Iron - can you overload your horse’s diet?

Dr Mark Barnett (PhD), MTB Equine Services, Consultant to Benchmark Horse Feeds.

A simple but important question for the modern day horse, especially here in Australia.
Why here in Australia more so than many other places in the world? Simple. Our soils are very old and haven’t received the level of precipitation (rainfall, snow, ice, etc.) that many other countries have experienced, particularly those in the Northern Hemisphere and Tropics. As a result, our soils are very high in certain minerals, like iron and manganese, but are extremely deficient in others like copper, zinc and selenium. With the majority of our soils in Australia being high in iron, this mineral will naturally be present in higher-than-average concentrations in the plants, grasses, seeds and grains grown in these soils.

Established recommendations set by the National Research Council back in 2007 stated that the average sized horse (about 500kg) in minimal activity needs just 500mg/day to function normally. Unfortunately, with our soils and, therefore, our pastures and grains, being naturally high in iron, this recommended daily intake of 500mg is easily surpassed. In some cases, the daily intake can exceed 2000mg/day. The big question is “can you feed too much iron and create an iron overload”?

The short answer to this is “No, most likely not”!

Why is iron necessary for horses?
Iron is an essential trace mineral meaning it is important for various functions within the body (red blood cell formation, brain and muscle development, etc.). The issue is that should too much iron enter the body, it can cause negative health issues like a build up in the liver of iron deposits called haemosiderin, or even result in a whole-body overload of iron called haemochromatosis. It is highly NOT recommended for any horses to receive an iron supplement unless there is a medically related (and diagnosed) condition such as anaemia, excessive blood loss, internal parasitic burden, etc. It is essential in the daily diet as horses generally need somewhere between 500 to 900mg/day for normal, healthy physiological functioning. Remember, the maximum safe level of dietary iron for horses, as set by the National Research Council in 2007, is 500mg/kg of diet (approximately 5,000mg/day for a 500kg horse)

Can too much iron in the diet cause problems?
Simply put, yes. Too much iron in the daily diet can cause disruptions to the nutritional balance within the body of the horse. Iron shares similar transport mechanisms and metabolic pathways as other essential minerals such as calcium, manganese, and zinc. Providing an amount of iron in the diet that far exceeds the optimal physiological requirement of the horse will most likely result in a deficiency of these key minerals, minerals essential for a vast number of important and essential functions within the body. This can very easily be fixed by providing a feed or supplement with good levels of trace minerals like copper and zinc.

So, why are modern diets most likely not going to result in an iron overload?
Firstly, haemochromatosis is a very rare condition in horses. Why? Because all mammals, including horses and humans, work on what’s called a negative feedback loop. You see, horses (and humans, cats, dogs, sheep, etc.) do not regulate the conditions within their bodies through positive or reinforcing conditions, they do it through the detection of negative or unbeneficial conditions. Simply put, should there be too much or not enough of something, the body; through a series of chemical, hormonal and neural signals; responds by either stopping the production or absorption of something, or prompting for more to occur or enter the body.

When there is an excess of iron in the diet, e.g. grazing pastures all day or ad lib access to hay (of any kind), the body detects this excess of iron and does its best to self-regulate (re-enter homeostasis). It can do this in several ways.

Reduce absorption from the gut – not all iron in the diet is naturally well absorbed because iron can come in a variety of different forms. Two general forms are called Haeme (Heme) and Non-Haeme (Non-Heme). Haeme iron sources, e.g. meat, are very high in their bioavailability (how much is naturally absorbed into the body), while non-haeme iron sources; such as grains, grasses, pastures, and hays; are very low in their bioavailability. On average, about 15% of the iron in the horse’s daily diet is absorbed into the body. When the level of iron in the diet is excessive, this absorption rate can fall to as low as 2%.
Hepcidin – high iron concentrations in the blood can be regulated through a peptide hormone produced and secreted from the liver. Its role is to regulate the entry of iron into the blood. Hepcidin degrades an iron transporter called ferroportin, blocking its binding action with free iron and, thereby, preventing absorption of iron into the body. This action locks some of the iron molecules found in the diet inside the enterocyte cells which are the absorptive cells that line the luminal wall of the digestive tract. The locked-in iron and the old enterocyte cells eventually slough off, leaving the body with the faeces.
Inhibitors in the diet – plants contain natural inhibitors of the absorption of iron in the diet. Compounds such as phytases and polyphenols; found in abundance of a lot of grains, grasses and pastures; actively inhibit the uptake of iron into the body by either blocking the binding sites to the transport receptors on the epithelial lining of the gut or binding the iron molecules, making them unavailable for absorption.
Minerals preventing absorption – a few minerals which are found usually in abundance in the modern equine diet, have been shown to have a negative impact on the absorption of iron into the body. Calcium, a very common mineral and often over-suppled in the daily diet, has been shown to interfere with the transfer of iron across the luminal wall of the gut into the body. Additionally, minerals like manganese and zinc share the same intestinal absorption pathways. High levels of either of these (Australian soils and grasses/pastures are usually abundant in manganese) minerals will cause competition for both the transporters as well as the internal metabolic pathways.

So, summing everything up:

Iron is an essential trace mineral in the equine (and mammalian) diet.
It needs to be provided daily in the diet.
You can feed too much which could result in some health issues and possible mineral deficiencies for the horse, but these issues are usually easily overcome by adjusting the diet.
BUT, you are not likely to cause an “overload” of iron in your horse, causing severe health issues for the animal, due to its internal (and external) “fail safe” mechanisms.

Dr Mark Barnett PhD
Equine Nutritional Physiologist
MTB Equine Consultancy Services