The Role of Vitamin D in Horse Health
Vitamin D is known as the ‘sunlight’ vitamin, in general humans obtain majority of their vitamin D from our skin being exposed to UV radiation from the sun, we can also obtain some from our diet.
There are two forms of vitamin D – vitamin D2 (found in plants) and vitamin D3 (the form produced in the skin and that is also found in animal material). Vitamin D has a number of very important roles in the body, with one of the main roles being assisting calcium and phosphorus absorption from the intestines, therefore having a significant role in bone development and strength. It also supports muscle function and immune health.
Interestingly, horses seem less reliant on vitamin D for calcium absorption than humans and other mammalian species and have lower circulating levels of vitamin D in their blood than other species.
They also seem to be reliant on UV radiation for vitamin D in a different way to humans and other mammals. Some recent studies have indicated that horses produce a limited amount of vitamin D3 from UV exposure to their skin but are rather more reliant on plant sources for vitamin D2. Plants are also reliant on UV radiation to produce vitamin D, so it seems the horse has become reliant on plants producing vitamin D to meet their requirements, rather than producing it themselves.
While horses may obtain vitamin D differently, it is still an important nutrient in their diet.
A study conducted in ponies with no vitamin D supplementation or sunlight exposure had poorer bone growth and development than those supplemented with vitamin D and a study in sick foals indicated a lack of vitamin D could be a contributing factor to their illness. The National Research Council also states that young, growing horses and those in work should be supplemented with vitamin D to support healthy bone growth and development.
How horses are managed will also influence their vitamin D intake.
Multiple studies comparing stabled horses supplemented with vitamin D3, to horses managed on pasture found that those stabled had higher plasma concentrations of D3 (due to supplementation) and lower concentrations of D2, while those on pasture had undetectable levels of D3 and higher concentrations of D2, which they obtained from forage.
While the difference between supplementation with either D2 or D3 still needs to be investigated, neither study noted health concerns in either group of horses. However, these studies do indicate that if horses have limited access to forage with UV exposure, they will benefit from supplementation with vitamin D. While this is an area that needs a lot more research, so far this points to another benefit of allowing our horses free access to forage and ensuring a balanced diet.
