In light of some confusion regarding the safety and usefulness of potassium in supplements, it is worth reviewing the science behind potassium, so that all horse owners have the information, and confidence, to use supplements containing potassium.
Potassium is an important electrolyte, vital for maintaining normal function of muscles and nerves. In fact, every cell in the body contains potassium, as it is vital for maintaining cell volume and electrical activity. Potassium assumes the role as the major positively charged electrolyte inside cells, particularly heart and muscle cells. It works very closely with the other major electrolyte, sodium, in this regard.
Even relatively small changes in total body potassium can negatively affect athletic performance. Because nerve and muscle function is based on maintaining a fairly constant gradient between potassium inside cells and sodium outside cells, the body has sophisticated methods of keeping potassium levels within a narrow normal range.
Exercise in particular, causes large amounts of potassium to leave cells and enter blood. Blood tests only register the potassium levels in circulating blood, and don’t give any indication of potassium levels inside cells. Potassium levels in blood return to normal relatively quickly after exercise, so high blood potassium levels in a normal horse with normal kidney function are usually only transient. In contrast, low blood potassium levels almost certainly reflect low cellular concentrations of potassium as well (hypokalaemia), and performance will almost certainly be reduced or compromised in these horses.
Horse sweat contains calcium, magnesium, some trace elements and protein, and relatively high levels of sodium, chloride and potassium. It is possible to roughly estimate electrolyte loss in sweat, and typical losses in different activities are estimated below:
It is apparent that heavily sweating horses experience large electrolyte losses, as well as large fluid losses. It is also apparent that different work activity produces different electrolyte losses.
The electrolytes have many functions, including maintenance of acid-base balance in body fluids, nerves and muscles. Large electrolyte losses (leading to an electrolyte deficiency) can result in several neuromuscular and systemic disturbances including muscle cramps, tying up, synchronous diaphragmatic flutter (thumps) and systemic alkalosis. Horses with large electrolyte losses may also have reduced sweating rates leading to a reduced ability to manage excess body temperature. In addition, abnormal blood electrolyte levels may play a role in the horse’s thirst reflex and desire to drink. Ironically, when horses sweat a lot and lose a considerable amount of sodium, the thirst response may be depressed, and the horse may not drink sufficient water to maintain adequate body fluid levels.
An excess of electrolytes such as potassium are invariably passed out in urine in the normal horse with normal kidney function, unless horses have genetic diseases such as HYPP in Quarter Horses. An excess of potassium is rarely a problem to horses as the normal feed supply of horses provides a wide variation in potassium levels, and the body has sophisticated metabolic processes to maintain cellular potassium levels within fairly narrow physiological limits.
There are many electrolyte supplements available for horses. Generally, if a horse is eating a balanced diet and is not experiencing extreme electrolyte loss such as with heavy sweating, intense or long exercise periods, humid tropical conditions, or diarrhoea, then electrolyte supplementation may be of questionable value in many horses. If heavy electrolyte losses are anticipated then a supplement can be of significant value, but it must be remembered that electrolytes are not stored in the body, so if they are administered in balanced supplements, then the body will recognise any excess electrolytes as excess to requirements and they will be eliminated by the kidney. It is important that electrolyte supplements be administered while the loss of electrolytes is occurring, especially in long endurance work, for example. In this regard, it is particularly important to supplement potassium because of the significant potassium loss during exercise. Any balanced electrolyte supplement will also contain high levels of sodium, which will usually be in excess because of the sophisticated reabsorption conducted by the kidneys to conserve sodium, and any excess sodium may then be passed out in urine with the excess potassium.
High concentrations of potassium are present in the normal roughage diet of horses, and large amounts are found in forages such as pasture grasses and hays. Oat hay contains approximately 1.4% potassium, lucerne hay approximately 2.5%, for example. This doesn’t sound a lot, but 2kg of good lucerne hay can provide 50g of potassium in just one meal, using these figures. At the same time, a horse receiving 40mL daily of a supplement such as Dynavyte (which provides 2g/litre of potassium) is only receiving 80mg of potassium per daily dose - an insignificant level compared to the potassium intake from roughage.
The major electrolytes sodium and potassium are responsible for the establishment and maintenance of proper electrical gradients (membrane potentials) across cell membranes, and calcium and magnesium also have a role in this function. Any electrolyte deficiencies or imbalances can thus impair nerve and muscle function.
