Dietary Iron Unlikely to Cause Insulin Resistance in Horses.
Authors: McLean Nancy L, McGilchrist Nerida, Nielsen Brian D
Journal: Animals : an open access journal from MDPI
Summary
# Editorial Summary: Dietary Iron and Insulin Resistance in Horses Racehorses frequently receive iron supplementation beyond their documented requirements, yet insulin resistance remains remarkably uncommon in this population—a disconnect that prompted investigation into whether excess dietary iron might be causative. McLean and colleagues surveyed 120 American Thoroughbred trainers representing nearly 2000 horses across multiple regions, quantifying daily iron intake from forage and concentrates alone, then factoring in supplemental contributions. The analysis revealed a striking disparity: racehorses consumed an average of 3900 mg iron daily from hay and grain—nearly ten times the NRC 2007 recommendation of 400 mg for a 500 kg working horse—with supplements adding a further 500 mg, yet not a single confirmed veterinary diagnosis of insulin resistance emerged from the surveyed population. This absence of disease in the face of such elevated iron intake strongly suggests that dietary iron, at least at these concentrations, is unlikely to function as an independent driver of equine insulin resistance, thereby questioning the rationale for iron restriction protocols proposed by some equine nutritionists. For practitioners, these findings support the notion that other metabolic and management factors warrant greater scrutiny when insulin resistance develops, rather than attributing the condition to routine dietary iron levels.
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Practical Takeaways
- •Iron supplementation in racehorses may not be necessary or beneficial given that typical diets already exceed recommended daily requirements by 9-10 fold
- •Excess dietary iron should not be assumed to cause insulin resistance in horses based on current epidemiological evidence in Thoroughbred populations
- •Review current iron supplementation protocols with clients, as unnecessary supplementation adds cost without apparent performance or health benefit
Key Findings
- •Surveyed racehorses consumed an average of 3900 mg iron daily from hay and grain alone, exceeding NRC 2007 recommendations of 400 mg for a 500 kg working horse by nearly 10-fold
- •Additional iron supplementation increased daily intake by an average of 500 mg, bringing total to approximately 4400 mg daily
- •No confirmed veterinary diagnoses of insulin resistance were found among 1978 Thoroughbreds from surveyed trainers despite excessive dietary iron intake
- •The rarity of insulin resistance in Thoroughbred racehorses despite widespread iron excess suggests dietary iron is unlikely to be an independent causative factor in equine insulin resistance