The effect of season, management and endocrinopathies on vitamin D status in horses

Background: Vitamin D deficiency is common in humans and is increasingly linked to the pathogenesis of a multitude of diseases including obesity and metabolic syndrome. The biology of vitamin D in horses is poorly described; the relative contribution of the diet and skin synthesis to circulating concentrations is unclear and associations with the endocrine disease have not been explored. Objectives: To determine the relationship between management, season and endocrine disease and vitamin D status in horses. Study design: Cross-sectional cohort study. Methods: Plasma concentrations of 25-hydroxyvitamin D ₂ (25(OH)D ₂) and D ₃ (25(OH)D ₃) were measured by liquid chromatography-tandem mass spectrometry in 34 healthy unsupplemented grazing ponies and 22 stabled Thoroughbreds receiving supplementary vitamin D ₃ in feed. A nested group of 18 grazing ponies were sampled on long and short days (>12 and <12 h of light/day) to determine the effect of sunlight exposure. In addition, the relationships between age, sex, adiposity, serum insulin, adrenocorticotropic hormone and vitamin D status were assessed in a mixed group of 107 horses using a linear regression model. Results: All animals had a measurable level of 25(OH)D ₂ (median 10.7 nmol/L) whilst 25(OH)D ₃ was only detected in Thoroughbreds receiving D ₃ supplementation. Thoroughbreds had lower concentrations of 25(OH)D ₂ than ponies (7.4 vs. 12.6 nmol/L, p < 0.01). In grazing ponies, 25(OH)D ₂ concentrations were significantly higher on long days compared to short days (14.4 vs. 8.7 nmol/L, p < 0.01), whilst 25(OH)D ₃ was undetectable. Measures of increased adiposity, but not basal insulin, were associated with higher 25(OH)D ₂ concentrations, conversely to humans. Increasing ACTH was associated with lower 25(OH)D ₂ (p < 0.01). Main limitations: Vitamin D ₂ concentrations were not measured in grass or forage. Conclusions: In horses 25(OH)D ₂ is the predominant vitamin D metabolite, and there is an apparent lack of endogenous vitamin D ₃ production. The relationship between vitamin D and endocrine disorders in horses does not reflect that of other species and warrants further investigation.