Some horses and ponies show symptoms of Pre-Cushings (PC) or Cushings-like syndrome. The condition is characterized by lumpy abnormal fat deposits on the neck, above the tail head and on the shoulders. Unlike a true Cushings sufferer these horses will usually have a normal ACTH response but may well show elevated insulin.
The horse may or may not have the extra thick hair coat but the hair will appear to be harsh and staring. There will be either a tendency to founder or a history of founder. These PC horses respond well to additional Mg in their diets.
It should be noted that blood Mg levels normally rise following glucose ingestion or following a meal high in simple carbohydrates, indicating that Mg is involved with the action of insulin to clear the glucose from the blood. The Pre-Cushings horses can be considered the equivalent of a human type II diabetic. Type II diabetes is a resistance to insulin, rather than a true insulin deficiency. The PC horse may also show elevated insulin levels following a carbohydrate meal. These horses will do best if also fed a low glycemic diet (one low in simple carbs; see accompanying box), but the supplementation of Mg will be beneficial even if the low glycemic diet is not fed.
Magnesium may help to protect the horses from founder by protecting peripheral circulation; by preventing the arterioles from contracting and causing the arterio-venous shunt.
Grass in the early spring is often low in minerals and high in sugar content -- this may be why spring grass is often associated with founder in grazing horses and ponies.
Magnesium Physiology and Biochemistry
Clinical laboratories typically measure Mg as total serum Mg, of which approximately 50% is bound to either plasma protein, or is complexed with ions such as phosphate or citrate. And about 50% is present in the ionized or active form. Although serum Mg usually does not accurately reflect the overall body Mg balance, deficiency symptoms correlate well with serum levels of
Magnesium, commonly written as Mg++ is a bivalent cation. It is found in the periodic table in the same family as Calcium, another important mineral it is closely associated with.
Mg is an important ion required by the body in relatively large amounts. It is essential for the optimal function of over 300 key enzymes involved in energy transformation, protein synthesis and nucleic acid metabolism. It is also essential for the stability and normal function of the cell membranes of excitable tissues, such as nerve and muscle. Thus Mg deficiencies or abnormalities have a profound effect on neuromuscular function and cardiac tissue. In addition, a normal body Mg content is necessary for the maintenance of electrolyte balance, particularly for Calcium (Ca++) and Potassium (K+).
As well as being important for the function of nerve and muscle, Mg is involved in the formation of Hydroxyapatite, one of the principle components of bone and a major "hardener" of bone. About 50% of the body content of Mg is found in the skeleton.
Mg is very important as a co-factor in enzymes that control the metabolism of carbohydrates, most specifically the enzymes that are involved in insulin response. Cells that are deficient in Mg show impaired CHO metabolism and a reduced insulin response.
Magnesium and RER
Within the muscle cell, Mg and Ca have antagonistic functions. The Ca ion is released during muscle contraction and binds to the actin-myosin complex, 'locking' it in the shortened or contracted state. When relaxation of the muscle is required, Mg ions are released and 'knock' the Ca from the binding site allowing the actin-myosin complex to relax back to the inactive or non-contracted state. Hence, low muscle levels of Mg are associated with tetanic muscle states, e.g., muscle spasms or muscles that cannot relax or return to the normal state. An excess of Ca or a deficiency of Mg can both cause a temporary muscle tetany in horses called tying-up or Recurrent Exertional Rhabdomyolosis (RER).
In horses who suffer from RER, characterized by a chronic tying-up during work sessions, supplementation with Mg will help to prevent the symptoms. It is important to distinguish RER from the tying-up that is caused by an inability to handle simple carbohydrates, called Equine Polysaccharide Storage Myopathy (EPSM), as this condition is unlikely to respond to Mg. EPSM should be handled by a low glycemic diet. Clinically, Mg has been used to treat cases of spontaneous smooth muscle contraction in humans. This includes conditions such as asthma, preeclampsia and eclampsia, seizures and other problems with muscle spasms and arrhythmias.
Mg is easily absorbed from the GI tract, and may have at least one carrier that is Mg specific and one that is competitive with calcium. Hence, Mg and Ca in the diet need to be balanced with the ratio ideally being 1:1. In practical fact, Ca is usually well supplied in horse diets and Mg tends to be low, so it is considered better to supplement the Mg and assume that the diet will provide sufficient Ca.
Modern day diets are often low in magnesium and the high stress lifestyle of some equines leads to an increased need for magnesium. Areas with acid soils and soft water may not provide enough Mg in water and soil for the needs of performance horses. In a study at See page 14 VA Tech some years ago, blood was drawn from foals in all parts of Virginia, all breeds and types, from the Blue Ridge to the Chesapeake. The blood was assayed for mineral status, across the board; the Mg levels were 1.8-1.9 mg/dl, with some as low as 1.7 mg/dl. This indicates a statewide incidence of low magnesium. This hypomagnesemia is more prevalent up and down both coasts of the USA and anywhere there are clay soils or a granite base. Areas with a limestone base or alkaline soils have a less acute problem with low Magnesium.
Magnesium can be supplied in several different forms. There are several Mg salts readily available, but the best supplements offer the chelated forms. Chelation means the Mg is bound to a highly absorbable molecule like an amino acid. As the amino acid is actively picked up from the GI tract, the Mg comes with it. The non-organic forms of Mg salts are well absorbed but can be interfered with by the presence of other bivalent cations like Calcium. The non-chelated forms are usually cheaper to buy, and can supply useful magnesium at low cost, but for clinical use the chelated forms are much more effective.
The most common form available is Magnesium sulfate, commonly called Epsom salts. Epsom salts can be a useful source of magnesium on an emergency basis, but are not suitable for regular feeding. One side effect of regular feeding of Epsom salts is high GI tract motility (aka diarrhea). The usual form used in animal feeds in Magnesium Oxide, a fine white powder. MgO is about 50% absorbable depending on the relative levels of Ca also present.
Magnesium is non-toxic and is impossible to overdose by mouth. The GI tract absorbs what it needs and the rest is dumped. Overfeeding Mg in the form of salts usually just results in extensive manure. Excess MgO or MgSO4 produces a transient diarrhea, as the GI tract dumps the excess. The main pathway for the deposition of Mg is via the kidneys. Salts of Mg are very water-soluble and Mg is easily excreted, but if renal function is impaired there may result an inability to dispose of Mg and hence an overdose. Thus, Magnesium salts should not be fed to horses with impaired kidney function or those with restricted water intakes.
This response to Mg in Pre-Cushings horses has not been determined by any proper scientific studies; so far, all evidence is anecdotal. However, there is some historical precedent for it, as there is an old horseman's trick of giving magnesium salts (usually mag sulfate, aka Epsom salts) to horses that are foundering or in danger of foundering. We have fed the Mg supplement to more than 1,000 horses so far and most have shown positive results. While we have not done any statistical analysis of the work, in such numbers the evidence must count for something!
For further information, contact Dr Melyni Worth at firstname.lastname@example.org . Or call 540-942-4500.