Tying up some loose ends on rhabdomyolysis - can nutrition help treat or prevent tying up?

A horse with raised muscle enzymes is always a cause for concern for trainers, whether it is a single isolated incident or a regular occurrence.

There has been a slow but steady increase in our knowledge of this disease or syndrome, which in more recent years has been helped by the application of genetic-based tests for some forms. An increase in our understanding of the metabolic basis for the syndrome is imperative and will help us to better manage these horses in terms of nutrition and training.

Catherine Dunnett (European Trainer - issue 29 - Spring 2010)

Pilates For Horses - enhancing performance and reducing injuries

Pilates is increasingly used amongst professional athletes as a method to enhance athletic performance and to reduce injuries.

Nicole Rossa (European Trainer - issue 21 - Spring 2008)

Racing power - supporting muscular effort through nutrition

The powerhouse for a horse in training is found in its large muscle mass. Whilst genetic makeup within the Thoroughbred breed has a large impact on a horse’s innate racing ability, dietary factors will also influence subsequent performance. There are many elements found in a racehorse’s diet that will help to support muscle function. Hydrolysable carbohydrate (sugar and starch), assisted by fermentable fibre, will help to maintain important muscle stores of glycogen (a carbohydrate fuel).

Dietary electrolytes, which are integrally involved in muscle contraction, are essential to offset electrolyte loss in sweat. Key dietary antioxidants such as vitamins E and C and also antioxidant co-factors, such as copper, manganese, zinc and selenium, are also important as part of the body’s antioxidant team which strives to reduce the formation of free radicals or reactive oxygen species, and to limit their damaging effects on the body. Free radical damage has previously been implicated in the process of exercise induced muscle damage.

GLYCOGEN STORES MUST BE REPLENISHED FOLLOWING EXERCISE

One of the most important functions of the diet is to replenish the horse’s energy stores in muscle on an ongoing basis. A racing ration needs to support the synthesis of glycogen to maintain the store of this important fuel, which is used in increasing amounts during exercise. Glycogen, which consists of a large branched chain of glucose units, is stored in both skeletal muscle and the liver and it represents one of the largest potential energy stores in the body. Horses being natural athletes, have a relatively large muscle glycogen store when compared to other species. As the glycogen content of horse muscle is influenced by the proportion of different muscle fibre types present, this means that there is a genetic influence on the overall glycogen content. Fast twitch fibres (Type IIb), which are found in increased numbers in talented sprinting horses, store relatively more glycogen than the slower type I and type IIa fibres. However, both diet and training can influence the level of glycogen stored in muscle. Exercise training for example has been reported to increase muscle glycogen content by 30-60% in horses. Logically, diet should have a significant effect on the storage of muscle glycogen as it provides the building blocks for glycogen synthesis. Glycogen can be synthesised efficiently from dietary starch, which is another polymer of glucose found in cereals. Glycogen can also be produced from certain glycogenic amino acids, released from the protein content of feed. In addition, propionic acid, which is a significant volatile fatty acid produced in the horse’s hindgut during the fermentation process, can also ultimately be converted to muscle glycogen.

In terms of the day to day diet, starch is by far the most direct and most efficient precursor for glycogen and so it is therefore not surprising that cereals, which are high in starch, have been the mainstay of racing diets for many years. In recent years we have seen the introduction of racing feeds that are lower in starch and sugar than traditional racing rations, with a greater emphasis being placed on digestible fibre and oil as energy sources. Whilst there are many health benefits attributable to this type of diet, the effect of changing the level of starch in the diet on muscle glycogen should always be considered.

MUSCLE GLYCOGEN - AN IMPORTANT FUEL BUT NOT THE KEY FACTOR IN FATIGUE

Muscle glycogen is a major source of energy (ATP) to working muscle during intense exercise, which is characteristic of racing. The amount of muscle glycogen used during training or racing will depend on its rate of utilisation, which in turn is affected by the speed and duration of the exercise undertaken. In general terms, the higher the speed, the faster muscle glycogen is broken down and used.

The duration of fast exercise is normally curtailed, which limits the overall amount of glycogen used. During slower work, although the rate of glycogen utilisation is much lower, exercise can usually be continued for a much longer time allowing more glycogen to be utilised overall (see figure 1). Total muscle glycogen content can be reduced by about 30% during a single bout of maximal exercise in horses. However, as muscle is a mix of different fibre types, the depletion of glycogen in individual fibres may be greater than this depending on the pattern of fibre recruitment during the exercise. Studies, however, have shown that even the IIB muscle fibres, which use glycogen at the fastest rate, are not totally depleted of glycogen following racing.