In addition, the kidneys are of prime importance in maintaining electrolyte balance. Some electrolytes are lost in sweat and faeces, but most of the fine-tuning is done by the kidneys. In general terms, sodium is highly conserved (very little is excreted in urine), and substantial quantities of potassium and calcium are excreted on a daily basis by the kidneys. This is partly due to the fact that intake of potassium and calcium generally tends to be high. (Any excess potassium found in normal supplements will be relatively insignificant when compared to that commonly available in many feeds).
Lon D. Lewis, DVM, Ph.D, in the veterinary text Equine Clinical Nutrition: Feeding and Care, Lea& Febiger, 1995, states of Potassium, in the section entitled ‘Minerals for Horses”, page 37:
“Forages contain 1 to 4% potassium, and cereal grains may contain 0.2 to 0.7% potassium, but usually have 0.3 to 0.5%, as compared to 0.4% recommended in the horse’s diet. Although 0.25% potassium may be adequate in the idle horses diet, more than 0.6% may be needed for the horse in training, or with frequent physical activity sufficient to result in substantial sweating. Thus, even during these periods, the horse’s potassium requirements are met by most practical diets consisting of commonly used feeds. Excess potassium intake is not harmful, unless renal excretion is decreased, as excesses are readily excreted in urine. Renal excretion of potassium is highly dependent on renal glomerular filtration rate. If adequate water is not available, or renal function is sufficiently impaired to decrease potassium excretion, the horse won’t eat; so even then, excess dietary potassium is not a problem, except in horses with potassium-induced periodic paralysis (HYPP), which is a genetic disease uncommon except in certain lines of Quarter Horses.
Thus, excess potassium intake is rarely a problem. In contrast, a potassium deficiency may occur and is quite detrimental.
Sweating increases potassium loss both in sweat and in the urine. Sweating-induced water and sodium losses result in increased aldosterone secretion. Aldosterone increases renal reabsorption of sodium and excretion of potassium. Increases in urinary potassium losses are aggravated by the use of diuretics such as furosemide (Lasix), which is commonly used to try to decrease exercise-induced pulmonary haemhorrage (bleeders). The probability of a potassium deficiency occurring as a result of these increases in potassium losses is more likely if a low potassium diet, such as a high grain / low forage diet, is fed. In one study, a net loss of body potassium occurred in horses exercised daily and fed a diet consisting of 1/3 grass hay and 2/3 oats - a diet typical of that often fed to horses in training and use. Increased potassium losses also occur in horses with many diarrhoeal diseases who, in addition, may have a reduced potassium intake because they don’t feel well and decrease their feed intake; as a result, a potassium deficit may occur.
Prolonged or frequent physical activity, such as endurance racing or training, particularly in warm and/or humid environments, may also result in a potassium deficit, which can be a major factor responsible for causing post-exercise fatigue.
Fatigue, muscle weakness, lethargy, exercise intolerance, and decreased water and feed intake are the major effects of a potassium deficit. Increased restlessness and timidity to noise have also been reported. Foals fed an experimental potassium deficient diet gradually decreased their feed and, as a result, their potassium intake. They consequently lost weight, became unthrifty in appearance, and had a moderately low plasma potassium concentration.
Muscle weakness and fatigue might be expected as a result of a potassium deficit since about 75% of the body’s potassium is in skeletal muscle and is necessary for its function, as it is for all body tissues. If the potassium concentration is less than 2.8mEq/L in plasma or 150mg/dl in urine, the potassium:creatinine-nitrogen ratio is less than 1:1, or the renal creatinine clearance ratio of potassium is less than 15-23% in the normally hydrated horse, it indicates that a potassium deficit is present.
Acidosis causes an increased movement of potassium out of cells, which tends to maintain or increase the plasma potassium concentration and to increase renal excretion of potassium even when a substantial total-body potassium deficit is present. Conversely, dehydration decreases the renal clearance ratio of potassium to as low as 2% in one study.”
Randolph Stewart, DVM, MS, in “Considerations in Fluid and Electrolyte Therapy”, page 192, Equine Internal Medicine, WB Saunders 1998 , Eds: SM Reed & WM Baily, outlined potassium activity as follows:
“The Na+ - K+ pump located in the cell wall actively transports potassium into the cell and sodium out of the cell. These actions maintain a high extracellular and low intracellular concentration of sodium, as well as low extracellular and high intracellular concentrations of potassium. Sodium is therefore the main extracellular cation and potassium is the principal intracellular cation. The relationship of extracellular and intracellular fluid volumes is thus dependent on the body’s content of freely exchangeable sodium and potassium.
