Electrolytes – necessary or not?

by Dr Derek Cuddeford,  Royal (Dick) School of Veterinary Studies, University of  Edinburgh

The short answer is yes in terms of bodily functions. They are contained in all feed types in varying proportions and sometimes, they must be added to the daily ration.

What are they? Well, any substance that contains free ions and can conduct electricity. Animals, humans, etc cannot live without them and the main electrolytes are sodium (Na+), potassium (K+), chloride (Cl-), calcium (Ca2+), magnesium (Mg2+), hydrogen carbonate (HCO3-) and hydrogen phosphate (HPO42-). These are substances that ionise/dissociate when placed in a suitable solvent such as water. For example, ordinary salt (NaCl) dissociates as follows:

NaCl= Na+ + Cl-

It should be apparent from the foregoing that electrolytes are charged (+ or -) and they do not all carry the same electrical charge and that the quantities of charge vary. When chemicals dissociate they release charged ions but when they recombine, there is no charge (neutral). The electrolytes are distributed throughout the horse’s body mostly in the liquid phase. About 20% of the horse’s bodyweight is composed of the extracellular fluid. So what constitutes this fluid? There are contributions from various different body fluids including that contained in the joints, the lymph, the plasma, gut fluids and that surrounding the brain. The intracellular fluid accounts for 40% of the horse’s bodyweight! The remaining 40% of the bodyweight is made up of tissues such as bone and muscle. Essentially a horse is a bag of water (80% of its bodyweight) and it is a wonder to me how it can get along… however, your bodily constitution is much about the same so when riding you must both just get into the flow of things!

There is an electrolyte balance between the extracellular and intracellular fluid which is carefully regulated to ensure that a precise gradient exists between the two in order that both nerves and muscles can function correctly. Furthermore, a balance is required to control blood pH (acidity/alkalinity) and to maintain hydration. Tissues such as nerves and muscles are activated by the flow of electrolytes between extra- and intracellular fluid. They enter or leave the cells via specialised routes known as ion channels. Muscle contraction is an example of tissue activity controlled by the animal’s own electricity supply. It depends on the flow of calcium, potassium and sodium ions. As most people are aware muscle contraction can also be achieved by passing an electric current through a muscle thus confirming the electrical nature of body function.

Electrolyte balance or homeostasis is under hormonal control with excesses of electrolytes (eg., Na+) usually lost via the urine. Of great relevance to the horse is a disturbance (or as the Americans might say “a perturbation”) to the system such as dehydration which, if not treated quickly, can give rise to a serious medical emergency. The loss of “water” from the horse’s body means that a lot of important electrolytes (Na+, K+ and Cl-) are lost simultaneously causing significant electrolyte imbalance as well as likely clinical complications. Obviously, under “normal” conditions diet will provide all the electrolytes needed by the animal. Sometimes electrolytes are administered orally via syringe when horses are sweating excessively as in an endurance event. If they become dehydrated it may be necessary to intubate the horses and administer electrolytes and water via this route. However, a clinical incident might require intravenous administration of an electrolyte solution in order to try to restore hydration and electrolyte balance within the body. So we begin to see the crucial role played by electrolytes within the horse’s body; no electrolytes or imbalance=death!

Common electrolyte problems are excesses of sodium and potassium or their deficiencies but other electrolytes such as calcium can be involved. A thumping noise coming from the abdomen of an exercised horse back in 1831 and reported by a vet at that time gave rise to the term, “Thumps”. The condition is also known as synchronous diaphragmatic flutter. This is often recorded in Arabian horses following a long distance endurance event when sweat losses have caused an electrolyte imbalance. Low plasma calcium is often associated with thumps but magnesium, sodium, chlorine and potassium should also be checked.

The electrolyte balance of a food can markedly affect how it is used by the animal. For example, the Dietary Cation Anion Balance or DCAB is normally composed of the electrolytes Na+ + K+-Cl- but of course one can expand this to include other cations and anions such as Ca2+ and SO42-. Some people use slightly different terminology and refer to the Dietary Cation Anion Difference or DCAD. Both DCAB and DCAD give an indication of the acidity or alkalinity of a food/diet. To give you some impression of the effect of this on its “electrical charge” consider forage. Forages are universally high in K+ and thus their DCAB is high in terms of its charge measured as milliequivalents/kg dry matter (mEq/kgDM). Taking a forage like alfalfa we find that its DCAB is very high (>400) and thus it is a highly basic feed. Dairy farmers routinely manipulate the DCAB of their cow’s diet to control calcium metabolism which is always a worry since hypocalcaemia is often a metabolic disease problem amongst high yielding dairy cows. They are fed anywhere between -150 and -250mEq/kg DM prior to calving and this creates a mild acidosis in the body which tries to correct itself by releasing calcium from the bone and thus improving calcium availability. One needs to know calving dates with some precision otherwise this technique, if applied too soon, can exacerbate the problem.

But you are not interested in dairy cows are you? So what does this all mean for the horse? We know that horses would naturally, if given the chance, consume only green forage. What is the DCAB of grass? It contains anywhere between 300 and 400 mEq/kg DM because grass contains so much K+. So it is really no surprise that horses reared on grass alone have the best bone because a basic diet (grass) maximises calcium retention in bone. The best hunters come from Ireland, the Emerald Isle, where they graze grass 24/7 until sold to be trained. Most Thoroughbreds however, apart from a brief spell at grass with mum, will be fed concentrate-based diets (DCAB<100mEq/kg DM) with much less emphasis on forage. The net result is that these animals are consuming an acidic diet because cereals etc are cation poor (eg., K+, Ca2+) and anion rich (eg., Cl-, HPO42-). In other words, they are being fed an acidic diet just like the dairy cow prior to calving resulting in mobilisation and loss of calcium from the bone. This is the very last thing that we want to happen in a growing horse or, in a competing animal. This situation cannot be offset by pouring calcium down their throats as animals fed an acidogenic diet will just excrete any additional cations via the kidneys into the urine.

In conclusion, it should be apparent that electrolytes are very necessary for ourselves and our animals. However, it is not enough just to have the electrolytes in the horse’s diet per se but rather, the balance of electrolytes must be right as it can affect the overall nature of the animal’s diet (DCAB/DCAD) which can have serious physiological consequences for the horse in terms of bone formation etc. Finally, if you find all of this rather hard going then perhaps a drink would help as Dave Berry (an American humourist) saw fit to say “Electricity is actually made up of extremely tiny particles called electrons that you cannot see with the naked eye unless you have been drinking.”

Author: Dr Derek Cuddeford

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