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 producti