Written By Dr. Kris Hiney
Last month we discussed the usefulness of fat in the equine diet, as well as some examples of typical feeds which contain fats. Fats are an easily digestible source of calories which can readily supply the extra energy that performance horses may need. Fat may lower the heat load on the horse compared to traditional diets, which may aid in performance in hot climates. Finally, fat may even help calm the horse compared to when they are fed high starch diets. But is there any other reason to feed fats that may help you get to the winner’s circle?
When horses are fed fat in the diet, their body responds by increasing the number of enzymes that are involved with lipid metabolism. These include the enzymes needed to remove fat from the bloodstream and enter muscle or adipose tissue, and those that ultimately oxidize the fatty acids. Feeding fat to horses results in a lowering of plasma triglycerides which is believed to be caused by a decrease in synthesis of triglycerides in the liver. The horse becomes more efficient at utilizing dietary fats for energy, rather than needing to use carbohydrate or protein. This adaptation has repeatedly been shown to take at least three weeks after the change in diet. Complete adaptation may take as long as 2-3 months. Therefore, if switching your feeding regimen, don’t expect to see instantaneous results.
Exercise and Fuel Sources
When fatty acids are oxidized in the body for fuel, their final metabolic pathway involves the Tricarboxylic cycle (TCA)* or Kreb’s cycle. This cycle is dependent on oxygen (through its connection to the electron transport chain) in order for it to work. The TCA cycle supplies the bulk of Adenosine Triphosphate (ATP)** for horses when they are working aerobically, or at lower intensities. Technically, aerobic work is at a low enough intensity that the requirement of ATP can be met by the slower metabolic pathway of the TCA cycle. At low intensities of exercise, fat typically supplies up to 50-60% of the calories needed. All dietary energy sources – fats, carbohydrates and protein – can be utilized in aerobic metabolism, provided there is sufficient intake of oxygen. That means that the horse’s heart and lungs can keep up in the race to deliver oxygen to the tissues. However, when the horse’s muscles are contracting faster or harder than the ability of the cardiovascular system to keep pace, they then enter into anaerobic metabolism. The horse must then switch to a different supply of fuel, primarily carbohydrate metabolism. They are simply working too hard for the aerobic system to keep up with the demands of the muscles for ATP. Therefore, horses undergoing intense exercise, or sprinting type of activities, must rely on their carbohydrate stores for energy. These include blood glucose, liver and muscle glycogen, and the body’s ability to perform gluconeogenesis (make glucose from other sources).
Can Fat Save Glucose?
It is presumed that due to the adaptation of the horse to become more efficient at fat metabolism, they are less reliant on their carbohydrate stores (blood glucose, muscle and liver glycogen) to supply their energy needs. This should allow the horse to work longer before turning to carbohydrate metabolism. This may be advantageous for two reasons. One is that carbohydrate stores in the body are much more limited in comparison to lipid stores, and two, usage of carbohydrate through anaerobic metabolism can result in the production of lactic acid. This may contribute to the onset of fatigue, due to depletion of energy sources or the accumulation of lactic acid. Therefore, fat fed horses may have some advantage in their resistance to fatigue.
Most studies of horses fed high fat diets have reported an increase in resting muscle glycogen stores. However, there have been a few reports which have shown an opposite effect of lowered muscle glycogen. In these studies, the horses were either untrained or receiving low intensity exercise. In studies which exhibit an increase in resting muscle glycogen, the horses received more intensive training, including sprinting exercise. This may be the key in seeing a response to the fat added diet. In addition, the amount of fiber and starch in the rest of the diets differed between studies, which also clouds interpretation. If horses do have higher glycogen stores at rest, it is unclear if this results in an increase in glycogen utilization during exercise. Some researchers found an increase in glycogen utilization while, again, others have found no change in glycogen metabolism during race simulations or long term sub-maximal exercise. But would an increase in glycogen utilization improve performance? Again results are mixed. Horses fed 12% fat for four weeks improved their run time to fatigue in a high intensity exercise bout on a treadmill. Others have found increased performance in sprinting exercise and in a simulated cutting event, while some have found no clear advantage to feeding fat in improved performance.
Is Anything Consistent?
In studies looking at blood metabolites in exercising horses fed a fat added diet, some consistent results have been seen. Feeding fat does decrease the exercise related drop in blood glucose. This is seen simultaneously with an increase in serum triglycerides and free fatty acids. Presumably these horses have indeed shifted toward a more efficient utilization of fatty acids during exercise, sparing their glucose stores. This seems to be supported by data which shows that horses on fat supplemented diets have a higher blood pH during exercise versus non-supplemented controls. The above effects are seen at lower intensities of exercise. When the horse increases its ATP demand, they will need to draw more from anaerobic metabolism and must shift to carbohydrate usage.
So with all of these conflicting results, what should you believe? It is clear that feeding performance horses fat rather than carbohydrates is a much healthier alternative. High carbohydrate diets carry with them the risk of laminitis, colic, ulcers and insulin resistance. As of now, no negative effects of feeding fats to horses have been found. The potential benefits are many, including a potentially calmer horse, a decrease in reliance on blood glucose (at least at lower intensities), and a possibility of increased performance in anaerobic activities. With little to lose, and benefits to gain, it is no wonder fat added diets are so popular in the equine industry.
Next month – The usage of fat added diets in metabolic diseases.
* TCA – Tricarboxylic cycle, also known as Citric acid Cycle, has been described as the “central metabolic hub of the cell”. A sequence of reactions taking place in mitochondria where acetyl units attached to CoA are degraded to carbon dioxide and the electrons produced transferred to the coenzymes NAD⁺ and FAD.
**ATP – adenosine triphosphate, an adenine nucleotide used as the energy currency in metabolism. The free energy released when ATP is hydrolyzed is used to drive reactions in cells.