maintain an adequate vitamin D status, according to recent research.
Vitamin D is an essential nutrient for horses, as it is for many other animals and humans. It plays a crucial role in maintaining overall health and well-being by regulating various physiological processes. The primary function of vitamin D is to help the body absorb and utilize calcium and phosphorus, which are vital for strong bones, teeth, and proper muscle function.
Unlike some animals that can synthesize vitamin D in response to UV radiation on their skin, horses have little or no ability to produce vitamin D this way. Consequently, they rely heavily on dietary sources to obtain an adequate supply of the vitamin. Horses primarily obtain vitamin D from their diet, including fresh forage like pasture grasses and certain types of hay, which naturally contain vitamin D. Additionally, commercial horse feeds and supplements may be fortified with vitamin D to ensure the animals receive enough.
What effect does limited grazing have on vitamin D status in horses?
In a study conducted by Miranda Dosi and colleagues, the vitamin D status of racehorses in training from both Hong Kong (HK) and the United Kingdom was assessed. A full report is published in the journal Animals. The main objective was to investigate whether the management practices and athletic activity of these horses made them more susceptible to having low vitamin D status, and to determine if appropriate dietary vitamin D supplementation could help mitigate this risk.
The study encompassed 101 Thoroughbred horses in training, 79 of which were from Hong Kong (HK) and 22 from the United Kingdom (UK). The HK horses did not have access to grazing and received minimal sunlight exposure, usually less than 30 minutes daily, during training and in-hand walking. In contrast, the UK horses had the opportunity to graze for at least an hour each day.
The research team assessed the serum concentrations of three active forms of vitamin D, namely 25-hydroxyvitamin D2 (25OHD2), 25-hydroxyvitamin D3 (25OHD3), and total 25-hydroxyvitamin D (total 25OHD).
Forty-one of the HK horses were sampled twice to investigate the impact of their duration in HK on their vitamin D status. By taking samples at two different time points, the researchers could assess how the length of time the horses spent in HK influenced their vitamin D levels.
According to the research findings, the HK horses had notably lower serum concentrations of total 25OHD and 25OHD2 compared to the horses from the UK. Additionally, 15 out of 79 HK horse sera showed undetectable levels of 25OHD2. The researchers also found that the serum concentrations of 25OHD2 decreased with the length of time the horses had spent in Hong Kong.
Furthermore, the research team observed an inverse relationship between 25OHD2 and 25OHD3 in the study subjects. This finding suggests that oral D3 supplementation might have a negative impact on the serum concentrations of 25OHD2. In other words, increasing the intake of vitamin D3 through oral supplements could potentially lead to a reduction in the levels of 25OHD2 in the blood. They suggest that this highlights the need for further investigation and consideration when administering vitamin D3 supplements to the horses.
For more details, see:
Thoroughbred Racehorses in Hong Kong Require Vitamin D Supplementation to Mitigate the Risk of Low Vitamin D Status
Miranda C.M. Dosi, Chris M. Riggs, Jessica May, Adele Lee, Eugenio Cillan-Garcia, Joe Pagan, and Bruce C. McGorum.
Animals (2023) 13, no. 13: 2145.