Iron is a trace mineral that is required in the horse’s diet. Iron is involved in many bodily processes, including transporting oxygen in the blood and producing energy in cells. [1]

Supplementation with this mineral is usually not recommended because it is naturally abundant in feeds, forages, and even drinking water. Horses are more likely to have excess iron intake than be deficient in this mineral.

Excess iron is not excreted by the body and accumulates in the liver. Over time this can lead to symptoms of iron overload, which is associated with insulin resistance in horses and other animals.

This article summarizes the functions of iron in the horse’s body, nutritional requirements for this mineral, and the link between iron and horse health.

Iron Functions in the Horse’s Body

In the horse’s body, iron is found in many cells and tissues, but it is mostly contained in the muscle, spleen, liver, bone marrow, and blood. [2] The distribution of iron in the body is approximately: [1][2]

  • 60% in hemoglobin, the part of red blood cells that binds to oxygen
  • 20% in myoglobin, an oxygen transport protein found in muscle
  • 20% in proteins, such as ferritin and hemosiderin that store and transport iron
  • 0.2% in enzymes within the mitochondria of cells and elsewhere

Iron is an important component of several enzymes and proteins that are essential for bodily functions. Oxygen transport is one of the most well-studied functions of iron for horses. However, this mineral’s role in diverse processes such as energy metabolism and the immune system are gaining more attention.

Oxygen Transport

Hemoglobin is the protein in red blood cells that binds to oxygen and carries it throughout the body. Each hemoglobin contains four iron atoms with a high binding affinity for oxygen.

When red blood cells pass through the lungs, where there are high levels of oxygen, the iron in hemoglobin quickly binds and captures oxygen.

From there, red blood cells circulate throughout the body, releasing oxygen into areas where the oxygen concentration is lower. [3] Red blood cells then circulate back to the lungs to repeat the process.

It’s important to note that although iron is an critical constituent of red blood cells, supplementing with additional iron will not result in better oxygen carrying capacity of red blood cells. [1]

Cellular Energy Production

Iron is also involved in energy production within all cells by forming iron-sulfur clusters in the mitochondria. Mitochondria are cellular organelles often described as the “powerhouses” of the cell.

In the mitochondria, the iron-sulfur clusters accept electrons and shuttle them along the electron transfer chain. The movement of these electrons generates ATP (adenosine triphosphate), the main energy carrier in cells. [4]

ATP is required for many processes in the body, including muscle contraction, nerve signaling, and protein synthesis.

Immunity

macrophages, which help to protect against pathogens. [2]

Iron is also an important component of the protein lactoferrin, which provides immune support in the mare’s mammary glands and transfers immunity to nursing foals. [2]

In addition to supporting immunity for mares and foals, iron is involved in both innate and adaptive immunity in all life stages of horses.

Innate immunity

The innate immune system is the body’s first line of defense against pathogens. It is non-specific and does not confer any long-lasting protection against a specific pathogen or have a memory component.

The innate immune system includes:

  • Physical barriers (skin, mucus)
  • Chemical barriers (stomach acid)
  • Cellular defenses (macrophages, neutrophils)

As part of the innate immune response, the horse’s body sequesters iron by actively removing it from the bloodstream and storing it within immune cells and storage proteins. The body also reduces iron absorption in the intestine.

This effectively reduces the availability of iron to invading pathogens like bacteria and viruses, which need it to grow and multiply. By restricting access to iron, it prevents the infection from spreading, thereby protecting the horse. [5]

In addition, iron regulates enzymes that produce free radicals, which are used by immune cells to defend against pathogens. [5]

Adaptive immunity

The adaptive immune response is a specialized part of the immune system that develops targeted responses to specific pathogens and retains a memory to provide better protection against future infections.

Although slower to respond, the adaptive immune system confers greater protection by generating antibodies designed to eliminate specific pathogens.