Because the Na+ - K+ pump continuously moves potassium ions into the cell and sodium ions out of the cell, the plasma potassium concentration is relatively low and not a good indicator of intracellular or total body potassium content.
For example, hyperkalaemia associated with acidosis may occur in the presence of normal or reduced total body potassium content. Hypokalaemia occurs in association with alkalosis, elevated levels of plasma insulin, or total body potassium depletion.
The clinical signs associated with the deficit or excess of potassium ion reflect the very important role it plays in the excitability of nerve and muscle. The clinical effects of hypokalaemia include cardiac arrythmias, myocardial dysfunction, muscle weakness and intestinal ileus. A hyperkalaemic syndrome in horses referred to as HYPP (Hyperkalaemic Periodic Paralysis) causes a myotonia characterised by muscle spasms and fasciculations followed by episodes of muscle weakness, recumbency and cardiac arrythmias. Current research and references all agree that performance deficits are far more likely in conditions of hypokalaemia. There is almost no reference material describing hyperkalaemia and its negative effects on performance unless discussions centre on HYPP.
The high roughage diet consumed by most horses provides a very high level of dietary potassium - a significant portion of which is excreted in urine. This high urinary potassium excretion is only moderately reduced in horses that are deprived of food, and its continued loss results in depletion of total body potassium. It is therefore very important, when evaluating a fluid therapy plan for a horse with significantly reduced feed intake to provide adequate potassium supplementation. Diarrhoea and excessive salivary or sweat loss can also contribute to potassium depletion.”
This is a subject of some controversy in Quarter Horse circles. An inherited disorder characterised by elevated potassium levels, this is a similar disorder to one seen in humans. Affected horses are genetically linked to a leading Quarter Horse sire, “Impressive”.
Symptoms are that the horse has muscle cramping, quivering muscles, difficulty breathing, and paralysis. Death can occur from cardiac or respiratory failure due to high blood potassium levels. It is estimated that about 2% of Quarter Horses carry this gene. HYPP symptoms need not occur during exercise, as with tying up related conditions. Horses are commonly at rest, or have eaten a meal high in potassium levels.
What is actually happening in affected horses? For some reason not yet understood, sodium leaks into muscle cells, and at the same time, potassium leaks out of cells into interstitial fluid and blood. This creates an electrical charge that overstimulates the muscles, plus an elevated blood potassium level.
One proposed therapy has been to decrease dietary potassium levels as a preventative. This seems odd, because by far the greatest amount of potassium ingested by all horses is in forage. Lucerne hay, for example, is high in potassium. Horses with HYPP should avoid high potassium feeds. Lower potassium levels are found in Bermuda grass, oaten hay and some fescues, and are recommended for these horses.
We know that potassium is the main ion found inside cells, which functions to maintain cell volume and electrical activity. Potassium metabolism is very closely regulated by hormones of the kidneys, adrenal glands, thyroid gland and pancreas. In normal circumstances potassium is almost 100% absorbed from the stomach and small intestine. It is excreted by kidneys, and smaller amounts are lost in sweat, faeces, and even from sloughed skin cells.
We know that non-exercising horses require 0.3 - 0.4% of dry matter intake of potassium daily, and that the requirement can double in hard working horses, particularly in hot, humid conditions where sweating may be copious.
Common horse feeds supply 0.3% to 6% potassium, and horses regularly ingest up to 10 times their requirement of potassium. Normal horses readily excrete any excess potassium. But horses with HYPP have been shown to exhibit symptoms when their total dietary potassium is greater than 1.1% (To put that figure in perspective, if a horse with HYPP eats 10kg feed daily, it will ingest 110g of potassium from that daily feed before symptoms of HYPP are seen).
Episodes of HYPP can very often, and easily, be misdiagnosed as rhabdomyolysis (Tying-Up), and even colic.
Contrary to popular opinion, high plasma potassium levels cannot be used to diagnose HYPP, because all horses show a post-prandial (after meal) variation in plasma potassium (the severity of which depend on the potassium concentration in the meal). Symptoms of HYPP have been shown to begin at 3.79mmol/l, which is in the normal range for potassium in plasma.(Have a close look at an equine blood count form, and you will see that the normal range for potassium is generally 3 to 5mmol/L).
