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Magnesium is used as a food supplement in human diabetes patients and may be beneficial in horses with insulin resistance as well.
So far, magnesium supplementation (30 mg/kg) is routinely used in high performance horses but dose-effect relations have not been determined.
Therefore in this study the magnesium concentration in blood lymphocytes, serum, urine and the fractional excretion in the urine were measured in five horses before and after oral supplementation for seven days with 15, 30 and 60 mg/kg magnesium as magnesiumaspartate-hydrochloride (MAH). All horses were exclusively fed with hay, its magnesium concentration was measured each week. All horses were examined daily, including signs of colic and soft feces.
There were no significant changes in the magnesium concentration in the diet. Signs of colic, soft feces or other side effects were not observed. The serum magnesium concentration after supplementation with 60 mg/kg magnesium as MAH was significantly higher than the baseline (Wilcoxon signed-rank test) and exceeded the reference range in four of five horses. After 15 and 30 mg/kg magnesium as MAH the serum magnesium concentration showed a significant increase over time (Friedman test). Neither a significant increase of the intracellular magnesium concentration nor significant changes in urine magnesium concentration and fractional excretion were observed. The dosages of 15 and 30 mg/kg magnesium as MAH require a longer supplementation and observation period but seem so far effective to raise the serum magnesium concentration and safe to use. 60 mg/kg magnesium as MAH could lead to side effects after long-term supplementation.