Adaptive immunity involves lymphocytes (B-cells and T-cells) that have the ability to recognize and remember pathogens, providing long-term protection.

In the adaptive immune system, iron is important for replication of lymphocytes. As a mediator of DNA synthesis, iron is critical for allowing B-cells and T-cells to divide quickly, improving the protection against pathogens. [5]

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Iron Digestion and Absorption

The typical iron intake for a horse usually exceeds their nutritional requirements. Despite this, iron absorption from the gut is closely regulated to maintain consistent levels in blood.

Iron digestion begins in the stomach, where acid converts iron in feed into a soluble form that can be absorbed by the horse’s body. Feed then moves to the small intestine, which is the primary site of iron absorption in horses and other species. [2]

Iron is then absorbed by intestinal cells and transferred into the blood. This process involves two key transporter proteins:

  1. Divalent metal transporter 1 (DMT1): Brings iron into the intestinal cell
  2. Transferrin: Shuttles iron in the blood to other parts of the body

It is estimated that only 15-20% of the iron ingested by horses is absorbed by their bodies. However, horses are highly efficient at retaining iron, making deficiencies in this mineral rare. [1][2]

In addition, research in other species shows that fibre fermentation in the large intestine improves the availability and absorption of iron. [6][7][8] This is an uninvestigated and potentially significant avenue for iron absorption in hindgut fermenters, such as the horse.

Regulation

Iron levels in the horse’s body are maintained within a narrow range to support optimal physiological function and prevent diseases related to iron deficiency or overload.

Hepcidin is a hormone that is produced in the liver and plays a key role in maintaining iron homeostasis (balance). Hepcidin controls how iron is absorbed, utilized, and stored, influencing its release from the intestines and distribution around the body. [2][9]

High Iron Intake

When horses consume high amounts of iron, their bodies initially increase the production of hepcidin. This reduces the release of iron from the intestines into blood circulation.

Hepcidin works by binding to a protein called ferroportin, which is located on the surface of cells lining the intestine and also on iron-storing cells in the liver and spleen.

Ferroportin acts as a gatekeeper, allowing iron to exit these cells and enter the bloodstream. Hepcidin causes ferroportin to be degraded, effectively closing the gate and preventing iron from being absorbed into the bloodstream.

Under normal conditions, this mechanism helps regulate iron levels effectively. However, if horses consume too much iron, it can overwhelm hepcidin’s ability to regulate iron absorption, leading to excessive iron accumulation in the body. [2]

Low Iron Levels

It is rare for horses to develop low iron levels. Unlike many other minerals, iron is not actively excreted from the body in large amounts, unless there’s significant blood loss.

Horses are also very efficient at storing and recycling iron. When red blood cells are broken down, the iron from their hemoglobin is recycled by macrophages and either stored or released back into the bloodstream as needed.

However, if horses are injured and experience severe blood loss or if red blood cells are destroyed due to infection, it can lead to anemia. Anemia occurs when the rate of red blood cell destruction exceeds the body’s ability to replace them, resulting in decreased oxygen-carrying capacity in the blood and potential health complications.

To avoid such conditions, iron absorption from the gut will be increased to maintain iron levels without needing additional dietary supplementation. [10]

Inflammation and Infection

Inflammation and infection also influence iron regulation in the horse’s body by increasing levels of hepcidin. This increases the sequestration of iron into macrophages to support immune function and mitigate the potential for iron to contribute to oxidative damage or bacterial growth. [2]

However, horses with chronic inflammatory conditions may develop anemia due to insufficient circulating iron for maintaining red blood cell production. [2]

Mineral Interactions

High iron intake also affects the absorption of other minerals in the horse’s diet, such as zinc and copper. All three of these minerals utilize the same transportation pathway in the cells lining the horse’s intestines to enter the bloodstream. [11]

DMT1 (Divalent Metal Transporter 1) is responsible for transporting various divalent metals, including iron, zinc, and copper, from the intestines into the body. When there is an excess of one mineral, such as iron, it can saturate the DMT1 transporter, reducing the absorption capacity for other minerals like zinc and copper.