A series of studies at Texas A & M University in 1998 tried to determine the relationship between dietary potassium content, plasma potassium concentration, and HYPP symptoms. Horses were fed a variety of diets with 1.1%, 1.9% and 2.9% potassium content. These trials determined that there were no differences between normal and HYPP horses in potassium excretion or absorption, so any potassium related problems were not from alterations in absorption or excretion. There was a post-prandial (after a meal) pattern of increased plasma potassium from higher potassium diets, but not from a 1.1% potassium diet. Interestingly, all horses adapted to high potassium diets within 3 weeks. There was an increased average plasma potassium level from a 1.9% potassium diet.
No horses showed increased HYPP symptoms on a low 1.1% potassium diet, but HYPP symptoms increased as dietary potassium levels increased from 1.9% to 2.9%. The onset of HYPP symptoms correlated with plasma potassium concentration regardless of dietary potassium concentration, and a considerable adaptation to high potassium diets had occurred by day 14.
It is well known that exercise results in increased plasma potassium concentrations, since potassium enters the blood from exercising muscle fibres. Increased dietary potassium also increases plasma potassium levels. It would be logical to assume that that there would be an additive effect on HYPP symptoms when horses consumed large amounts of potassium and were then exercised. This is not the case. In fact, walking and trotting tend to lessen HYPP symptoms.
HYPP horses often show symptoms in the absence of exercise. This could be because of dietary potassium being absorbed from the gut after a meal. People tend to associate effects with things that occur directly before them (for example, exercise), rather than things that occur 2-5 hours previously (a meal). The peak of plasma potassium concentration from a meal occurs at about 2-5 hours after a meal. For that reason, researchers recommend that an HYPP horse not be exercised during peak postprandial plasma potassium concentration periods; that is, within 2-5 hours of a meal), since they are unlikely to perform well during this period.
In the Texas A & M studies, when mares were fed twice daily with meals containing 33gm potassium per meal, they remained asymptomatic. However, when fed 58g/meal and 89g/meal, they showed symptoms of HYPP 52% and 67% of the time respectively. Therefore, the objective with HYPP is to feed less than 33g potassium per meal, and also to feed in multiple meals or allow horses to eat continuously.
Potassium is the major intracellular cation ( concentration is approximately 140mEq/l) and is important for maintaining the normal resting membrane potential of cells. 60-75% total body potassium is found inside muscle cells, with the remainder in bone. Only 5% of potassium is found in extracellular fluid, thus potassium levels in blood are not often a reflection of total body potassium levels. Plasma potassium concentration is tightly regulated: fairly small changes can have marked effects on organ function.
Ingested potassium is absorbed non-selectively in the stomach and small intestine. Regulation of plasma potassium is by renal excretion and movement of potassium from extracellular to intracellular fluid. If these mechanisms (Na+- K+ Pump) are functioning normally, the amount of potassium ingested has little effect on plasma potassium concentration. However, if one or more of the regulatory mechanisms is faulty, then the amount of potassium ingested can exacerbate abnormalities in plasma potassium levels. Urinary excretion of potassium is largely by secretion of potassium into urine by the distal tubules.
The National Research Council, (NRC) guideline entitled Nutrient Requirements of Horses, 5th Revised Edition 1989, notes on potassium, on page 12:
“Hintz and Schryver (1976) estimated that mature horses required 0.06g potassium/kg of bodyweight/day, or approximately 0.4% of diet. Because forages usually constitute a significant portion of the diet, the potassium requirements for horses should be met easily. If necessary, potassium chloride and potassium carbonate are effective sources of supplemental potassium. Jarrige and Martin-Rosset (1981) indicated that optimal potassium concentrations of equine diets were 0.4 to 0.5% for light to medium work, 0.4% for late gestation, 0.6% for 6-12 month old foals, and 0.8% for horses 18-24 months of age. Further, Drepper et al (1982) estimated the daily potassium requirements for a 600kg horse to be 22g for maintenance, 32g for light work, 43g for medium work, 53g for heavy work.
