A horse needs strength – stamina – muscular response – for a successful performance. Energy is a necessity for all of these elements of success, whatever equine sport the horse is involved in from amateur to professional, dressage to racing and all in-between.
What is Energy?
The respiratory, cardiovascular and musculoskeletal systems all work together in the energy cycle: The respiratory system is responsible for the uptake of oxygen, and then the cardiovascular system distributes it. Sustained energy production requires good blood supply. Oxygen from the lungs is delivered to the muscles where it is used to produce the chemical energy that powers locomotion.
Strength, stamina and muscle response are ultimately dependent on the conversion of chemical energy within the muscle fibres. During energy conversion, glycogen (from feeding carbohydrates) and free fatty acids undergo a chain of enzyme controlled reactions, the final product of which is the chemical adenosine triphosphate (ATP). ATP is the unit of biochemical energy in the same way a dollar is the unit of currency.
Where does Energy Come From
Carbohydrate, fat and protein are the dietary energy sources available (via the conversion process to ATP) to the muscles. The extent to which each is used to power muscle contractions depends on both the intensity and duration of exercise. Carbohydrate, stored in the muscles as glycogen, is the most important energy source. Stored with it are high concentrations of the enzymes necessary for its rapid breakdown to glucose. Liver stored glycogen is a secondary source of carbohydrate.
Fat is the second major source of energy. Fat provides approximately two and one half times more energy compared to the equivalent weight of carbohydrate. Horses can utilise 85 to 90% of the energy in fat compared to around 60% from carbohydrate sources such as grain.
Protein is the third major energy source. During strenuous exercise muscle protein is broken down to amino acids which in combination with those derived from feed, can be used to produce energy. Protein combustion is not an efficient process; the overall net gain is approximately twenty per cent less than from glycogen combustion. The difference is the amount of heat produced; performance horses fed high protein diets often sweat or blow excessively after exercise in an effort to expel this extra heat. Other complications can occur from high protein level, so it is not recommended to increase protein in the diet.
While all three sources contribute to the total energy pool, glycogen is by far the major source of energy for performance horses. Under most competitive situations muscle glycogen reserves are able to meet energy demands. As the duration of exercise increases the contribution from fat becomes more significant.
How it Works Inside the Horse
Whilst from the food source there are three “fuels” created, the phosphocreatine is a start up fuel (literally seconds) and has no lasting effect therefore we will leave it out of the discussion. The utilisation of (the other two), glycogen and fatty acids for ATP production can be carried out anaerobically (without oxygen) or aerobically (with oxygen), that is two systems that convert feed to energy. Also there are two types of muscle fibres: slow twitch and fast twitch. As the name suggests slow twitch are slower to contract and stretch, fast twitch faster to act. Also slow twitch have a good blood supply and fast twitch has a limited blood supply and the proportion of those different muscles vary from horse to horse – genetically predetermined. Hence some breeds, like quarter horses, move fast over short distances as they have a greater number of fast twitch fibres. Training can influence the fast twitch fibres by increasing the aerobic capacity.
Anaerobic metabolism (low oxygen/fast twitch) is the faster producer of ATP, but is more energy expensive – it uses thirteen times more glycogen to produce the same amount of ATP as aerobic metabolism. It also produces more lactic acid, which accumulates in the muscles, causing fatigue – “muscle burn”. Aerobic metabolism (high oxygen/slow twitch) is a slower producer of ATP and is associated with endurance and longer riding time.
The significance of each of these systems to energy production is important, that both systems operate simultaneously but the level of exercise determining the relative contribution of each to overall ATP production. One system doesn’t just switch off and the other takes over. During sustained effort a point is reached when oxygen demand outstrips supply and anaerobic metabolism has to contribute more to overall energy production. This point is called the anaerobic threshold. For horses competing in a more strenuous sport (rather than speed), having an alternative energy source is an advantage. Adding fat to the diet will effectively save glycogen stores and result in a 30% drop in glucose utilisation. The result is a delay in carbohydrate depletion and the associated onset of fatigue. So to provide for the two systems a balance in the diet is required in that carbohydrate is needed to supply the glycogen for the fast twitch muscle fibres and the quick short term energy and fat is needed to provide the fatty acids for the slow twitch muscles and longer term energy. The ratio is determined by the type of horse and the type of work required.
Glycogen loading used by human athletes, (over feeding carbohydrates) is not a safe procedure for horses due to the risk of laminitis and muscle tye-up. Maintenance of optimal glycogen stores is dependent on a balanced daily energy intake, relative to the work being done.