In horses, high iron intake has been shown to decrease zinc absorption, resulting in lower levels of zinc in the body. [12] In rats and other animals, high iron has also been shown to reduce copper absorption. [13] It is likely that this effect occurs in horses as well.

Given the relatively high iron intake observed in typical horse diets, it is important to ensure a balanced intake of zinc and copper to support optimal absorption of all three essential minerals. [14]

Iron Sources in Equine Diets

Iron is present in varying amounts in nearly all forages, feeds, and supplements for horses. While some sources contribute only trace amounts of this mineral, others provide more than enough to meet the daily nutrition requirements of horses.

Grass and hay provide varying levels of iron, depending on factors such as soil composition and plant species. [2] The best way to determine iron levels in your horse’s forage is to submit a sample for analysis.

In addition to natural sources, several horse feeds and supplements are formulated with iron as an added ingredient. Iron supplements for horses may contain this trace mineral in the form of iron oxide, iron sulfate, or dextran iron. [15]

Due to the risk of iron overload, these products should be avoided unless they are used under veterinary supervision for the treatment of iron deficiency anemia in horses.

Typical iron content of feeds and supplements [1]

Iron Source Iron Concentration
Forages 100 – 250 ppm
Grains <100 ppm
Milled Concentrate Feeds 500 – 1400 ppm
Calcium & Phosphorus
Supplements
20,000 – 30,000 ppm
2 – 3%

 

Iron Requirements of Horses

The daily iron requirements for horses depend on various factors such as age, activity level, and physiological status. Dietary requirements for horses are estimated to be: [1]

  • 40 mg per kg of dry matter intake for mature, sedentary horses
  • 50 mg per kg of dry matter intake for growing foals, pregnant mares, and lactating mares

These amounts are provided as concentrations of the diet based on the dry matter intake of the horse. This refers to the amount of feed consumed by the horse after removing the moisture content.

Adult Horses

For an average-sized 500 kg (1100 lb) adult horse, the daily iron requirement is equal to:

  • Sedentary horse: 400 mg per day
  • Heavily exercising horse: 500 mg per day
  • Pregnant mare: 500 mg per day
  • Lactating mare: 625 mg per day

Iron requirements are higher for pregnant and lactating mares to support the development of growing foals. [1]

Growing Foals

The iron requirement for growing foals is calculated based on their estimated total feed intake. For example, a typical yearling (12 month old) horse that is expected to reach 500 kg (1100 lb) at maturity is estimated to consume 8 kg of dry matter per day.

Based on this total feed intake, the yearling’s dietary iron requirement is calculated to be 401 mg per day. This is equivalent to 50 mg per kg of dry matter intake.

Typical Iron Intakes of Horses

Horses can easily meet their iron requirement from forage alone. According to Equi-Analytical’s database of forage analyses, the average iron levels found in forages are: [16]

A mature horse on a hay-only diet is expected to consume 10 kg (22 lb) of hay dry matter per day.

At that intake, a horse would consume 4280 mg of iron from a legume hay or 2080 mg from a grass hay, which is between 4 – 10 times their daily requirement. Assuming an absorption rate of 20%, this would result in the absorption of approximately 416 to 856 mg of iron into their body each day.

Iron Deficiency

Iron deficiency due to low dietary intake is very unlikely because forages naturally contain enough iron to meet a horse’s requirements. In fact, iron deficiency anemia has never been reported in an adult horse.

However, iron deficiency can develop in cases of severe blood loss, such as during significant injury or internal bleeding. In such cases, horses are at risk of becoming anemic due to low red blood cell levels.

