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Iron is the fourth most abundant element in soil and is vital for the transport of oxygen by red blood cells, along with other roles in immune function and metabolism.
Around a third of the “functional” iron in the horse is in haemoglobin inside red blood cells in the circulation or stored in the spleen (and released during exercise). Another third is in myoglobin in muscle (myoglobin moves oxygen inside muscle cells), but this has led to many myths, including that extra iron in feeds or supplements can act as an energy booster or to help horses recover from illness.
Iron supplementation in horses is almost only ever required when there has been significant blood loss and sub-clinical iron toxicity (not initially apparent from clinical signs) is many many times more common than iron deficiency. You may then ask, if all of this is true then why do so many supplements boast high iron content for “energy”, as a “tonic” or “to aid recovery”, to “improve performance”, “boost red blood cell numbers” or for “horses in hard work”?
Either these companies are ignorant of the science on use of iron in horses or they have products that sell well and don’t wish to educate horse owners.
You may also have seen that some of the brands with a strong science-based approach do not add iron to their feed and supplements.
So, why should you be avoiding products with added iron?
1. Iron deficiency is very rare
UK soils and forage are generally high in iron (›100mg iron per kg dry matter) and most horses get more than the recommended daily intake (400-500mg per 500kg per day; NRC 2007) from forage alone.
The estimated daily requirement of a 500kg horse in light work is 400mg of iron per day and rises to 500mg for a horse in hard work. Two-year old horses have a requirement around 530mg/day and the highest requirement is for lactating mares: 625mg/day. Forages tend to provide around 200-400mg iron per kg of dry material.
2. Most horses are over-supplemented with iron
Within the EU the inclusion of iron is actually limited to a maximum of 750mg per day in a complete diet (OJ L317/23. Commission Regulation (EC) No 1334/2003.). Even back as far as 20 years ago, some work in my group at the Animal Health Trust showed that event horses supplemented with iron had plasma total iron concentrations 2-3x higher than the typical normal range (Mills & Marlin, 1996).
3. Once ingested iron cannot effectively be excreted from the body
The horse has no mechanism to excrete iron once taken up from the gastro-intestinal tract other than via a very small amount excreted in urine. Once in the circulation, excess is removed by the liver or spleen and stored linked to protein either as ferritin or haemosiderin.
4. Iron acts as an oxidant increasing oxidative stress and inflammation
Iron in the body can act as a pro-oxidant, producing free-radicals which in turn cause tissue damage and inflammation. These damaging effects of free-radicals can be moderated by vitamins such as vitamins E and C or by antioxidant enzyme defences such as catalase, superoxide dismutase and glutathione peroxidise. However, this requires an increase in Vitamin C and E turnover and the need for a higher dietary intake to prevent depletion and secondary problems such as decreased immune function or increased respiratory inflammation. The damaging effects of high iron intake are made worse by exercise, stress and thermal environmental stress (hot or hot and humid weather or very cold weather). Mills et al. (1996).
5. Iron toxicity occurs at low intakes
Iron toxicity occurs at relatively low levels, perhaps only 2-3 times the recommended daily intake.
If a forage fed at 5kg dry matter provides 1000mg of iron (200mg/kg) and an additional 500mg per day comes from hard feed and supplements, this is already at three times the recommended intake. Initially the signs of toxicity may be unspecific and not readily recognised as being related to iron intake. Iron is corrosive and signs may include gastric irritation, gastric pain, gastric ulceration as well as damage to the small and large intestinal lining. Other signs associated with iron toxicity include diarrhoea, dehydration, depression and reduced immunity. There is also an increased risk of bacterial infections as bacteria use iron as an energy source. In more severe cases of iron toxicity, liver failure can be induced, which may be fatal. In people, iron supplementation can lead to undesirable overgrowth of bacteria in the small intestine. Excess iron is especially toxic to young foals and death due to excess supplementation has been reported.
6. Iron does not boost red blood cell numbers
Feeding iron supplements will not increase the number of red blood cells. Fact.
7. Iron does not correct anaemia (low blood count)
Feeding iron supplements will not correct anaemia (low red blood cell count) unless this is due to high blood loss. Fact. Here is a fact sheet on anaemia.
8. Iron does not increase energy
The theory for iron as an energy booster seems to be related to the fact that people who are anaemic have low energy levels and giving them iron supplements can restore their energy levels. But as we have already said, horses are rarely anaemic and if they are it is rarely due to iron deficiency and iron supplements will have no effect unless there has been heavy blood loss.
9. Iron interferes with absorption of other minerals
Supplemental iron can decrease zinc and copper uptake from the diet and over time can lead to deficiency and other health problems such as poor coat and hoof quality.
10. Iron may be linked to equine metabolic syndrome (reduction in the normal response to insulin)
In people there are studies that show iron plays a role in metabolic syndrome (Mojominiyi et al. 2008) and obesity (Zafron et al. 2011). Nielsen et al. (2012) also identified a link between iron and insulin resistance in horses, which appears to be becoming more common.
Bottom line: You should use feeds with added iron or use supplements with iron only on the advice of a qualified nutritionist or your vet, and ideally following confirmation of low iron content of your horse’s diet and clinical evidence of iron deficiency. Most horses are receiving far too much iron and this has a high potential to cause long-erm health problems that are rarely ever attributed to high dietary iron intake.