Vitamin K Supplementation for Horses

The actions of vitamin K on blood clotting, bone metabolism, and bone resorption have been known for some time. However, new research from Japan has shown that the most effective way to supplement horses with vitamin K is in the form of K3, or menadione.

The various forms of vitamin K are similar in structure, but each can have significantly different actions. Phylloquinone (K1) is present in plants, with high levels found in green grass growing in sunlight. Levels of K1 in grass drop when it is wilted, deprived of sun, or made into hay. Because it is tightly bound with chloroplasts, absorption of K1 in the digestive tract is poor. Menaquinone (K2) is synthesised by the bacteria in the hindgut and is present in some fermented foods. Absorption of K2 can be impaired by conditions such as hindgut acidosis.

Menadione (K3) is the synthetic form of vitamin K that can be found in some commercial vitamin and mineral premixes, and is added to feeds and supplements. It is thought that in order for the horse to absorb and utilize K1, it is first converted to K3 in the small intestine. The most potent analogue for bone formation is menaquinone-4 (K2-4), which is formed when K3 is metabolized in the liver. In humans, K2-4 has been found to be far more active than other forms of vitamin K in stimulating bone formation.

In a recent Japanese study, four groups of four horses were used: a control group, a K1 group, a K2 group, and a K3 group. The basal diet supplied 16 mg of vitamin K per day, and the treatment groups supplied 25 mg of each type of vitamin K for 7 days.

Supplementation with K1 increased K1 levels but not K2-4 levels, and supplementation with K2 did not increase K1 or K2-4 levels. The horses given K3 were the only horses to have a significant (4x) increase in K2-4 levels. Therefore, because of the high conversion efficiency, K3 is the only form that increased the plasma concentrations of K2-4 and is the most effective source of vitamin K for bone health in horses.

Injectable menadione has been shown to cause kidney damage in horses and is associated with colic, muscle stiffness, and death. Injectable menadione should be administered under veterinary supervision and only as prescribed.

This research was reported in Journal of Animal Science 89:1056.