Does Your Horse Need a Low-Starch Diet?

Conditions that benefit from low starch levels and how to make the change

My students at Midway University often ask my if their horses should be on low-starch (starch being a type of carbohydrate) diets. This has led to some great discussions about low-starch and low-carbohydrate diets and how all horses must consume carbs— they just need to be the “right” ones. It’s also important to realize that a low-starch diet is not ideal for all horses but, rather, depends on their caloric requirements, the disciplines they perform, and existing health conditions.

“Low-carb for humans essentially means restricting all easily digestible sugars, starches, soluble fibers, and other digestible fibers from our diet to lose weight,” says Rhonda M. Hoffman, PhD, PAS, Dipl. ACAN, professor and director of the Horse Science Program at Middle Tennessee State University, in Murfreesboro. “Horses, in contrast, rely on carbohydrates as the largest portion of their diet, especially soluble and insoluble fibers.”

Forages, which should be the basis of any diet, are 75-90% carbohydrates. The types of carbohydrates found in pasture and hay fall into two very broad categories: nonstructural and structural.

Carbs, Sugars, and Starches in Your Horse’s Forage and Feed

Nonstructural carbohydrates (NSCs) in forages are simple sugars and fructans, which horses can readily digest or ferment. They are produced in plants during warmer weather through photosynthesis and tend to be higher in more immature forages. Plants store these carbohydrates during the day, then use them at night for metabolic processes. Pasture is usually lowest in NSCs in the early morning, unless overnight temperatures are cold and cause metabolic processes to slow.

These carbohydrates can affect some of the disease processes we’ll describe in this article, so you’ll need to monitor how much your horse consumes.

The components of the typical concentrate portion of a horse’s diet also contain carbohydrates, including simple sugars and starch. Grains such as corn and oats are seeds and, as such, are high in NSCs, mostly starches. Starches are long chains of attached sugar molecules. These sugars get broken apart during the digestive process and the simple sugars (glucose) readily absorbed. Most horses can digest and absorb sugars and starches in the small intestine, through a process called hydrolysis. From there, glucose in the bloodstream triggers the pancreas to release insulin; the glucose molecules then move into cells for storage as glycogen.

However, sometimes starch makes it into the cecum, which is the first part of the hindgut (everything after the small intestine), where it’s rapidly fermented. This usually only occurs if there is more starch in a single meal than the horse can digest.

The byproduct of this fermentation is lactic acid, which lowers hindgut pH, killing the good bacteria that reside there. The endotoxins released from microbial death can contribute to both colic and laminitis (when the laminae that suspend the coffin bone within the hoof capsule become damaged and inflamed).

There is no single definition of a “low- starch” diet. “Carbohydrates by definition would include sugars, starches, and some fiber, which is why I will not use the term low-carbohydrate to describe a horse’s diet,” says Amy Parker, MS, manager of technical services and equine nutritionist for McCauley Bros. Inc., in Versailles, Kentucky. “By this definition, a low- carbohydrate diet could mean a low-fiber diet, which would not be healthy and would likely prove fatal for the horse.

When discussing feedstuffs, I prefer ‘low-sugar and -starch’ or ‘low-nonstructural- carbohydrates.’ Discussing both the sugar and starch concentrations is important, as describing only one or the other can be misleading.”

Hoffman explains these terms further: “Low-starch is generally a label put on grain concentrates to guarantee that starch (and sugar) content is low enough to limit glycemic response in horses,” she says. “Usually this content is under 10-12% starch—but really starch and sugars. The recommendation for horses with metabolic issues is not low-starch but low-nonstructural carbohydrates. The NSC is a laboratory measure containing starch plus all water-soluble carbs (sugars and fructans).”

In general, a traditional sweet feed with grains such as corn and oats as its base might be 40-60% starch. Although this might seem high, horses without underlying issues can consume it without issue. A “mid-level” starch content is usually between 20 and 25%. A “low-starch” feed usually contains less than 15% starch, but some companies might classify low-starch as any feed below 20%.

While some companies do include starch information on their labels, you can talk with your feed manufacturer directly to determine the starch content of its feeds.

Conditions of Concern

Indeed, for horses with some conditions, veterinarians and nutritionists might recommend a low-starch diet to help maintain blood glucose levels at a steady level. These are conditions that cause horses to become more sensitive to sugars and starches, requiring owners to reduce these levels in both forages and concentrates. Individual horses have variable responses related to a variety of factors, including age, body condition, fitness, metabolic status, and disease status.

Obesity and equine metabolic syndrome (EMS)

Conditions that fall under the metabolic umbrella are of major concern, say Hoffman and Parker. Some breeds are “metabolically thrifty” and can readily convert glucose into fat for storage.

These horses need an overall reduction in calories consumed, and not just from starches. They would benefit from a lower-quality forage and no concentrates, except for possibly a ration balancer to provide vitamins and minerals.

Insulin dysregulation (ID)

In these horses insulin is not effective at trans- porting glucose from the bloodstream into cells, so both remain elevated. This can lead to increased susceptibility to

laminitis. Horses with this condition are very sensitive to starches and, therefore, should be on as low a starch and, specifically, soluble-carbohydrate diet as possible. Insulin dysregulation is considered a component of EMS.

