Do all horses respond to vitamin E the same way? New research suggests not...
A 2025 case study led by Carrie Finno suggests that horses may respond very differently to vitamin E supplementation - and genetics could be the reason why.
Why vitamin E matters
Vitamin E is a powerful antioxidant. In simple terms, it helps protect cells from oxidative damage caused by free radicals. This protection is particularly important for:
- Muscle function
- Immune health
- Reproductive performance
Some horses are more likely to need additional vitamin E, including:
- Horses with limited access to fresh pasture
- Those consuming low-quality or long-stored forage
- Horses on high-fat (oil-supplemented) diets
- Performance horses
- Older horses
- Breeding stock
Fresh grass is naturally rich in vitamin E. However, levels decline rapidly once forage is cut and stored, meaning many stabled horses may not meet their requirements from hay alone.
What about genetics?
In human medicine, researchers have already identified several genetic variants that influence how efficiently vitamin E is absorbed, transported, and metabolised. In fact, at least five variants are known to significantly affect vitamin E handling in people.
Given the number of equine conditions linked to vitamin E deficiency, particularly neuromuscular disorders, researchers wanted to explore whether similar genetic differences may exist in horses.
The 2025 case study
The study focused on a horse diagnosed with Vitamin E Responsive Myopathy (VEM). VEM is a condition associated with chronic vitamin E deficiency, and clinical signs can include:
- Muscle weakness
- Tremors
- Muscle wasting (atrophy)
- Difficulty moving
The case involved an 8-year-old Percheron gelding presented to the UC Davis School of Veterinary Medicine with severe weight loss, muscle wastage, and movement difficulty.
Blood tests were performed to measure serum vitamin E levels, and muscle biopsies were taken. The biopsy samples were evaluated by Stephanie Valberg, a leading expert in equine muscle disease.
Interestingly:
The gluteal muscle appeared normal.
The sacrocaudalis dorsalis medialis muscle (located just above the tail) showed clear atrophy and muscle fibre damage - consistent with VEM.
Supplementation and response
The horse was treated with vitamin E at a dose of 10 IU/kg once daily for three weeks. The results were dramatic - clinical signs improved significantly within this short period.
However, when the dose was reduced to 5 IU/kg:
- Clinical signs returned within three days.
When the higher dose was reinstated:
- Symptoms again resolved rapidly.
Blood testing during this period revealed something particularly interesting. At 10 IU/kg, the horse’s serum vitamin E levels became extremely high compared to typical responses, indicating a very unusual sensitivity to supplementation.
What did researchers conclude?
The researchers concluded that:
- This horse required a higher-than-normal serum vitamin E concentration to prevent neuromuscular disease.
- The unusually high requirement was likely due to an underlying genetic variation affecting vitamin E transport or metabolism.
This finding aligns with earlier research. A 2017 study by Brown et al. Showed that even when horses started with very similar baseline vitamin E levels and were placed on the same supplementation protocol, their responses varied significantly.
In other words, horses do not all respond the same way.
Why this matters for horse owners
Vitamin E deficiency can affect overall health long before obvious neuromuscular symptoms appear. Many deficient horses show no clear outward signs initially, but long-term insufficiency may still have consequences.
Important practical considerations include:
1. Forage quality matters.
Stored hay can lose up to 50% of its vitamin E content per month after cutting.
2. Pasture access makes a difference.
Fresh grass is the most natural and reliable source.
3. Form of vitamin E matters.
Natural vitamin E (d-alpha-tocopherol) has significantly better bioavailability in horses compared to synthetic forms (dl-alpha-tocopherol).
4. Individual variation is real.
Two horses on the same yard, same diet, and same supplement may not achieve the same blood levels.
The bigger picture
More than 200 genetic variants have already been identified in and around vitamin E–related genes in horses. While we are still at an early stage of understanding what this means in practice, the research suggests that genetic profiling could one day help guide more personalised supplementation strategies.
For horse owners who value science-led care, this study highlights an important takeaway:
Vitamin E requirements may not be “one size fits all.”
Monitoring serum levels, choosing bioavailable forms of supplementation, and staying informed on emerging research may help ensure your horse receives optimal support, particularly if they are in work, ageing, breeding, or not on fresh pasture.
Discover more about the research here: Finno, C. J. (2025). How nutrigenomics impacts equine health - A case study of vitamin E. Journal of Equine Veterinary Science, 148, 105421. Https://doi.org/10.1016/j.jevs.2025.105421
Written by Anna Jones BSc (Hons) Equine Science, Customer Care & Marketing Associate