In our last article, we learned that our horses may not be getting as much Vitamin E as we thought due to degradation caused by summer heat (available here). This week, we are discussing the efficacy and absorption of the various types of Vitamin E.
◆︎ How much Vitamin E does your horse need?
The NRC indicates that horses need 1 IU per kg of body weight for basic maintenance. A standard 500 kg (1,100 lb) horse would need 500 IU Vitamin E daily.
Natural pasture provides roughly 30 to 100 IU per kg of dry weight. So, if a 500 kg horse is eating 2% of their body weight in grass, they will consume 300 to 1,000 IU of Vitamin E per day. In this article, we will use that 1,000 IU of Vitamin E amount as the benchmark for what a horse should ideally receive per day.
◆︎ The Problem with Hay
While fresh grass gives us plenty of Vitamin E, it is quickly degraded when the grass is cut. Timothy hay can lose as much as 60% of its Vitamin E just four days after being cut. Alfalfa hay loses as much as 54% to 73% within the first three months of storage. With a 60% loss, your horse's Vitamin E intake drops to just 120 to 400 IU per day. The older the hay is, the less Vitamin E you can expect it to have.
◆︎ The Problem with Supplementation
Knowing that hay loses its Vitamin E, we can just supplement it, right? Yes, but there are important caveats to know about the different forms of Vitamin E available.
➡ Is it Natural or Synthetic?
Natural: The natural form is listed as d-alpha-tocopherol. This is the most biologically active form and is the form the horse relies upon.
Synthetic: This is labeled as dl-alpha-tocopherol. The "dl" indicates it is a synthetic mixture of eight different molecular shapes. When your horse digests this, their liver actively filters out and rejects the majority of these synthetic shapes, preferring the natural "d" form. Because so much of the product is rejected by the liver, you have to feed more of the synthetic powder just to achieve baseline blood levels.
➡ Oil vs. Dry Supplements
Powdered Vitamin E is highly stable. Most commercial feeds contain the standard powdered form because of its excellent stability and inexpensive cost. However, standard Vitamin E must be associated with an oil before it can be broken down into microscopic droplets (emulsified) and absorbed by the gut.
A study showed that Vitamin E mixed with a high-fat diet was 84% absorbed, whereas dry Vitamin E on an empty stomach was only 29% bioavailable. The dry Vitamin E that didn’t “find” a fat is simply flushed down the digestive tract, missing its opportunity to combine with a bile salt for absorption. Because of this vast difference in bioavailability, it’s generally recommended that you add oil to standard dry Vitamin E powders.
➡ The Missing Gallbladder Issue
Humans and dogs have a gallbladder. When we consume a large, high-fat meal, the gallbladder squeezes out a massive bolus of bile salts to instantly break those fats down into small droplets for absorption.
Horses, however, do not have a gallbladder. Instead, their liver trickles bile salts continuously into the gut. This works fine for a horse eating low-fat grass all day long. But when we feed a bucket of food containing Vitamin E mixed with a few ounces of oil, the slow trickle of bile is completely overwhelmed. There are not enough bile salts available at that exact moment to bind to all the fat molecules. Those unemulsified fat molecules (and the Vitamin E trapped inside them) wash right through the small intestine and are flushed out in the manure, losing their opportunity for absorption.
◆︎ The Solution: Microencapsulated Vitamin E Zsorb
Vitamin E Zsorb was engineered to overcome these exact biological hurdles.
☛ It uses natural Vitamin E, the d-form.
☛ It is microencapsulated and readily emulsifies in the stomach — completely bypassing the need for added oil or the horse's limited bile salts.
☛ Once the pancreatic enzymes trim the protective acetate cap off the molecule, it is immediately ready for absorption.
◆︎ The Master Chart: Comparing the Bottlenecks
Knowing all the biological hurdles your supplement has to jump over, how do the different formulations actually compare in the real world? This chart compares what you have to feed to achieve that 1,000 IU baseline target.
◆︎ Consult Your Veterinarian
Your veterinarian can test your horse's actual Vitamin E serum levels. High-dose Vitamin E supplementation is critical for the clinical treatment of:
▪ Equine Neuroaxonal Dystrophy (eNAD)
▪ Equine Motor Neuron Disease (EMND)
▪ Vitamin E Deficient Myopathy (VEM)
Always consult your veterinarian for specific therapeutic dosing guidelines if you suspect your horse is dealing with a neurological or muscular condition.
We do the best we can for our horses. Understanding our horse's needs is a part of that.
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Disclaimer: Statements regarding dietary supplements have not been evaluated by the FDA and are not intended to diagnose, treat, cure, or prevent any disease or health condition.
©Joan Kulifay, MSc. 2026