Serum 25(OH) Vitamin D Concentrations in Horses: Effects of Age, Gender, Breed, Skin Colour and Season.

Background: The optimal amount of vitamin D required for the proper functioning of the immune system differs from the amount necessary for bone homeostasis. Furthermore, vitamin D metabolism varies among horses. Nevertheless, there is a dearth of information regarding reference values for vitamin D in horses, particularly in the Turkmen breed. The primary objectives of this study were to determine the concentrations of 25-hydroxyvitamin D (25(OH)Vit D) in Turkmen, Thoroughbred and mixed Turkmen × Thoroughbred horses and to explore the effects of various factors on it and its relationship with serum biochemical variables and signalment data.

Methods: For the measurement of 25(OH)Vit D, 90 healthy horses (min age: 6 months; max age: 10 years) that were stable in the north of Iran (37°17' N, 55°18' E) were selected for clinical examination and complete blood biochemistry analysis. The horses were categorised into different groups based on sex, season (spring, n = 45; autumn, n = 45), breed (Turkmen, Thoroughbred and mixed Turkmen × Thoroughbred), age (< 2 years, 2-8 years and > 8 years) and skin colour. Sampling was performed only once from each horse.

Results: In this study, the median serum concentration of 25(OH)Vit D in the sampled horses was 17.42 ng/mL (IQR: 9.82-30.85). The serum 25(OH)Vit D concentration was significantly lower in autumn (median: 15.83 ng/mL and IQR: 8.67-54.68) than in spring (median: 18.02 ng/mL and IQR: 13.77-27.54) and was also lower in Turkmen horses (median: 15.83 ng/mL and IQR: 11.63-23.12) than in mixed breed horses (median: 17.72 ng/mL and IQR: 8.94-51.67) (p ≤ 0.05).

Conclusion: According to the results of the present study, the season had a significant effect on the serum 25(OH)Vit D concentration, and this difference was also observed between Turkmen horses and mixed-bred horses (p ≤ 0.05). It is unclear whether the time of sun exposure or vitamin D content of diet causes the seasonal difference of 25(OH)Vit D concentration.