Fructans are a storage form of carbohydrate and a component of the non-structural carbohydrates (NSC) found in cool-season grasses, such as tall fescue, timothy, orchardgrass, and perennial ryegrass.

Fructans are indigestible by horses, passing through the foregut to the hindgut where they are rapidly fermented by bacteria to supply energy to the horse. [1]

There is an ongoing debate about the effects of fructans on equine health. Some researchers suggest that diets high in fructans predispose horses to health conditions such as insulin resistance, laminitis, or leaky gut syndrome.

Other researchers argue that fructans do not cause laminitis because they do not trigger insulin secretion. Laminitis has only been triggered experimentally by administering very large doses of chickory root fructan by stomach tube, with the resultant syndrome the same as with grain overload in the hind gut.

Fructan concentrations in forage can vary anywhere between 3 – 50%, depending on the type of grass, time of year, weather, and growing conditions. [2] However, there has never been a documented case of laminitis linked to the fructan level of pasture or hay.

What are Fructans in Forage?

Fructans occur naturally in many plants, including grasses. They serve as an energy storage form for plant cells. [1]

Fructans are polysaccharides composed of short chains of fructose molecules terminating with a glucose molecule. The length of a fructan depends on the number of fructose molecules linked together. [3]

Cool-season grasses such as tall fescue and ryegrass tend to store excess energy in the form of fructans. In contrast, warm-season grasses and legumes do not produce fructans. Instead, they store excess energy as starch (a complex chain of glucose molecules).

There are three main categories of fructans: graminans, inulins, and levans. These fructans differ in the number of fructose molecules and by the bonds connecting them. Graminans and levans are the main forms of fructans found in forages. [4][5][6]

All fructans are types of non-structural carbohydrates, used primarily as energy storage for plants. NSCs also include simple sugars (glucose, sucrose, fructose) and starches.

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Fructans in the Equine Diet

Fructans are complex carbohydrates that are rapidly fermented in the horse’s hindgut. This process generates volatile fatty acids that are absorbed from the hindgut and used for energy by the horse. [1]

Depending on environmental conditions and plant species, fresh forages can contain anywhere from 32 – 439 g of fructans per kg of dry matter. [2] It is concentrated in the lower stems of the plant.

Assuming a 500 kg / 1100 lb horse consumes 10 kg of dry matter per day, high fructan pasture could result in over 4 kg / 9 lb of fructans consumed in a day. However, slow intake over a 24-hour period is not the same thing as a bolus by stomach tube. In those studies chickory fructan was used, which is not the same thing as the levan most predominant in grasses.

In addition, the Dairy One database of over 8,000 North American pasture grass samples shows a maximum fructan content of less than 20 grams per kg of dry matter. [36]

The very high pasture fructan levels are mainly found other parts of the world, such as the subarctic or regions that grow a lot of high-fructan ryegrass. In these grasses, the simple sugars (ESC) are also very high; well above the threshold for triggering laminitis in a horse with metabolic syndrome. [2]

The best way to determine the carbohydrate content of your horse’s forage is to submit a sample for analysis.

A hay analysis will provide levels of water-soluble carbohydrates (WSC), ethanol-soluble carbohydrates (ESC), and starch.

High fructan levels in grasses correlate with greater energy supply to the horse. [7] While forages with high digestible energy may be appropriate for horses in heavy exercise or with high-calorie requirements, they are not recommended for easy keepers or overweight or obese horses.

Horses with insulin resistance and Equine Metabolic Syndrome also need to avoid forages with high hydrolyzable carbohydrates (HC) – the ESC (sugar) + starch portion of NSC. These horses should be fed a diet with less than 10% total HC content.

Fructan Digestion in the Horse

Horses digest and absorb simple sugars in the foregut, but they cannot digest fructans because they lack fructanase enzymes. These enzymes are responsible for breaking down fructans into fructose molecules. [3]

Without fructanases, fructans remain intact and pass through the foregut undigested and enter the hindgut where they are fermented by gut bacteria. [3]

This makes fructans similar to structural carbohydrates, such as cellulose, which are also exclusively digested by microbial fermentation in the horse’s hindgut.

Fructan Fermentation

The horse’s foregut includes its stomach and small intestine, which are responsible for the enzymatic digestion of proteins, lipids, and some hydrolyzable carbohydrates such as starch from grains. [5]

The hindgut is the part of the gastrointestinal tract that comes after the foregut. It includes the cecum where most fermentation occurs and the colon where fermentation continues, and nutrients are absorbed.

