The vitamins and minerals required by a horse’s body do not function in isolation. They are part of a complex metabolic network in which some nutrients enhance each other’s effects (synergy), while others compete for the same pathways or inhibit each other’s utilisation (antagonism). Understanding these interactions is important, especially when supplements are used or feeding strategies are adjusted.
Key vitamin–mineral interactions in horses
1. Vitamin E and selenium (Se)
Synergistic effect – cellular protection and muscle health
They work together in the body’s antioxidant system, protecting cells from oxidative stress. Selenium is part of an enzyme (glutathione peroxidase) that requires vitamin E to function effectively. This interaction is particularly important for sport horses, growing foals, and broodmares. A deficiency in one increases the requirement for the other, which is why they are often supplemented together.
2. Vitamin D and calcium (Ca) + phosphorus (P)
Essential interaction – skeletal health and mineral balance
Vitamin D (in its active form, calcitriol) regulates the absorption and balance of calcium and phosphorus. Vitamin D deficiency can lead to hypocalcaemia and weakened bones, even if dietary calcium and phosphorus intake is adequate. Growing horses are particularly sensitive to this relationship. The recommended Ca:P ratio in horses is approximately 1.5–2:1, depending on age and use.
3. Vitamin A and zinc (Zn) + iron (Fe)
Synergistic effect – skin, mucous membranes, and blood formation
Vitamin A supports the integrity of mucous membranes and skin, especially in combination with zinc. Iron influences vitamin A storage in the liver and its transport in the bloodstream. Deficiencies may appear as dry skin, reduced immunity, or anaemia.
4. Vitamin C and iron (Fe)
Synergistic effect – iron absorption and collagen
Vitamin C enhances iron absorption by converting it into its reduced form (Fe²⁺). Horses produce vitamin C themselves, but requirements may increase during stress, illness, or ageing. Vitamin C is also important for connective tissue and blood vessel health.
5. Excess fat-soluble vitamins (A, D, E, K)
Toxicity and mineral disturbances
Excessive intake of vitamins A or D can disrupt mineral balance (e.g. excessive calcium levels, liver strain). Vitamin A overdose may inhibit vitamin E utilisation and lead to skeletal disorders or skin problems.
| Vitamin | Affects mineral | Type of interaction | Effect |
| Vitamin E | Selenium | Synergistic | Antioxidant effect, muscle health |
| Vitamin D | Calcium, phosphorus | Synergistic | Skeletal development, mineral balance |
| Vitamin A | Zinc, iron | Synergistic | Skin, mucous membranes, immune system, blood formation |
| Vitamin C | Iron | Synergistic | Iron absorption, collagen synthesis (connective tissue, blood vessels) |
Sources:
- National Research Council (NRC). (2007).
Nutrient Requirements of Horses (6th Revised Edition).
- Geor, R. J., Harris, P. A., & Coenen, M. (2013).
Equine Applied and Clinical Nutrition.
- McDowell, L. R. (2003).
Vitamins in Animal and Human Nutrition (2nd ed.).
- Hand, M. S., Thatcher, C. D., Remillard, R. L., et al. (2010).
Small Animal Clinical Nutrition (5th ed.).
- Kentucky Equine Research (KER) – Nutrition Library.
- Alltech & Zinpro – Technical Papers.