Anemia in horses can arise due to: [9]

The most common cause of anemia in adult horses is anemia of chronic disease (ACD), in which elevated hepcidin levels block the absorption of iron from the diet. In ACD, the hormone that stimulates new red blood cell production from bone marrow is also inhibited, resulting in reduced levels of red blood cells. [17] Fortunately, this form of anemia resolves on its own when the underlying disease is controlled.

Older horses also commonly have a mild form of anemia that is likely a physiological response to aging and low levels of activity. [18]

Iron Deficiency in Foals

While not a major concern in adult horses, iron deficiency can be a concern for young foals. These horses have a higher demand for iron to support rapid growth and expansion of blood volume.

Most foals consume enough iron through their feed and by occasional consumption of soil, such as in a pasture or a dirt floor stall. [1]

However, foals reared on concrete floor stalls without access to soil are at risk of developing iron deficiency anemia. [1][19] This is a well-known problem among piglets raised in concrete housing and is remedied by supplying them with pans of dirt. [20]

If your young foal does not have access to soil, consult with your veterinarian to determine whether iron supplementation is necessary.

Assessing Iron Deficiency

Horses suspected of being deficient in iron should have this confirmed through several blood tests, including:

  • Serum iron
  • Total iron binding capacity
  • Ferritin levels

Consult with your veterinarian to determine whether these tests are appropriate for your horse. In the US, these tests can be done at the Comparative Hematology Laboratory at Kansas State University.

Anemia should not be suspected as related to iron deficiency unless both the Mean Cell Volume (MCV) and Mean Cell Hemoglobin Concentration (MCHC) are low.

Low levels of these two indicators suggest that the red blood cells are smaller and contain less hemoglobin than normal, which are characteristic features of iron-deficient erythropoiesis (red blood cell production).

Iron Overload

Since most horses get more than enough iron from their diet, supplementation with this mineral is generally not recommended. Some supplements targeting performance horses advertise iron as an added ingredient, but there is no evidence to suggest that feeding more iron positively impacts athletic performance.

Horses are more likely to experience adverse effects related to consuming too much iron as opposed to too little. Excess iron intake is of particular concern for newborn foals, who are more efficient at absorbing iron from the diet.

Even small amounts of oral iron supplementation in foals can result in too much iron accumulating in the liver. This can lead to liver damage and serious health consequences, including death. [1]

Symptoms of iron toxicity in foals include: [1]

Upper Tolerable Intake

The upper tolerable intake level (UL) is the maximum amount of a nutrient that can be consumed daily without posing a significant risk of adverse health effects for most individuals in a particular population group.

For mature horses, the UL of iron is 500 mg per kg dry matter in the diet. This is equivalent to 5,000 mg per day for a typical 500 kg (1100 lb) horse. [1]

While some forages contain high amounts of iron, there are no reported cases of iron toxicity in horses from the consumption of high iron forages alone. [1]

Iron toxicity is more common in cases where iron is supplemented on its own, administered in an injectable form, or ingested in water.

Iron Accumulation in the Liver

When horses consume excess iron, it can accumulate in the liver and cause liver damage.

In one study, a relatively small dose of an oral iron supplement (0.6 mg per kg body weight) resulted in iron toxicity in a mature Thoroughbred horse after six weeks. Clinical signs of toxicity included high blood iron levels, abnormalities in liver enzymes, and formation of liver lesions. [21]

The clinical signs resolved after removing the iron supplement. However, the researchers noted that it was surprising to observe signs of toxicity with such a low dose. The researchers suspected that excess iron in other feeds, miscalculation of the dose, or previous administration of intravenous iron – a common practice on racetracks – may have affected the results.

In another study, a much higher dose iron supplement (50 mg per kg body weight) fed daily for eight weeks did not cause toxicity in mature horses. [22]

Chronic Excessive Intake

Research studies on experimentally induced toxicity often look at the effects of high iron doses supplemented to horses over short periods of time. However, this may not accurately reflect the effects of sustained high iron intake over months or years.