Fructans are quickly fermented by microbes in the cecum, producing volatile fatty acids and lactic acid. [5][6]

Consequences of a High-Fructan Diet

Diets containing excess fructans can lead to several health concerns for horses due to the impact of fructan fermentation on the digestive tract.

Experimental overdose of fructan could increase intestinal permeability (leaky gut) and contribute to endotoxemia (toxins in the blood). [3][8]

Fructans in forages can also contribute to weight gain, particularly in ponies, easy keepers and horses at risk of obesity. [10]

Hyperinsulinemia (high blood insulin) together with characteristics of metabolic syndrome are the only things that have been linked to pasture laminitis risk. [19][27][28]

Hindgut Acidosis

Fermentation of fructans produces lactic acid, which lowers the pH in the hindgut making it more acidic. [11]

The horse will develop subclinical hindgut acidosis if the hindgut pH falls to 6. [12]

A hindgut pH below 6 is considered clinical acidosis and may be accompanied by diarrhea and changes in the microbiome to favour lactic acid-producing bacteria and reduce fibre-fermenting bacteria. [2][12]

Gut Permeability

Severe hindgut acidosis can also result in hindgut ulcers and increased intestinal permeability, making it easier for substances to pass through the lining of the intestines into the bloodstream.

The acidic environment irritates and damages the lining of the horse’s gut, making it more permeable. This enables toxins and bacterial products from the gut to leak into the horse’s bloodstream. [6][11]

One study used normal ponies or ponies predisposed to develop laminitis. They were fed 3 grams of fructan per kilogram of body weight which reduced fecal pH but without inducing diarrhea and there was no evidence of increased gut permeability. [13] For a 500 g (1100 lb) horse, this is equivalent to feeding 1,500 grams of fructans.

Researchers believe short-chain fructans, like the oligofructose from chickory root used to experimentally produce laminitis, pose a greater risk for hindgut acidosis because they are fermented faster, producing more lactic acid than the levans and gramminans in grasses. [11][14]

Fructans and Endotoxemia

In experimental models, researchers believe that fructans cause laminitis by increasing gut permeability and triggering endotoxemia.

This is the same mechanism as in the “horse broke into the feedroom” scenario or with deliberate experimental overload of cornstarch. It is distinctly different from laminitis caused by high blood insulin levels.

The heightened gut permeability allows bacteria and toxins to pass from the intestines into the bloodstream and reach the hoof. [12][15]

Bacterial products trigger activation of the matrix metalloproteinase enzymes which break down the basement membrane which is anchoring the laminae. [9]

Toxins called vasoactive amines cause vasoconstriction, reducing blood flow in the hoof. [6][11] This cuts off the nutrient supply to the laminae in the horse’s hoof, which connects the coffin bone to the hoof wall.

The laminae become inflamed, causing the coffin bone to detach from the hoof wall and resulting in laminitis. [6][13]

Laminitis

Feeding experimentally high doses of fructans can induce laminitis in horses. In one study, all horses receiving a bolus of 10 g per kg BW (body weight) dosage of inulin-like oligofructose developed clinical laminitis. [9]

Other studies have induced laminitis in horses by administering a bolus of 7.5 g per kg BW of inulin. [9] However, another study found that 3 g of inulin per kg BW lowered hindgut pH but did not increase gut permeability, and did not result in laminitis. [13]

The feeding rates that triggered laminitis are equivalent to 3.75 to 5 kg of inulin for the average 500 kg (1100 lb) horse, equivalent to 37.5 to 50% of the dry matter of the diet which would not occur naturally.

Insulin Resistance

Insulin resistance occurs when cells fail to respond properly to the hormone insulin. This hormone helps your cells take up glucose from the blood and use it for energy or store it as glycogen or fat.

When cells become insulin-resistant, blood sugar levels rise, and the horse’s body continues secreting insulin in response. [17]

One study fed inulin (3 g per kg BW) to ponies predisposed to laminitis and found that the inulin caused insulin levels to rise suggestive of insulin resistance. [16]

However, that study was deeply flawed in that they replaced one-third of the tested, low HC/NSC forage with flash-dried ryegrass as a carrier for the inulin. Flash drying preserves the highest levels of sugar. Ryegrass, known to be very high in both HC and fructan, was likely the cause of the insulin spike.

