Nutritional Management of Horses With PSSM

Choosing a diet that is low in starch and sugar can help reduce excess muscle glycogen storage in horses with PSSM.
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Horses with PSSM often benefit from regular exercise and extended turnout. | iStock

Q: I have a young horse I bought for my breeding program that has PSSM1 (polysaccharide storage myopathy). She is only 17 months old, and I tested her at 6 months old. So far, I have not seen any signs. I have been careful about what I feed her but would like some advice on managing this.

A: As is often the case, how a condition like PSSM (a disease that causes muscle cramping in horses from abnormal glycogen storage in the muscles) affects an individual horse can vary widely. Some PSSM horses can remain asymptomatic for years (average age of first appearance of symptoms is 6 years) until they experience some change in their schedule, such as being laid up due to injury or not getting regular exercise due to weather. You might find that with good management, your filly will never show obvious symptoms.

Both diet and exercise are equally important in the management of this condition. Because PSSM1 is an issue of glycogen over-storage in muscle, the goal when managing these horses is to decrease the dietary supply of glycogen “building blocks.” Significantly reducing your horse’s starch and sugar intake and increasing the number of calories supplied by dietary fat will decrease both glucose uptake into the muscle and glycogen stores. For horses with high calorie requirements, you can choose from several high-fat/lower-starch options on the market. Because your horse is still young and growing, look for one that is specifically formulated for “growth” or for “all life stages.”

For horses with lower calorie requirements (and many PSSM horses tend to be easy keepers, making high-fat diets hard to feed without resulting in obesity), a ration balancer might be the right choice. Simply follow the feeding directions for growing horses, as they have higher nutrient requirements than mature horses. Feed either of these low-starch rations with good-quality grass hay or a maximum of 50% alfalfa hay. In the future, if your mare becomes symptomatic, you might need to manage her diet more intensively by selecting low-sugar hay, adding vitamin E, and potentially supplementing with additional amino acids

Regular turnout for as much time as possible is critical to managing PSSM horses successfully. They do not do well confined to stalls or missing days of exercise. Regular daily exercise has been shown to help manage serum CK (creatine kinase, a marker of muscle damage that increases in horses with PSSM) following exercise. Adjusting to a lower-starch, higher-fat diet will further support these results. Plenty of movement and turnout, the right diet, and careful observation usually helps horses with PSSM be comfortable and productive long-term.


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Anna Pesta Dunaway, PhD, is a nutritionist on the equine technical solutions team at Purina Animal Nutrition. She is responsible for helping bring innovative solutions from the research team out to the field. Pesta Dunaway spends most of her time providing technical consultations and support to the sales team on the East Coast, as well as speaking on equine nutrition at horse owner meetings and professional conferences. She earned her BS in animal science from Kansas State University and received both her MS and PhD in animal nutrition from the University of Nebraska. Her graduate research focused on the use of high-fat diets and manipulating the microbial community in the gut. Anna resides in Aiken, South Carolina, and is a lifelong equestrian with a special interest in the nutrition and development of the future sport horse.

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