Recent research shows that long-term exposure to high iron levels in drinking water can cause chronic liver disease in horses. Prolonged intake of excess iron, rather than short-term overfeeding, may be contributing to undiagnosed liver disease in horses. [23]

If you use ground water on your farm, consider submitting a sample for water analysis to determine the total iron content.

It’s also important to consider more subtle negative effects of high iron intakes, as opposed to overt symptoms of toxicity. Although not yet researched in horses, subclinical iron accumulation may cause oxidative damage that affects tissue function. [2]

Insulin Resistance

Insulin resistance is a condition where the body’s cells do not respond effectively to insulin, a hormone that regulates blood sugar levels. As a result, the body needs more insulin to help glucose enter cells, often leading to elevated blood sugar and equine metabolic syndrome.

The relationship between high iron levels and insulin resistance is well-documented in humans and laboratory animals. This link has also been identified in a wide range of species, including black rhinos and tapirs, two close cousins of the horse. [24]

Iron overload increases serum ferritin levels, which has been identified as an independent risk factor for type 2 diabetes mellitus in humans. [25] This condition is characterized by significant insulin resistance.

Research also shows that phlebotomy (blood letting), which reduces iron levels, improves insulin resistance in both animals and people. However, not all studies report the same result. [26][27][28]

Does High Iron Cause Insulin Resistance?

While there is a clear association between high iron levels and insulin resistance, it remains uncertain whether high iron directly contributes to insulin resistance or if both conditions are influenced by another underlying factor.

In research on mice, a high-fat and high-fructose diet results in iron overload before the onset of insulin resistance. [29] This suggests that iron overload may be contributing to insulin resistance.

A recent evaluation by the ECIR group of 33 horses with metabolic syndrome also found that 100% of the horses were iron overloaded. Similarly, horses with exaggerated insulin responses to a oral glucose challenge also had higher ferritin levels. [30]

However, mounting evidence suggests that the development of metabolic syndrome in horses is influenced more by genetics than diet. [31] While it seems unlikely that iron overload causes insulin resistance, the association observed warrants further investigation to fully understand the relationship.

Summary

  • Iron is an important mineral for many functions in the horse’s body, including oxygen transport, energy production, and immune defenses.
  • Most horses get enough iron to meet their nutritional requirements from forages and feeds in their diet.
  • Iron supplementation is not necessary or recommended for horses, except in rare cases under veterinary supervision.
  • Consuming too much iron can result in oxidative damage and adverse effects on liver health in horses.
  • More research is needed to understand how prolong high iron intake can contribute to subclinical symptoms as well as the relationship between iron overload and insulin resistance.

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References

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  2. Geor, R.J., et al. Chapter 10: Macro and Trace Elements in Equine Nutrition. Equine Applied and Clinical Nutrition: Health, Welfare and Performance. 2013.
  3. Rhodes, C.E., et al. Physiology, Oxygen Transport. StatPearls. 2023.
  4. Read, A.D., et al. Mitochondrial iron–sulfur clusters: Structure, function, and an emerging role in vascular biology. Redox Biology. 2021.
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  9. Remillard, R.L., et al. Chapter 8: Minerals. Equine Clinical Nutrition. 2023.
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  25. Qin, Y., et al. Association between systemic iron status and beta-cell function and insulin sensitivity in patients with newly diagnosed type 2 diabetes. Front Endocrinol (Lausanne). 2023.
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  28. Johnson, S.P., et al. Use of phlebotomy treatment in Atlantic bottlenose dolphins with iron overload. J Am Vet Med Assoc. 2009.
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  30. Kellon, E.M., and Gustafson, K.M. Possible dysmetabolic hyperferritinemia in hyperinsulinemic horses. Open Vet J. 2020.
  31. Stefaniuk-Szmukier, M., et al. Equine Metabolic Syndrome: A Complex Disease Influenced by Multifactorial Genetic Factors. Genes (Basel). 2009. View Summary