Oral magnesium supplementation alters blood and urine electrolyte concentrations in horses

Oral magnesium (Mg) supplementation in horses has been heavily promoted as a calmative by several sectors of the equine industry. However, its interaction with other major cations, especially calcium (Ca), should be considered as additional Mg may alter blood concentrations and/or urinary excretion. As part of a study into the potential efficacy of oral Mg supplementation as a calmative, the effects of Mg supplementation on Mg, Ca, potassium (K) and sodium (Na) concentrations in blood and urine were investigated. Six mature Standardbred geldings were used in a Latin square design with each horse acting as its own control. All horses were fed a control ration comprising low energy pellets (Cool Conditioner, CopRice) plus clover/ryegrass hay fed at 2.0% BW (total mean Mg: 11.2 ± 2.0 g). Three treatments were used in the trial: control ration, control ration with 2.5 g added Mg (as magnesium aspartate) and control ration with 10 g added Mg. Horses were fed each ration in 2 meals per day for 7 d. All horses were kept in individual dry lots that allowed free exercise. On Day 6, a series of blood and urine samples were collected at allocated times over 24 h to determine Mg, Ca, K and Na blood concentrations and urinary excretion. Repeated measures ANOVA of blood and urine concentrations at allocated sampling times showed an increase in total whole blood Mg concentrations between the control group and the treatment groups (P < 0.001) over the 24 h period, with the greatest difference between the control and the 10 g added Mg occurring one hour after consuming the Mg supplement. There were no differences in whole blood Ca and K but significantly lower Na concentrations for both Mg supplemented groups over the 24 h time period compared with the control group (P < 0.001). Urinary Mg excretion increased with Mg supplementation being significantly higher (P < 0.05) in 2, 4 and 6 h post feeding samples with 10 g supplementation and at 6 h post feeding with 2.5 g supplementation. Urinary Ca concentrations were also significantly higher (P < 0.02) over the 24 h sampling period for the 10 g supplementation with the greatest increase seen at 2 h post feeding. There was no effect on urinary K excretion overall with Mg supplementation. A clear diurnal excretion pattern was found for Na but no effect of the Mg supplementation. This study confirms that oral Mg supplementation has an effect on the blood and urine concentrations of other electrolytes as well as Mg itself. The potential implications of an increased urinary Ca excretion with oral supplementation of Mg above NRC recommendations need to be considered further.