Q: My Hanoverian gelding just turned 18 and doesn’t always have access to high-quality pasture, so I recently had his vitamin E levels checked. My veterinarian tried to explain the results to me, but I’m not sure I understand. Is it better for his numbers to be in the normal range or higher on the scale, and should I supplement with vitamin E if his levels are below normal?

— Melissa F., New York

A: Vitamin E is an often overlooked nutrient in the equine diet, but it serves as a crucial and potent antioxidant and helps combat oxidative stress, augments immune system responsiveness and supports the neuromuscular system. Vitamin E test results can be confusing for horse owners for a number of reasons. Daily requirements vary significantly and are based on your horse’s weight, activity level, diet and age. At maintenance, for example, an 1,100-pound horse should consume approximately 500 IU of vitamin E per day. If that horse is in some kind of exercise program, his requirement increases to more than 1,000 IU per day.

The best and most abundant source of vitamin E is found in green pasture. Horses with access to adequate, quality pasture for approximately 12 hours per day will naturally maintain an ideal serum vitamin E level of 3-6 µg/ml. With hay, the nutrient is rapidly depleted post-baling, so it’s safe to assume that horses not receiving adequate pasture intake will only receive vitamin E from their feed and/or supplements.

When and How Often to Test

For horses who receive little or no pasture, it’s highly recommended to test annually for serum vitamin E status. This is also a good practice as vitamin E is a sensitive compound and not always particularly stable in supplements and/or feed. Most companies do a sound job offering a stable form of the nutrient, but in my consulting practice I’ve seen cases of horses receiving far beyond their daily requirements yet still testing as deficient. Therefore, it’s essential to make sure the dietary source of the nutrient is viable. Your veterinarian can assess this through a simple blood draw.

For horses on quality pasture who also receive adequate dietary intake of vitamin E from their grain ration or supplements, regular testing is not as critical unless there are observed symptoms associated with deficiency. It’s also important to note that horses receiving the exact same diets can have individual variation in vitamin E metabolism, so it’s best to have the entire herd tested if there’s suspected deficiency.

How to Interpret Vitamin E Test Results

When reviewing your horse’s test results, it’s important to look at the serum value. Adequate serum concentration is ≥ 2 µg/mL (micrograms per milliliter). While 1.5-2 µg/mL is considered marginal, and < 1.5 µg/mL is deficient. Horses that score ≤ 2 µg/mL should undergo a vitamin E supplementation protocol to return serum levels to ideal. It’s best to consult with your veterinarian or an equine nutritionist to determine the most appropriate product and amount to supplement. Because horses on abundant pasture will naturally maintain serum at 3-6 µg/mL, this is a great target for “ideal.”

Vitamin E test results can be confusing because the scale goes up to 10 µg/mL (or 1000 µg/dL depending on the lab your veterinarian sends the sample to). Many owners believe their horses should be on the high end of the scale for best results in performance, health, etc. Based on the current data available, there isn’t a reason to believe that “high normal” (7-10 µg/mL) is better than the suggested 3-6 µg/mL range for serum.

If your horse doesn’t come back with a “high normal” value but is still within 3-6, there isn’t support for additional vitamin E supplementation. To address any further questions about your individual horse’s results or for help determining the ideal form or product for his specific needs, your best bet is to connect with your veterinarian and an equine nutritionist.

About Rachel Mottet, MS, PhD

Rachel Mottet, MS, PhD, is an independent equine nutritionist under her business Legacy Equine Nutrition. She earned her B.S. in Equine Science from the University of Wisconsin River Falls, her M.S. in Animal Science from North Dakota State University and her PhD in Nutrition and Animal Science from the University of Minnesota.

Professionally, Mottet has worked with horse owners all over the world as an invited lecturer, university professor and consulting equine specialist for numerous equine feed and supplement companies. She has worked with FEI and Olympic competitors in numerous disciplines and has successfully formulated diets for thousands of horses. She actively competes in hunter/jumpers and owns a farm outside of Ocala, Florida, where she lives with her two dogs and three horses.

For More on Vitamin E and Equine Health:

• Learn about vitamin E’s essential role in supporting your horse’s immune health here.
• Does your horse need vitamin E supplementation during the winter? Find out more here.
• A veterinarian explains the differences between natural and synthetic vitamin E and how to determine which is best for your horse here.