The Negative Effects of the Production of Energy
From the process of ATP production comes waste products, and these have the potential to interfere with muscle function. These include heat, carbon dioxide, lactic acid, ammonia and free radicals. Lactic acid levels rise during strenuous exercise proportional to the degree that anaerobic metabolism is used. Once the anerobic threshold has been passed the production of lactic acid is markedly increased. Accumulation has an adverse effect on enzyme function which shows physically as muscle fatigue. During low intensity exercise small amounts of lactic acid are constantly produced and removed by the circulation with no effect on cellular function.
Ammonia is a by-product of protein metabolism and also contributes to muscle fatigue. Where protein is given in excess of dietary requirements, it is detrimental to health and performance, as the unused amino acids are broken down in the muscles and liver to urea and ammonia and ammonia inhibits the enzymes involved in ATP production. Studies have shown a decrease in the race times of horses fed diets containing 2 to 3 times their protein requirement. Free radicals are oxygen molecules that accidentally lose an electron, becoming dangerously reactive in doing so and unless they are oxidised, they can cause irreversible cellular damage. Also research has shown that free radicals are a major factor in the development of cardiac muscle disease. Their role in equine skeletal myopathies including tye-up is suspected.
The Provision of Feed for Energy
Equine energy requirements increase as the intensity of training increases, also temperament and low environmental temperatures vary the demand, the level of condition needs to be assessed as unused energy will be stored. Adult horses (500kg) have a daily maintenance requirement of 17mCals with no work and this energy level is provided for in a good balance by good pasture – this increases up to 35mCals to meet the demands of training and competing. The golden rule feed according to work done applies. Understanding and providing the correct type of energy source – and the ratio carbohydrate/ fat is an important key to successful performance.
While it is tempting to try and totally reduce carbohydrate intake in sport horses in the hope that they will be calmer for less able riders, it is still necessary to provide some level so that it will maintain the balance in the ATP production. For example some glycogen (sourced from carbohydrates) is needed to utilise energy from fatty acids, it is the starter for fat conversion. Understanding carbohydrate sources and monitoring a balanced diet to suit an individual horse is vital. Temperament issues attributed to carbohydrates are usually a result from insufficient work or poor work pattern commensurate to the total feed given, rather than the myth that starch or grains are the “bad guys” and create the training problems. It is lack of knowledge on the management of the nutrition of sport horses that is the real culprit. For example ask a number of riders what is the weight of the components that they feed their horses? The majority will not know, so how then can a balanced diet be the result if the actual amount is a mystery. Adding fat in the form of meal (e.g. coprameal, soya bean meal), rice bran or vegetable oils to the diet are a way of increasing energy intake and leads to a reduced dependence on glycogen stores. The addition of trivalent chromium to the diets allows better utilisation of energy through its effect on insulin, the hormone that regulates the transfer of glucose and amino acid between blood and muscle cells. Human studies have shown that daily supplementation with chromium, in combination with physical training, increased muscle mass and reduced body fat. The improved uptake of glucose boosts ATP production while the increased amino acid uptake has an anabolic effect, improving the muscle to fat ratio. There are a number of vitamins directly involved in the conversion of glycogen to ATP, most are readily available to the horse and do not need supplementation, however, although B group vitamins are normally synthesized in the large intestine, high grain intakes lower the intestinal pH with a resultant drop in vitamin B production. Human studies have shown that when athletes were restricted to less than 50% of their daily requirement for vitamins B1, B2 and B6 there was a significant drop in performance.
Free radicals -the waste products of energy conversion can be neutralised by anti-oxidants. The nutritional anti-oxidants are mostly of plant origin. Common examples are vitamin A, vitamin C, vitamin E and selenium. Rice bran oil is a rich source of natural anti-oxidants. Selenium has a specific role as a component of the enzyme responsible for peroxide neutralisation within the cell. Vitamin E is present in every cell-membrane and is essential in maintaining cellular function, particularly in skeletal muscles where the risk of peroxidation damage is highest. Vitamin E and selenium are frequently used in the treatment of muscle tye-up.
Chromium has also been shown to reduce blood lactic acid levels thereby delaying the onset of fatigue. Studies in supplemented horses showed a 13% reduction after intense exercise. Gamma oryzanol is another antioxidant which has been scientifically proven to not only oxidise the dangerous free radicals but assist in reduction of ulcers, often generated by high grain intake. Omega 3 and omega 6 oils are involved in the production of cellular hormones known as eicosanoids which play an important role in many cellular processes. The ideal dietary support product for performance horses is one that contains rice bran (which provides omega 3 and 6 in correct ratio), vitamin E, gamma oryzanol, fatty acids, abundant antioxidants and trivalent chromium.