior.”

The authors noted that oral magnesium supplements are used by horse owners and trainers to enhance systemic health and, in general, is considered a low risk of causing hypermagnesemia because doses are low.

“One can speculate that chronic hypermagnesemia in horses could alter calcium and electrolyte homeostasis as well as neuromuscular activity, however, high magnesium doses for a long time would be required. This would be a valid question to investigate to further understand magnesium biology in healthy and sick horses.”

They continued: “It is important to emphasize that intravenous administration of magnesium sulfate to horses to give a competitive edge is a nefarious and dangerous practice.”

The authors noted that while no effect of hypermagnesemia on plasma sodium, potassium and chloride concentrations was documented, its evident effect on urinary electrolyte excretion indicates that, in the short term, other homeostatic systems could maintain their concentrations.

The full Ohio State University study team comprised Schumacher, Ahmed Kamr, Jeffrey Lakritz, Teresa Burns, Alicia Bertone and Ramiro Toribio.

Schumacher SA, Kamr AM, Lakritz J, Burns TA, Bertone AL, Toribio RE (2021) Effects of intravenous magnesium sulfate on serum calcium-regulating hormones and plasma and urinary electrolytes in healthy horses. PLoS ONE 16(6): e0247542. https://doi.org/10.1371/journal.pone.0247542

The study, published under a Creative Commons License, can be read here

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