This supports the notion that although glycogen is an important fuel source for racehorses, glycogen depletion is not the most important factor in fatigue. However, exercise studies do suggest that power output and exercise performance can be decreased in horses where muscle glycogen has failed to be adequately replaced following a previous race or piece of hard work. This was the conclusion drawn by Lacombe and co-workers (2001) who reported that horses with replete muscle glycogen stores were able to run for longer periods during a maximal exercise test compared to horses whose muscle glycogen level remained low following a previous exercise bout. Whilst there are always horses that will buck the trend, this research emphasises the need to allow a suitable period of time between races, but also between bouts of fast work and subsequent racing to allow muscle glycogen stores to be replenished.

In contrast to human athletes, muscle glycogen replenishment in horses is relatively slow. Following racing or a hard work, research suggests that muscle glycogen can take up to 72 hours to return to pre-exercise levels when a traditional high cereal racing ration is fed. Certainly research carried out in the past 3 years would suggest that a high glycemic racing ration would be better placed to support glycogen replenishment more quickly following racing or hard work. There are many factors that affect the glycemic response to feed, which in simple terms describes the relative rise in blood glucose following feeding.

The starch and sugar content of a feed, however, is one of the most significant factors affecting glycemic response. Feeds that are high in starch and sugar e.g. a high cereal-containing mix produce a greater glycemic response compared with feeds that are very low in starch and sugar e.g. a forage only ration. Rate of glycogen synthesis following a glycogen depleting exercise bout was significantly higher in horses fed a high glycemic diet compared to those fed a very low glycemic control diet (Lacombe et al 2004, Lacombe et al 2006). In addition, absolute glycogen concentration in muscle was significantly higher both 48 and 72 hours following exercise in the high glycemic group compared to the control horses and muscle glycogen concentration had returned to pre-exercise levels following 72 hours. The benefit of a high glycemic diet for glycogen repletion does, however, appear to be time dependent. Jose-Cunelleras (et al 2006) reported a minimal difference in glycogen repletion in the first 24 hours following a glycogen depleting exercise bout between horses that were fed a high glycemic feed compared with a group where feed was withheld for 8 hours and another group of horses where only hay was fed.

A recent study also concluded that the route of administration of carbohydrate given post-exercise significantly affects the rate of glycogen replenishment. Horses that were given an intravenous infusion of glucose following exercise exhibited significantly greater glycogen storage rates and glycogen concentration in the first 6 hours following exercise compared to horses fed a similar quantity of glucose orally. In fact, the repletion of glycogen in response to oral glucose was minimal over this time period compared to the unsupplemented control horses (Geor et al 2007). Whilst it is difficult to draw direct comparisons with feeding practices used in racing, it is worth appreciating the possible differences in the rate of glycogen repletion when very high glycemic feeds are fed compared to very low glycemic feeds. The reality in many training yards I would suspect lies somewhere between these two extremes.

LOW GLYCEMIC DIETS CAN OFFER RACEHORSES MANY BENEFITS

There are many health-related benefits to feeding a ration that is lower in starch and sugar. However, one should be mindful of muscle glycogen when considering horses that are consistently fed a low glycemic diet. Specifically horses may be fed this type of ration because they are behaviourally more manageable, or because a specific condition such as the muscular disease recurrent exertional rhabdomyolysis (tying up) (RER) is present. A low starch diet is actively encouraged for horses that suffer from RER. McKenzie (et al 2003) reported that plasma creatine kinase activity (CK), elevations of which can indicate muscle damage, was significantly reduced following exercise in RER horses fed a low starch high fat diet versus a high starch low fat diet. In addition, lower resting heart rates have also been reported in horses fed a low starch high fat diet compared to the reverse.

A lower resting heart rate may be beneficial especially in RER horses where it reflects a calmer horse as stress has been implicated as a trigger factor for the condition. The current thinking on feed for horses with RER continues to be a low starch and sugar diet supplemented with oil. It is also important that the diet is well balanced, especially with respect to calcium and phosphorus. Adequate electrolyte provision is equally important, as is the intake of antioxidants such as vitamin E and other related trace minerals such as selenium. Any potential individual limitation in mineral or electrolyte absorption and retention should be investigated further with veterinary assistance in order that individualised adjustments can be made to the diet.

A SUPPORTING ROLE FOR PROTEIN IN MUSCLE RECOVERY

Whilst we are all no doubt aware that the amino acids that make up protein are important for muscle development and repair, protein and its constituent amino acids have received very little attention in horses in terms of their potential to limit exercise induced muscle damage and aid muscle recovery. In human athletes, co-consumption of a protein and carbohydrate drink during and after exercise appears to limit exercise induced muscle damage, ultimately allowing faster recovery (Baty et al 2007; Saunders et al 2004). Recent introduction of ingredients containing partially hydrolysed protein may improve absorption of these amino acids and peptides possibly offering further benefit. Finally, some nutraceutical ingredients including carnitine and creatine have been hailed as being beneficial to muscle function and recovery in human athletes. Creatine, which has been studied in the horse, has failed to offer any great advantage, largely due to its poor absorption. Likewise, carnitine has been reported to improve muscle blood flow during exercise in humans, helping to reduce muscle damage. However, this aspect has not as yet been investigated in horses and previous dietary studies with carnitine were not unequivocal about the ability of oral carnitine to increase muscle carnitine content.