Polysaccharide storage myopathy (PSSM)

Horses with the muscle disorder PSSM often have a normal glucose/ insulin metabolic process, but they are unable to use the form of glucose stored in their cells as energy. These horses are also susceptible to tying-up (exertional rhabdomyolysis, or the breakdown or destruction of skeletal muscle cells associated with exercise).

Laminitis

Horses that are prone to laminitis should also consume diets low in soluble carbohydrates and starch.

Remember that elevated levels of starch, especially if they reach the hindgut, can lead to hindgut acidosis, effectively killing off the good microbes there. Then endotoxins get released, which can negatively affect enzymes involved in maintaining the integrity of the laminae in the hoof.

This can lead to laminitis. These horses should not be allowed to graze immature or lush, rapidly growing pasture routinely found in spring and early fall.

Pituitary pars intermedia dysfunction (PPID, formerly equine Cushing’s)

This endocrine disease mostly affects horses over the age of 15. It develops when the neurons in the hypothalamus at the base of the brain deteriorate and fail to regulate the pituitary gland’s par intermedia. Some (but not all, cautions Hoffman) PPID horses require a lower-starch diet.

“PPID horses that are also insulin-dysregulated are the ones which will benefit,” she says. “Some PPID horses are thin, non-insulin-dysregulated, and they need calories … and a more traditional NSC level is fine for these horses.”

Gastric ulcers

Horses prone to ulcers and hindgut acidosis might also benefit from a low-starch diet, says Parker. Signs of this condition include poor performance, poor attitude, and mild colic.

Low-starch diets are beneficial to horses prone to gastric ulcers, because chewing and consuming the fibrous carbohydrates produces more saliva, which can help reduce acidity in other parts of the digestive tract, such as the stomach.

Reactivity

Horses that are anxious or hyperactive might benefit from less starch in their diets. Researchers on multiple studies (Bulmer et al., 2019; Destrez et al., 2015) have focused on diet’s effect on behavior. Recently they’ve explored more of the “why” behind this. Glucose is a sugar that easily crosses the blood-brain barrier. Elevated glucose levels are associated with increased dopamine production, which can lead to elevated awareness or hyperexcitability.

Going Low-Starch

If your horse does not have one of the above conditions, then he probably doesn’t need a low-starch diet. In fact, some performance horses, such as Thoroughbred racehorses, benefit from a diet with readily available carbohydrates. These horses need to be able to recover after a training bout or race, which includes replacing the stored glycogen in skeletal muscle.

“Performance horses, especially every horse that has anaerobic exercise (short bursts of high-intensity training) as a component of its work, require some starch in their diet to replenish muscle glycogen,” says Hoffman. “For example, I’ve known a limited non-pro reining horse that the owner, in good faith, put on a low-starch, low-NSC diet … and it literally ran out of gas. When muscle glycogen is low, the muscle adapts by slowing contraction rate and power in order to conserve fuel (glycogen). This is obviously not desired in a competition horse.”

Horses that are in good body condition (not too fat or too thin), are fit for their discipline, and have high caloric demands are likely fine on a more traditional feed containing higher starch levels. Horses that are hard keepers might not be good candidates for low-starch feeds, either, unless you also increase fat content in the diet. These horses need more readily available calories than the fibrous ingredients often used in low- starch feeds.

For the horse that needs a low-starch concentrate, ideally, you’ll transition him the same way you would to any new diet. You must make a slow, gradual change, so as not to upset the hindgut microflora—those microbes must have time to adjust to a new diet. Most nutritionists recommend making the complete transition over about two weeks. Start with a meal that is ¼ of the new feed and ¾ old feed, and stay at this level for four days. Move up to ½ and ½ for another four days. Then switch to ¾ new feed and ¼ old feed for another four days. By the end of this period, you should be able to feed a full meal of the new feed. If you are concerned about NSC levels in forage, too, you can limit pasture access when they’re elevated (e.g., during spring grass growth) and soak hay before feeding. Just remember to discard the soak water, so the horse doesn’t drink it.​

Hoffman and Parker both note, however, that the transition to a low-starch diet often has to happen suddenly, such as after a metabolic event (e.g., laminitis) where the horse is moved from lush pasture to a drylot or begins to wear a grazing muzzle. In these cases you don’t have days or weeks to make the change. Observe your horse carefully for signs of digestive disturbance such as loose stool. If signs develop (and if possible), back off the transition rate and make the conversion more slowly.

If you have more than one horse and one requires a low-starch diet, you might wonder if you can feed all the horses that same feed. Doing so might make feeding more convenient, but you must consider individual horse needs.

“As long as the horse’s nutrient requirements are being met, there is no harm in a horse being on a low-sugar/starch diet,” says Parker. “Owners need to work with a nutritionist to (confirm this). The only drawback may be cost, because low-sugar/starch feeds are usually more expensive due to higher ingredient cost as compared to traditional grain-containing feeds.”

Hoffman, on the other hand, believes that if a horse does not fit into one of the recommended profiles for a low-NSC diet, the owner should not feed it as such.

Take-Home Message

A low-starch diet might be in the picture for your horse, depending on his health. If not, however, it’s likely not necessary and might even reduce performance. Before making the change, talk to your veterinarian and a qualified equine nutritionist. It is also important to know how your feed company uses the term “low starch,” so you are making decisions based on the best information available.