Vitamin D study sheds light on horse health

Some stabled racehorses may be predisposed to low vitamin D status owing to their lifestyle, a study has shown.

Scientists who studied levels of both types of vitamin D – D₂ and D₃ – in Thoroughbreds in Hong Kong and the UK found that the former group, which had no access to grazing, had lower vitamin D levels overall.

The finding, which is attributed to the horses’ diet, lifestyle and management, sheds light on an understudied area of health and wellbeing in horses, which are unable to produce vitamin D from exposure to sunlight.

The study highlights the need for dietary supplementation for horses that are not grazing, and are therefore unable to obtain vitamin D from grass.

Vitamin D2, the form occurring in naturally grazing horses, may be the most important type for horse health and athletic performance, but more work is needed to determine how best to administer supplements to horses, the outcomes suggest.

Researchers also discovered that raised levels of either type –  vitamin D₂ or D₃ – may lead to a drop in levels of the other type.

Lifestyle factors

A team from the Royal (Dick) School of Veterinary Studies and The Hong Kong Jockey Club Equine Welfare Research Foundation compared vitamin D levels in the blood of groups of Thoroughbred racehorses in Hong Kong and in Scotland.

They anticipated that the horses’ limited access to grazing, and time spent indoors, combined with their intense activity, might lead to relatively low levels of the essential nutrient.

Their results showed that Thoroughbred racehorses should be given vitamin D supplements in their feed.

The study also highlighted the need for further work to understand how horses process the vitamin, and the optimum dose and type of supplementation.

Health insights

The study suggests that vitamin D₂ may have a more significant impact on horse health and athletic performance than the D₃ form, and that the inverse relationship identified between the two means that oral D₃ supplementation may reduce D₂ levels.

The outcomes offer novel insight into equine vitamin D biology. The study confirms previous findings that horses’ vitamin D levels are considerably lower than in other species.

Further study is required to determine the threshold concentrations for vitamin D deficiency, and whether supplements can improve racehorse health and athletic performance.

Non-grazing racehorses in Hong Kong should have vitamin D supplementation, but research is needed to determine optimal dose and form of supplementation.

The study was published in Animals, in collaboration with colleagues from Kentucky Equine Research, US, and funded by the Fiona and Ian Russell Seed Corn Fund for Companion Animal Research and the Hong Kong Jockey Club.

These findings add to our understanding of vitamin D biology in equine athletes, and in horses overall. Further research could add to this by helping define the optimum levels for equine health and performance.

Professor Bruce McGorum

Scientific publication

_{Image credit: Keith Luke / Unsplash; Ronnie Macdonald  via Wikimedia Commons}

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The Royal (Dick) School of Veterinary Studies is a one-of-a-kind centre of excellence in clinical activity, teaching and research. Our purpose-built campus, set against the backdrop of the beautiful Pentland Hills Regional Park, is home to more than 800 staff and almost 1400 students, all of whom contribute to our exceptional community ethos. 

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