Electrolyte Balance – vital to the proper functioning of a racehorse's system

Electrolytes are essential components of the racehorse’s diet as they are vital to the proper functioning of the body’s basic physiological processes, such as nerve conduction, muscle contraction, fluid balance and skeletal integrity. The major electrolytes, sodium, potassium, chloride, calcium and magnesium are widely distributed within the body, but can be more concentrated in particular organs and tissues. For example, the level of potassium is very high in red blood cells but quite low in plasma, and the level of calcium in blood is low, but comparatively very high in bone and in muscle cells. The body has in-built mechanisms that work to maintain the correct electrolyte balance within the tissues, fluids and cells. These modify the absorption of electrolytes in the gut, or their excretion by the kidneys. These mechanisms are not foolproof however, and electrolyte loss through sweat can be a major issue for Thoroughbreds. The sweat of the equine athlete, unlike its human counterpart, is hypertonic; meaning that horse sweat contains higher levels of electrolytes than the circulating blood plasma. Consequently, the horse loses comparatively large quantities of electrolytes through sweating.

Although the electrolyte composition of equine sweat varies between individuals, on average a litre would contain about 3.5g of sodium, 6g of chloride, 1.2g of potassium and 0.1g of calcium. From this we can see that the majority of the electrolyte lost is in the form of sodium and chloride or ‘salt’. The amount of sweat produced on a daily basis and therefore the quantity of electrolytes lost differs from horse to horse and depends on a number of factors. As sweating is primarily a cooling mechanism, how hard a horse is working, i.e. the duration and intensity of exercise and both the temperature and humidity of the environment are all significant. Horses can easily produce 10 litres of sweat per hour when working hard in hot humid conditions. Stressful situations can also cause greatly increased sweating.

For example, during transport horses can lose a significant amount of electrolyte through sweating and the opportunity for replenishing this loss through the diet may be less as feeding frequency is reduced. Use of electrolyte supplements either in the form of powders or pastes is advocated before, during and after travel, especially over long distances. Jim Paltridge from IRT (UK) Ltd, (International Racehorse Transport), says, "we use a powdered electrolyte supplement added to the feed on a regular basis given for the 3 days prior to travel. We find this helps offset much of the loss normally incurred during transport and subsequently the horses arrive at their destination in better shape. We feel this electrolyte supplementation is a valuable attribute in the ongoing battle to reduce in-flight dehydration".

Electrolytes lost from the body in sweat must be replenished through the diet. All feeds, including forages, have a natural electrolyte content and in concentrate feeds this is usually enhanced by the addition of ‘salt’, which is sodium chloride. Forages such as grass, hay, haylage or alfalfa (lucerne) naturally contain a large amount of potassium, as can be seen from the table 1 below. In fact, 5kg of hay for example, would provide in the region of 75g of potassium, which largely meets the potassium needs of a horse in training. It is therefore questionable whether an electrolyte supplement needs to routinely contain very much potassium unless forage intake is low. Calcium is another important electrolyte, but it is lost in sweat in only very small amounts and its availability in the diet tends to be very good.

Calcium is particularly abundant in alfalfa with each kilogram of the forage providing nearly 1.5g of calcium. A kilo of alfalfa alone would therefore go a long way towards replacing the likely calcium loss through sweating. In addition, the calcium found in alfalfa is very ‘available’ to the horse in comparison to other sources, such as limestone. Calcium gluconate is another very available source of calcium, however, it has a relatively low calcium content compared to limestone (9% vs. 38%) and so much more needs to be fed to achieve an equivalent calcium intake. Interestingly, there is great variation between individual horses in their ability to absorb calcium, however, scientific studies carried out at Edinburgh Vet School showed that this variability was considerably less when a natural calcium source in the form of alfalfa was fed.

By far the most important electrolytes to add to the feed are sodium and chloride or ‘salt’. The levels of sodium and chloride found in forage are quite low and due to manufacturing constraints only limited amounts of salt can be added to traditional racing feeds. A typical Racehorse Cube fed at a daily intake of 5kg (11lbs) would provide only about 20g of sodium and 30g of chloride. As can be seen from table 2 this is a fair way short of meeting the daily requirements for these particular electrolytes by a racehorse in hard work.

