Written By Dr. Kris Hiney
Last month we discussed the potential performance-enhancing benefits of feeding fats to exercising horses. These included a lowering of the thermal load on the horse, increasing its aerobic capacity, and perhaps even increasing their anaerobic energy stores in the form of muscle glycogen. Clearly, feeding fat has many advantages for the average horse. Fat can even be used as a nutritional management technique for horses that may suffer from repeated bouts of tying up. This month we will examine two particular types of muscle disorders which may actually benefit from the addition of fat to the diet.
Tying up in horses is usually seen as stiffness in the back or hindquarters, reluctance to move, cramping of the muscles or muscle fasciculation’s, profuse sweating, and may be accompanied by head nodding or pawing. The horse is extremely uncomfortable and should not be forced to continue to exercise.
Tying up syndrome may have a multitude of causes. Horses may tie up due to electrolyte imbalances from prolonged exercise or sweating, or even if they have HYPP. However, some horses may chronically tie up, usually after they have been given a period of rest. Traditionally this syndrome was referred to as Monday morning sickness or Azoturia. It was seen in work horses which would experience muscle cramping soon after beginning work following a weekend of rest. However, now we know much more about this disease, its underlying causes, and its treatment.
Horses which tie up chronically typically fit into separate breed types. In Thoroughbreds, this is commonly seen when the horse is already fit, and may be under a period of stress. It also occurs following time off or rest. Thoroughbreds typically have a syndrome referred to as “recurrent exertional rhabdomylosis, ” or RER. This disease is also seen in Arabians and Standardbreds, but less frequently than in Thoroughbreds. The underlying cause in these horses is that the calcium channels in the muscle do not work properly. The release of Ca results in muscle contractions following nerve stimulation. However, in these horses, the threshold of muscle contraction is due less to the abnormalities in the calcium channels.
Another disorder which is frequently seen in stock horse breeds and draft
horses is related to storage of muscle glycogen. This disease, characterized by abnormal accumulations of glycogen in the muscle, is referred to as “polysaccharide storage myopathy,” or PSSM. Owners may initially not even know their horses have this disease, as the average age of first clinical symptoms is 6 years, with a range from 1 year of age to 12 years. Horses with PSSM have increased insulin sensitivity, combined with an abnormally high rate of activity of the enzyme which produces glycogen. When presented with glucose from nonstructural carbohydrates in the diet, these horses rapidly clear glucose from their blood and store it in the muscle. Due to their abnormal metabolism, they also seem to be unable to properly mobilize their own lipid stores. Ironically it is during aerobic exercise that these horses experience clinical symptoms, usually within 20 minutes of the beginning of exercise.
Although PSSM and RER horses have different disorders which lead to their tying up, they do share similarities in their management. Confinement without exercise should be avoided in these horses. If they do need time off, turn out is a must. However, if your horse merely stands at the gate waiting to come back in, alternative strategies should be developed. This could include lunging or providing a more active buddy which will encourage your horse to move around.
The diet of the horses should be changed, with more stringent requirements for the PSSM horse. Grass hays should be used with a low content of non-structural carbohydrates, ideally under 12% of the diet for the PSSM horse. For RER horses, a goal for the overall diet should be less than 20% of their caloric intake as non-structural carbohydrate. Traditional horse grains should be avoided, especially those containing molasses. Rather, they should be replaced with low starch, high fat concentrates, or, even, just add vegetable oil to their grain. Frequently the PSSM horses, which are usually easy keepers, can meet their digestible energy requirements by forage alone, but more heavily exercising horses may need fat to supply their calories. In addition, clinical signs of PSSM may not resolve unless fat is added to the diet.
Why does fat help?
For the horse with PSSM, adding fat to the diet gives the horse an available source of long chain fatty acids that can be metabolized during exercise. Remember that these horses do not seem to be as able to mobilize their own lipid stores due to abnormal feedback from glucose metabolism. In addition, feeding fat may help these horses adapt to using fat for fuel during aerobic exercise and help to prevent episodes of tying up. However, caution must be used with these horses to avoid obesity. While RER horses don’t have a glycogen disorder or have an inability to efficiently use fat, addition of fat to the diet of these horses also appears to be helpful. Presumably this may be due to the calming effect of fat in the diet, which may make these horses less reactive. As their tying up bouts are frequently associated with times of stress (when the horse is nervous or excited) it may just be a shift in behavior which helps prevent their tying up.
If your horse suffers from one of these diseases, it can be managed with diet and exercise. Avoid diets high in nonstructural carbohydrates, supplement the diet with fat, and be sure to balance properly for minerals and vitamins. Do not neglect these horses in their stall – regular exercise is key! With careful management, your horse can lead a normal, pain free life.
Next month: The benefits of Omega-3 fatty acids in the horse’s diet.