Another study fed 1 g/kg BW of glucose, fructose or inulin to both normal ponies and ponies predisposed to laminitis. They found higher insulin responses to glucose and fructose in the previously laminitic ponies but virtually no response to inulin in either group. [31]

This again demonstrates that the ESC and starch content of forage are of greater concern regarding laminitis risk than the fructan content.

Equine Metabolic Syndrome

Hyperinsulinemia is a component of equine metabolic syndrome (EMS).

Equine metabolic syndrome is characterized by obesity, regional fat deposits (such as cresty neck), insulin resistance and a high risk of laminitis. [18]

Horses with EMS should be fed diets with less than 10% hydrolyzable carbohydrates. [10] There is no evidence that fructan levels naturally found in forages are a concern for metabolic syndrome.

Colic

Like plant fibres, fructan is fermented in the hindgut rather than digested in the small intestine. However, unlike most other plant fractions its fermentation produces large amounts of lactate rather than volatile fatty acids.

This is similar to what occurs in grain overload where excess simple sugar and starch reaches the hindgut and produces lactic acid.

Because of the rapid fermentation and shifts in pH, introduction of high fructan items should be done slowly to avoid diarrhea and possible bloating and colic.

Can Fructans in Grass Cause Laminitis?

Outbreaks of pasture-associated laminitis often occur in the spring and fall, when fructans and simple sugars are highest in forage. But does this mean that high fructans cause laminitis?

Research shows that simple sugars directly cause laminitis, but there is no research proving that fructans in forage can directly trigger laminitis.

Horses typically consume 2 – 2.5% of their body weight in forage daily on a dry matter basis.

Research shows that forage can contain anywhere from 32 – 439 g of fructans per kg, depending on the plant species and climate conditions. [2]

This means that a typical mature adult horse weighing 500 kg (1100 lb) could consume up to 5.5 kg of fructans per day on a pasture-only diet. Of this a portion will be inulin, but the other fructans (graminans and levans) make up the majority of fructans in forages.

There are some key differences between the studies that induce laminitis by administering pure chickory root fructans and how horses consume fructans from forages:

  1. Dose: These studies delivered the high dose of inulin over a short period of time as a bolus. Horses consuming a high-fructan hay would consume an similar level of total fructans over a full day. [2]
  2. Type: These studies used purified short-chain inulin derived from chicory root. The fructans of forages consist of inulin, graminans and levans. Of these, graminans and levans are more abundant in forages. In vitro studies show that grass-derived fructans can alter pH and lactic acid content but the in vivo implications are unknown. [6]

Dr. Eleanor Kellon (DVM), argues that fructans are unlikely to contribute to laminitis in the same way as simple sugars because fructans do not directly influence insulin levels – the primary mediator of endocrinopathic laminitis. [2][20]

Excessive one-time doses of inulin alone can trigger laminitis, but horses would never naturally consume this much inulin in a short period of time when grazing on pasture grasses or hay.

Horses graze slowly throughout the day, and fructan intake is accompanied by other nutrients such as structural carbohydrates. The slower rate of fructan consumption makes it unlikely that the results seen in experiments would be seen in the real world.

It is unclear whether the types and concentrations of fructans found naturally in forage can trigger laminitis when evaluated over a typical daily feeding rate.

Limiting Starch and Sugar Accumulation in Grass

Now that we’ve established that hydrolyzable carbohydrates (ESC + starch) are the main culprits of pasture-associated laminitis, you can consider these when selecting forage for your horse.

Choose forage that provides adequate energy for your horse’s needs without over-supplying hydrolyzable carbohydrates that can contribute to health conditions.

Simple sugar content in grass varies by time of day, time of year, plant species, environmental conditions and more. Remember these factors when deciding when to allow your horse to graze on pasture and when to harvest hay.

Factors that influence sugar and starch levels in forage include: [2]

  • Grass height
  • Season
  • Temperature
  • Lighting
  • Nutrient availability
  • Plant genetics

Season

Simple sugars are produced during photosynthesis as a form of plant energy that is used immediately or stored. [21]

When the plant produces more sugars than it can use, the excess sugars are stored as fructans (polymers of fructose) or starch for future growth and survival. [3][22]

Fructan and starch levels in grass vary between seasons because of temperature changes and the energy demands of plants as they grow. [23][24]

Concentrations of starch and fructans are usually highest during the spring, peaking between late April and June. [11][25][26] Levels decrease through the late spring and summer as photosynthesis and plant growth slow.