It is therefore very important that supplemental sodium and chloride is fed. Ordinary table salt is by far the simplest and most economical electrolyte supplement, but the downside is the issue of palatability as the addition of larger quantities of salt to the daily feed can cause problems with horses ‘eating up’. As an alternative salt could be added to the water, but only when a choice of water with and without salt is offered. Salt should not be added to the water if it puts a horse off from drinking, as dehydration will become a problem.

Inadequate water intake can also contribute to impaction colic. Saltlicks are another alternative, although intake can be vary variable and we rely on the horse’s innate ability to realise its own salt requirements, which is questionable. So addition to the feed is by far the best route for adding salt or electrolyte supplements to the diet. Splitting the daily intake between two or three feeds can reduce problems with palatability.

Mixing salt and Lo Salt can make another simple DIY electrolyte supplement in the proportion of for example 500g to 250g respectively. Salt is sodium chloride (NaCl), whilst Lo Salt contains a mixture of sodium chloride and potassium chloride (KCl). This formulation provides 3g of sodium, 6g of chloride and 1g of potassium per 10g measure. This DIY mixture will replace these electrolytes in the approximate proportions that they are lost in sweat. What are the implications of a racehorse’s diet containing too little or too much of an electrolyte and how can we assess this? An inadequate level of certain electrolytes in the diet in some horses may simply result in reduced performance. In other individuals, it can make them more susceptible to conditions such as rhabdomyolysis (tying up), or synchronous diaphragmatic flutter (thumps), both of which are regularly seen in horses in training. Conversely, an excess electrolyte intake is efficiently dealt with by the kidneys and is ultimately removed from the body via the urine.

Therefore, the most obvious effect of an excessive electrolyte intake is increased drinking and urination. For this reason, the use of water buckets rather than automatic drinkers is preferred, as whilst the latter are far more labour efficient, the ability to assess water intake daily is lost. Excessive electrolyte intake can also be a causative factor in diarrhoea and some forms of colic. There is also some recent evidence in the scientific press that suggests that repeated electrolyte supplementation might aggravate gastric ulcers. However, these early studies used an electrolyte administration protocol typical of that seen during endurance racing, rather than simply a daily or twice daily administration, which is more commonly used in racing.

Supplements that contain forms of electrolyte that dissolve more slowly in the stomach, however, may be less aggressive to the sensitive mucosa. Unfortunately blood levels of sodium, potassium, chloride or calcium are poor indicators of whether dietary intake is sufficient or excessive unless it is very severe. This is because the body has effective systems for regulating the levels of these electrolytes in blood within very tight physiological limits. A creatinine clearance test, which measures the electrolyte content of a paired blood and urine sample is a much more useful indicator of dietary electrolyte adequacy.

There are a large number of commercial electrolyte products available, with a wide range in the breadth of ingredients that they contain. Consequently, they vary enormously in the amount of electrolyte that they deliver per recommended daily dose, as can be seen in table 3. In addition, whilst some glucose or other carbohydrate can help improve palatability, its presence should not compromise the amount of electrolyte that is contained within the supplement. In humans, it is recognised that the uptake of sodium from the gut is improved in the presence of glucose, while this effect in horses has not been firmly established. Electrolyte paste products are also often used either before and or after racing or travel.

These products are useful as they allow rapid electrolyte intake even when feed eaten may be reduced following racing. These electrolyte pastes often provide a more concentrated form of supplement and it is extremely important to ensure that the horse has access to water immediately following their use. Failure to do this may mean that the concentration of electrolytes in the gut actually draws water from the circulating blood, which can exacerbate dehydration. Another disadvantage with paste supplements is that if they are not formulated well, with an appropriate consistency, they can be difficult to dispense from a syringe and the horse may also be able to spit most of the product out after administration.

Some simple rules of thumb for choosing a good electrolyte are that salt should be one of the first ingredients listed on pack, as all ingredients are listed in descending order of inclusion. Additionally, be wary of supplements that taste sweet, as they may contain a lot of carbohydrate filler and little electrolyte. Some electrolyte supplements also contain many superfluous ingredients such as vitamins and trace minerals. The inclusion of these latter ingredients is largely unwarranted and their presence could cause issues with oversupply if the electrolyte is multi-dosed daily. Some electrolyte products specifically marketed towards racing may also contain bicarbonate.

The theory behind its inclusion is sound as ‘milk shaking’, whilst outside the rules of racing, has some scientific validity. However, the limited amount of bicarbonate contained in such electrolyte supplements is unlikely to have the positive effect on performance attributed to the former practice. Other extra ingredients such as pre-biotics may be more useful as they may improve the absorption of some electrolytes. In Summary, electrolyte supplementation in one form or another is essential within a racing diet. Ensuring that you are using a good electrolyte supplement is important and the quantities fed must be flexible and respond to changes in the level of work, degree of sweating and climate.