The importance of magnesium was ignored for many years but appreciation has grown with our understanding of the vital functions it fulfills.
Magnesium normally has a stabilizing effect in the nervous system
Like calcium and phosphorus in the skeleton, magnesium has an important structural role as a component of bone, cell membranes and chromosomes.
Minerals as cofactors are associated with enzyme systems, usually at their active centers. Magnesium performs this function for hundreds of different enzymes. It is required for the synthesis of the major energy compound, ATP, and for its stable storage as MgATP. Magnesium is also essential for the production of protein, carbohydrates, lipids, DNA, RNA and the antioxidant glutathione.
Magnesium exists in the body in two forms – bound and ionized. Bound magnesium is incorporated into bone’s structure or affixed to ATP, enzymes or blood proteins like albumin. The ionized form is magnesium in the blood or the tissues as a free magnesium ion, with two positive charges – Mg+2.
Ionized magnesium performs many important functions in the horse by competing with ionized calcium, which also has two positive charges – Ca+2. Ionized Ca is an excitatory mineral for nerves that release the chemical [neurotransmittor] acetylcholine. When ionized Ca+2 enters the nerve it triggers release of acetylcholine which in turn activates other nerves or causes muscular contraction, both heart and skeletal muscle. In addition, magnesium in the normal brain blocks access to a receptor called the NMDA receptor which is linked to hyperexcitability.
Magnesium also has important functions in the maintenance of both blood vessels and the lung’s bronchi in a normal, open state. If all of this is not enough, research in people and experimental animals has linked normal magnesium status with helping to maintain glucose and insulin homeostasis.
Obviously optimal intake of magnesium is important. Guaranteeing your horse has correct magnesium levels is a little tricky. Blood (serum) levels are OK for seeing the extremes of high intake or severe depletion but not very sensitive to levels in between which could still be interfering with normal function. Abnormally low levels can be present inside the cells or the fluid surrounding them when blood levels are normal.
A recent German study (Winter et al 2018) measured the magnesium level inside cells, specifically the lymphocyte white blood cells, which is a much more sensitive test. This work started to establish a normal range for horses but more work needs to be done before it is available as a commercial test.
The best test of the horse’s status to date is called fractional clearance of magnesium. This is determined by an equation after measuring concentrations of creatinine and magnesium in both blood and urine simultaneously. The tests are easy to run in any laboratory but obtaining urine is time consuming or invasive and urine obtained after stimulating urination with a diuretic cannot be used.
The best indicator of whether you horse should have supplemental magnesium is a diet analysis, including hay or pasture analysis. The National Research Council has established recommended minimum intakes for all ages and classes/uses. The horse may also benefit from having magnesium intake adjusted to a Ca:Mg ratio of 2:1.
Eleanor Kellon, VMD