Fructan and starch levels rise again in the fall when temperatures drop, but peak at a lower level than in the spring. During the winter, fructan and starch levels decrease as they are used for energy. [26]

These patterns vary considerably depending on grass species, temperature, and nutrient availability. This makes it difficult to accurately predict fructan and starch content without a forage analysis. [25]

Temperature

Temperature affects plant growth rates and how quickly plants use up their carbohydrate stores.

Warm temperatures (between 10oC and 20oC) encourage grass growth and cause fructans and starch to be used by plants. [11][25]

In contrast, plant growth slows in cooler temperatures (between -4oC to 10oC), causing fructans and starch to accumulate. Plants use less of the sugars they make through photosynthesis and instead store them. [11][25]

Lighting

Storage carbohydrates levels in forage rise in the presence of sunlight. [2]

Concentrations are usually lowest in the morning, rise during the day, peak in the afternoon and fall during the night. [2]

Plant Genetics

Genetics explains some of the variations in fructan and starch levels between different species of forages. Forages are broadly categorized as either cool-season or warm-season plants.

Cool-season forages (excluding legumes) store sugars as fructans. In comparison, warm-season plants and legumes typically store sugars as starches and contain few fructans. [2][22]

Warm-season plants primarily store starch in the leaves while cool-season forages accumulate fructans in the stems. [2] However, horses typically graze the upper portions of grasses, cutting them off two inches off the ground thus leaving behind the base where fructans are concentrated.

Fructan-accumulating plants include: [29]

  • Annual ryegrass
  • Perennial ryegrass
  • Timothy
  • Meadowgrass
  • Cocksfoot
  • Bluegrass
  • Oat
  • Barley
  • Orchardgrass
  • Bromegrass

Starch-accumulating plants include: [29]

  • Alfalfa (and other legumes)
  • Maize
  • Pangolagrass
  • Teff
  • Bahiagrass
  • Bermudagrass

Fructan content also varies between species of grass or legumes. For example, several studies have found that timothy grass accumulates more fructans and larger fructans than other grasses such as perennial ryegrass and meadow fescue. [21][30]

Tips to Prevent Excess Starch and Sugar Intake

Horses typically graze for 12-17 hours per day providing ample opportunity for starch and sugar consumption. Unrestricted grazing can allow susceptible horses to develop conditions associated with excess insulin stimulation. [2]

There are several ways to limit your horse’s hydrolyzable carbohydrate intake. Management strategies include: [2][32][33][34]

  • Restricting pasture turnout to early morning hours
  • Keep sensitive horses off pastures until they have grown to full height and dropped their seed
  • Using a grazing muzzle
  • Submitting your hay for analysis before feeding to verify that HC content is low
  • Fertilizing pasture with nitrogen to encourage the grass to use up carbohydrate stores for growth
  • Soaking hay in water for 30 minutes to remove some of the sugar [35]

If you suspect hindgut dysfunction related to high sugar and starch intake, consider feeding Mad Barn’s Optimum Digestive Health to support a healthy hindgut microbiome.

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  • Prebiotics, probiotics & enzymes
  • Support hindgut development
  • Combats harmful toxins in feed
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Keep in mind that forages alone will not supply all the vitamins and minerals your horse needs. Work with an equine nutritionist to ensure your horse’s diet is appropriately balanced.

Summary

Fructans are a type of non-structural carbohydrate found in grasses that could cause digestive issues in horses when ingested in large quantities.

There is an ongoing debate about whether fructans can cause laminitis in horses outside the experimental setting but there is currently no evidence to support this.

Cool-season grasses commonly found in the equine diet can contain anywhere between 3 – 50% fructans depending on growing conditions, plant species, weather, season, and other factors.

Metabolic horses generally should be fed a diet with less than 10% hydrolyzable carbohydrate (starch + ESC) content. Grass management, regulating turnout times, using a grazing muzzle and soaking hay are all ways to limit HC consumption.

If you suspect your horse may be sensitive to fructans, talk to your equine nutritionist about strategies to limit their intake or alternative forages that may be more suitable.

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References

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  3. Johnson, R. Fructokinase, Fructans, Intestinal Permeability, and Metabolic Syndrome: An Equine Connection? J Equine Vet Sci. 2013.
  4. Longland, A.C. et al. Comparison of a colorimetric and a high-performance liquid chromatography method for the determination of fructan in pasture grasses for horses. J Sci Food Agric. 2012.
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