Iron: Friend or Foe?

Recently I came across a Facebook post about beet pulp — an enormously helpful feed in many cases — flagging concerns about its iron content. It got me thinking: what is iron actually doing in the equine body, and is dietary iron overload even a real concern?

Iron is an essential mineral involved in numerous physiological processes — it is a critical component of both blood and muscle. While iron occurs naturally in all equine diets, its presence in feed does not mean it is available for absorption.

Because horses are herbivores, most of the dietary iron is in the ferric form (Fe3+). Before it can be absorbed, it must first be converted to the ferrous form (Fe2+) — the only form the body can take up. Absorption is limited to the early small intestine, specifically the duodenum and jejunum. However, because dietary iron is typically bound to other compounds, it often misses this narrow window of absorption entirely and isn't freed until much later in the GI tract — well past the point where uptake is possible.

This is the first layer of iron regulation — and it is remarkably effective.

The second layer of regulation is arguably the most powerful: hepcidin.

Hepcidin is a protein produced by the liver and serves as the master regulator of iron metabolism — the body's built-in gatekeeper. When iron levels are sufficient for healthy physiological processes, hepcidin rises and effectively shuts down intestinal iron absorption. When iron levels are low, hepcidin decreases and absorption is permitted.

In practical terms: the body is actively monitoring its own iron status and adjusting absorption accordingly. It is not a passive system vulnerable to whatever is in the diet.

As with any biological system, exceptions exist — certain disease states can disrupt hepcidin signaling. But in a healthy, mature horse, hepcidin is a remarkably sophisticated guardian of iron balance.

This is the second layer of iron regulation.

There is limited empirical evidence demonstrating that dietary iron overload leads to physiological problems in healthy horses. However, there is some evidence suggesting an association between iron and insulin dysregulation — and it is worth examining that evidence carefully.

A 2012 study by Nielsen, B.D. et al. investigating insulin dysregulation and iron levels identified one outlier with markedly elevated insulin and iron — but that mare had a pre-existing diagnosis of insulin resistance. The authors themselves noted that "inclusion of the data, clearly an outlier, was deemed prudent, because it alone suggests but does not prove a link" between high iron and insulin resistance. The elevated iron may have been a consequence of metabolic dysfunction, not a cause of it. This is an important distinction that is frequently lost in popular discussion.

Dr. Kellon and Dr. Gustafson have further explored this association, concluding that horses with metabolic conditions may have a potential for elevated iron levels. It is also worth noting that exercise, illness, and injury are known to raise serum ferritin, while pregnancy reduces it — meaning elevated iron markers can reflect a range of physiological states, not simply dietary intake.

Understanding the role of iron in the equine body is a legitimate and important area of inquiry. However, the leap from "iron may be elevated in metabolically compromised horses" to "dietary iron is dangerous for all horses" is not supported by the current evidence. More research is needed to understand the true relationship between iron, equine metabolic syndrome, and other conditions before broad dietary recommendations can be made with confidence.

Iron remains a valuable and evolving area of research in equine nutrition. But based on the current evidence, there is limited support for the claim that dietary iron intake causes iron overload in healthy horses.

It is also worth considering what happens when we overcorrect. Much of the current marketing around iron focuses on balancing zinc and copper ratios — but excess zinc can block copper absorption, and unabsorbed zinc and copper can be harmful to the environment. The solution to a poorly supported problem may be creating new ones.

I understand the impulse. We want to do everything possible for our horses. But currently, the evidence suggests our energy is better directed toward well-documented concerns — obesity, insulin dysregulation, and forage quality — rather than chasing a dietary iron problem that the horse's own physiology is remarkably well equipped to manage.

As always, if you have concerns about your horse's metabolic health, work with your veterinarian. And stay curious. The science is still evolving.

So…Iron: Friend or Foe? TBD

References

1.     A potential link between insulin resistance and iron overload disorder in browsing rhinoceroses investigated through the use of an equine model. Nielsen, B.D., et al. Journal of Zoo and Wildlife Medicine, (2012), 43(3s).

2.     Possible dysmetabolic hyperferritinemia in hyperinsulinemic horses. Eleanor M. Kellon* and Kathleen M. Gustafson. Open Veterinary Journal, (2019), Vol. 9(4): 287–293