Fructans are non-structural carbohydrates (NSC) found in cool-season grasses, such as tall fescue 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. Some studies showing a negative effect have used artificially high dietary levels of fructans.

Fructan concentrations in forage can vary anywhere between 3 – 50%, depending on the type of grass, time of year, weather, and growing conditions. [2] Monitoring when and how long your horse is turned out is important to prevent the over-consumption of fructans.

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.

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, forages can contain anywhere from 32 – 439 g of fructans per kg of dry matter. [2]

Assuming a 500 kg / 1100 lb horse consumes 10 kg of dry matter per day, high fructan hay could result in over 4 kg / 9 lb of fructans consumed in a day.

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. Fructans typically account for 55 – 75% of the WSC fraction. [2]

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 high-NSC forages. These horses should be fed a diet with less than 10% total NSC content.

Their turnout needs to be carefully managed to prevent grazing at times when fructan content in grass is known to be high.

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 carbohydrates (mainly sugars). [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.

Feeding a high-fructan diet could exacerbate insulin resistance, increase intestinal permeability (leaky gut) and contribute to endotoxemia (toxins in the blood). [3][8]

Some evidence suggests fructans may contribute to laminitis or colic, although this requires further study. [3][9]

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

Characteristics such as breed, genetics, management, and activity level also influence the risk of developing insulin resistance or laminitis.

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

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]

Studies show that feeding horses 3 grams of fructan per kilogram of body weight will induce hindgut acidosis and increase gut permeability. [13] For a 500 g (1100 lb) horse, this is equivalent to feeding 750 grams of fructans.

Researchers believe short-chain fructans pose a greater risk for hindgut acidosis because they are fermented faster, producing more lactic acid. [11][14]

Fructans and Endotoxemia

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

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

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 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 caused hindgut acidosis and increased gut permeability, but 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.

Insulin Resistance

Fructans do not directly stimulate the secretion of insulin. However, high-fructan diets may worsen insulin resistance and indirectly affect insulin levels. [2]

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

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]

Equine Metabolic Syndrome

Insulin dysregulation can result in equine metabolic syndrome (EM) – an endocrine disorder that is similar to Type 2 diabetes in humans.

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

Horses with EMS should be fed diets with less than 10% non-structural carbohydrates. [10] Fructan content in the diet should be carefully monitored to avoid worsening insulin resistance. [4][10]

Colic

Acute colic, also referred to as equine grass sickness, may be influenced by diets high in non-structural carbohydrates including fructans. [19]

Equine grass sickness is most common in the spring and fall when the grass is lush, and levels of fructans and NSC are high. [19]

Fructans are rapidly fermented in the horse’s hindgut, producing lactate and shifting the microbial population from fibre-fermenting bacteria to lactic acid-producing bacteria.

The sudden introduction of high amounts of fructans (such as when transitioning from winter hay to spring pasture) could result in hindgut acidosis, gut dysbiosis and colic. [19]

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 feeding 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 puried 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 Fructan Accumulation in Grass

When selecting forage for your horse, consider fructan content to prevent the over-consumption of these complex carbohydrates.

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

Fructan 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 fructan levels in forage include: [2]

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

Season

Fructans are produced during photosynthesis as a form of plant energy storage. [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 levels in grass vary between seasons because of temperature changes and the energy demands of plants as they grow. [23][24]

Concentrations of 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 levels rise again in the fall when temperatures drop, but peak at a lower level than in the spring. During the winter, fructan 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 content without a forage analysis. [25]

Temperature

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

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

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

Lighting

Fructan 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]

Nutrient Availability

Nutrient levels in the soil also affect the fructan content of forage. Location and soil quality should be considered when growing forage for horses or choosing a boarding facility where your horse will be turned out on pasture.

Using fertilizers, especially those containing nitrogen, encourage plant growth while reducing fructans in the grass. [1]

Although the effects of water supply on fructans have not been well studied, several observations suggest that fructans may rise following heavy rainfall. [14]

Fructans also play a role in preserving forages during drought or stress conditions. Fructans levels rise in forages during periods of drought to protect the plant. [27][28]

Plant Genetics

Genetics explains some of the variations in fructan 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]

Fructan-accumulating plants include: [29]

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

Starch-accumulating plants include: [29]

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

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]

Grass Height

Grass height can also impact fructan concentrations in forage. Fructans are stored in the leaves, stems and roots of forage, but the highest levels are found in the stems.

Over-grazed pastures are a risk factor for excessive fructan consumption.

Fructan levels decline in the grass after it has been cut or mowed because fructans are used for regrowth. Fructans are highest in first-cut hay and decrease in each subsequent hay cutting. [21][31]

Tips to Prevent Excess Fructan Intake

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

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

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Regulate Pasture Turnout

Avoid grazing your horse on pasture in the afternoon when the sun is high and bright. Instead, turn them out in the morning or overnight when fructan levels in grass are lower.

Provide free-choice hay for your horse when they are not turned out to enable natural foraging behaviours throughout the day. [2]

Pasture access in the spring and fall should be limited for horses sensitive to fructans. [2] To enable free movement, use a grazing muzzle or turnout your horse in a dry lot with appropriately selected hay.

Limit turnout in fields right after grass has gone to seed or after harvesting hay because fructan concentrations will be elevated. [2]

Maintain Low-Fructan Pastures

If possible, consider planting warm-season grasses that do not produce fructans.

Fertilizing your grass and keeping it short but not overgrazed can also minimize fructan accumulation. [2] If mowing grass, be sure to collect the clippings which may be high in fructans.

Soak Hay

Soaking hay in warm water for 30 minutes can reduce fructan concentrations by approximately 8 to 9%. Long periods of soaking (up to 12 hours) can reduce fructan levels by 38 to 58%. [35]

Keep in mind that soaking hay will also remove other nutrients (such as vitamins and minerals) and reduce the overall energy content of the hay. [35]

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 or metabolic issues in horses when ingested in large quantities.

There is an ongoing debate about whether fructans can cause laminitis in horses, but some research suggests that high-fructan diets can aggravate insulin resistance in susceptible horses.

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 need to avoid high-fructan forages and should be fed a diet with less than 10% NSC content. Grass management, regulating turnout times, using a grazing muzzle and soaking hay are all ways to limit fructan 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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