Feeding Type-1 PSSM Horses

Pasture turnout can help type-1 PSSM horses, because it allows constant movement; however, it might not be appropriate for all horses.

Photo: Thinkstock

Q. My veterinarian recently diagnosed my horse with type-1 PSSM. Do you have any advice on feeding horses with this condition? She’s such a nice horse and I’d like to do anything I can to keep her healthy and working.


A. Horses with the GYS1 mutation (type-1 polysaccharide storage myopathy, or type-1 PSSM) accumulate abnormally large amounts of glycogen (the storage form of glucose), as well as the abnormal sugar amylase-resistant polysaccharide, in their muscle tissue. This mutation is inherited, exists in many equine breeds, and affects as many as 35% of individuals within certain breeds. 

Researchers posit we might have unwittingly selected horses with this gene defect. That’s because when humans historically used horses for activities such as pulling ploughs and carriages, individuals with an ability to store more muscle glycogen would likely have performed better. The difference today is that we manage our horses differently. We tend to work our horses less intensely and feed them beyond their needs. This results in the development of clinical signs in those horses with the gene mutation. Nutrition is, therefore, a key component in successfully managing horses with type-1 PSSM. In fact, about 50% of horses with type-1 PSSM are reported to show improvement with diet changes alone. This might increase to 90% with additional changes in exercise protocol. 

The interesting thing about horses with type-1 PSSM is that despite having 1.8 times more glycogen in their muscle tissue than normal horses, they suffer from energy deficits when exercised. The GYS1 mutation causes the enzyme glycogen synthases to be overly active, particularly in the presence of insulin. This can result in an almost constant formation of glycogen. As the storage form of carbohydrate, glycogen is used by muscle tissue as a source of adenosine triphosphate  (ATP, the cell’s energy currency). Glycogen can be used during low-intensity aerobic work as well as during more intense work when muscles need to work anaerobically. 

Feed a Low-Startch Diet

After horses consume meals containing starch and sugar there’s more insulin present in the body, which can trigger glycogen synthesis and storage. Therefore, the most important part of feeding a horse with type-1 PSSM is limiting starch levels and readily available sugar from the diet.  When horses require additional calories to maintain a healthy body condition, these should come from fat rather than those carbohydrates. 

Many people think about avoiding grain but forget that most a horse’s diet is forage, and sometimes forage sources can be high in sugars. Forages should have less than 12% nonstructural carbohydrate (NSC) on a dry-matter basis. Ideally, owners should have hay tested to determine the NSC content before feeding it. If this isn’t practical, soak the hay for 30 minutes to an hour to remove some of the water-soluble sugar. Depending on the hay, soaking still might not result in a NSC below 12%.

Pasture turnout can help these horses, because it allows constant movement; however, it might not be appropriate for all horses. You must balance pasture access against the risk of horses potentially consuming a lot of high-sugar grass, especially when pastures are particularly lush. A grazing muzzle can be a useful management tool in these instances.

Type-1 PSSM horses are often easy keepers, and many are able to maintain their body condition purely from the forage in their ration. Staying away from feeding many of the traditional grains such as corn, oats, and barley is generally advised, because these are all high in starch and can result in elevated insulin and in turn greater accumulation of muscle glycogen.

Need More Calories? Fat’s the Best Choice

When extra calories are needed, fat is the source of choice for type-1 PSSM horses. This is where I often see mistakes being made in the diets of horses. I often encounter owners feeding high-fat (approximately 12-13%) feeds, yet because their horses are easy keepers they are feeding at well-below the manufacturer’s recommended daily intake. This can result in an unbalanced diet and a horse that’s not consuming enough key trace minerals and vitamins. Alternatively, some owners feed oil poured over hay pellets, which offers no diet fortification and, again, leaves the horse lacking key nutrients.

A better choice here is to feed a low-NSC ration balancer that has a small serving size and then, if necessary, feed a small amount of oil as a fat source. The amount of oil needed appears to vary from horse to horse. I have some horses that are symptom-free without any oil in their ration, while others need some.

Researchers recommend that less than 10% of the calories in the diet come from NSC and 20% come from fat. I recommend that you start off by feeding suitable forage along with the ration balancer and if your horse is still exhibiting clinical signs try adding a half cup of oil.  Some horses need as much as 2 cups of oil per day, but many do not.

Adding fat might mean you must reduce the forage you’re feeding to prevent undesired weight gain. Make sure, though, that you maintain a forage intake of at least 1.5% of body weight. Feeding a more mature, lower-calorie hay will help your horse maintain a higher intake, because such hays tend to have lower calorie contents.

Also keep in mind that most oils add nothing to the ration other than calories. Feeding too much oil might cause your horse to decrease forage consumption, which would result in a net reduction of many important nutrients.

As far as oil type, it doesn’t appear that the form of fatty acid has any particular benefit to helping type-1 PSSM. I like using oils that have other benefits beyond just offering a source of calories. For this reason, I like oil with a good amount of omega-3 fatty acid such as camelina or flax oil, which might help support a healthy inflammatory response. Note that camelina oil is more stable and doesn’t require refrigeration.

Some horses do well being fed some rice bran, which is about 20% fat and approximately the same level of starch. This might be too much starch for some horses, especially if you’re feeding larger amounts of rice bran.  

As fat in the diet increases so does the horse’s vitamin E requirement. Current recommendations suggest that for every cup of fat you feed you should supplement 600 IU of vitamin E. Consider having your horse’s vitamin E levels checked to ensure this is adequate. If necessary, supplement additional vitamin E in its natural form.

Other Options

Some owners feed their type-1 PSSM horses additional magnesium and acetyl-L-carnitine (ALCAR). Magnesium is thought to help with muscle relaxation if the horse is magnesium deficient, and the ALCAR is an amino acid that might assist with glucose clearance and encourage more efficient utilization of carbohydrates. No formal research exists on the impacts of either magnesium or ALCAR and horses with PSSM, although many people managing type-1 PSSM horses report good results. Some believe that using ALCAR allows them to feed less fat, which could benefit easy keepers.

Take-Home Message

Type-1 PSSM has no cure, and owners must carefully manage any dietary or exercise changes to avoid a flare up of clinical signs. However, through the careful implementation of a nutrition plan combined with an appropriate exercise protocol, horses with type-1 PSSM can lead active, healthy lives.

About the Author

Clair Thunes, PhD

Clair Thunes, PhD, is an independent equine nutrition consultant who owns Summit Equine Nutrition, based in Sacramento, California. She works with owners/trainers and veterinarians across the United States and globally to take the guesswork out of feeding horses. Born in England, she earned her undergraduate degree at Edinburgh University, in Scotland, and her master’s and doctorate in nutrition at the University of California, Davis. Growing up, she competed in a wide array of disciplines and was an active member of the United Kingdom Pony Club. Today, she serves as the regional supervisor for the Sierra Pacific region of the United States Pony Clubs. As a nutritionist she works with all horses, from WEG competitors to Miniature Donkeys and everything in between.

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