The NRC further commented, in Signs of Potassium Deficiency or Excess:
“Foals fed potassium deficient, pelleted, purified diets gradually refused to eat and, therefore lost weight, became unthrifty in appearance, and had moderately lowered serum potassium concentrations (hypokalemia). On addition of potassium carbonate to the purified diet, an immediate resumption of normal feed intake occurred (Stowe, 1971)
Excess dietary potassium is excreted readily, primarily via the urine, when water intake is adequate”
The NRC commented on Sodium Deficiency in Horses:
“In acute sodium deficiency, muscle contractions and chewing are uncoordinated and horses have an unsteady gait; serum sodium and chloride concentrations decrease markedly, whereas serum potassium increases. (Meyer et al, 1984)
Finally, the NRC noted that:
”In some instances in a hot or humid environment, oral electrolytes may be beneficial for heavily sweating endurance horses. Such supplementation should provide sodium and chlorine in roughly equal proportions, while the proportion of potassium should be one-third to one-half that of sodium. Calcium and magnesium may be added in small amounts”
Dynavyte is a liquid trace element and mineral supplement, and provides potassium at a level of 2g/litre. So at the recommended dose rate of 40mL daily per horse, Dynavyte will provide only 80mg per day! This level of potassium would be extremely safe even in a Quarter Horse with diagnosed HYPP, as this level is far short of the recommended low dose of 33gm potassium per meal for these horses. Even a very small meal of hay or roughage would provide perhaps 10 times that dose available from Dynavyte. In simple terms, Dynavyte administration would make absolutely no impact on total body potassium levels in normally fed horses.
If Dynavyte is providing 80mg potassium daily from one 40mL dose, this is well in line with the NRC recommendations of 60mg/kg bodyweight daily for an adult horse. In fact, this amount of potassium for an adult horse is relatively insignificant.
Hess et al (2005) compared the effects of oral supplementation on 46 horses with an experimental
potassium-free sodium abundant electrolyte mixture with that of a commercial potassium-rich
electrolyte supplement on acid-base status and plasma ion concentrations in horses during an 80km
endurance ride. Blood samples were collected before the ride; at 21, 37, 56 and 80km rest points,
and during recovery (30 minutes after the ride). Consumed electrolytes were recorded, and blood was
analysed for pH, PvC02, Hct, and plasma was analysed for Na+, K+, Cl-, Ca++, Mg++, lactate,
albumin, phosphate and total protein concentrations. Plasma concentrations of H+ and HC03- and
osmolarity were calculated. 34 (17 potassium-free, and 17 potassium-rich supplemented) horses
finished the ride. Potassium intake was 33g less, and Na+ intake was 36g greater for potassium-free
treated horses compared to potassium-rich supplemented horses. With increasing distance, plasma
osmolarity; H+, Na+, K+, Mg++, Po4, lactate, total protein and albumin concentrations, PvC02 and Hct
were increased in all horses. Plasma HC03-, Ca++ and Cl- concentrations were decreased. Plasma H+ concentration was significantly lower in potassium-free treated horses. Plasma potassium concentrations at the 80km inspection point and during recovery were significantly less in potassiumfree treated horses compared to potassium-rich treated horses. Conclusions were that increases in plasma H+ and K+ concentrations in this endurance ride were moderate and unlikely to contribute to signs of muscle fatigue and hyperexcitability in horses.
Hess, TM., Kronfeld, DS., Williams, CA., Waldron, JN., Graham-Thiers, PM., Griewe-Crandell, K., Lopes, MA., Harris, PA. Effects of oral potassium supplementation on acid-base status and plasma ion concentrations of horses during endurance exercise. Am. J. Vet. Res. 2005 Mar;66(3):466-73
The consensus of veterinary opinion is that potassium deficit is potentially far more serious than potassium excess. Potassium deficit reliably produces symptoms including muscle fatigue and poor performance. The equine metabolism is geared to manage excess potassium levels because of the wide variation in available potassium levels in standard feeds, and the significant potassium loss from muscle cells during exercise.
While there are suggestions that excess potassium levels may lead to muscle hyperexcitability, this only seems to occur in horses with confirmed genetic diseases such as HYPP, or possibly in horses with impaired kidney function.
It would be sensible to carefully consider any advice that excess potassium is detrimental to your horse. Evaluate
the scientific information and make your own mind up. Copies of any of the references listed are available on
The primary message from the potassium discussion is that a deficiency is potentially far more serious than an excess, leading to a range of muscle fatigue and performance problems, and that balanced electrolyte supplements will not compromise the horse - They will only be passed out rapidly in urine if they are in excess.