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

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.

To determine the relationship between management, season and endocrine disease and vitamin D status in horses.

Cross-sectional cohort study.

Plasma concentrations of 25-hydroxyvitamin D₂ (25(OH)D₂ ) and D₃ (25(OH)D₃ ) were measured by liquid chromatography-tandem mass spectrometry in 34 healthy unsupplemented grazing ponies and 22 stabled Thoroughbreds receiving supplementary vitamin D₃ 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.

All animals had a measurable level of 25(OH)D₂ (median 10.7 nmol/L) whilst 25(OH)D₃ was only detected in Thoroughbreds receiving D₃ supplementation. Thoroughbreds had lower concentrations of 25(OH)D₂ than ponies (7.4 vs. 12.6 nmol/L, p < 0.01). In grazing ponies, 25(OH)D₂ concentrations were significantly higher on long days compared to short days (14.4 vs. 8.7 nmol/L, p < 0.01), whilst 25(OH)D₃ was undetectable. Measures of increased adiposity, but not basal insulin, were associated with higher 25(OH)D₂ concentrations, conversely to humans. Increasing ACTH was associated with lower 25(OH)D₂ (p < 0.01).

Vitamin D₂ concentrations were not measured in grass or forage.

In horses 25(OH)D₂ is the predominant vitamin D metabolite, and there is an apparent lack of endogenous vitamin D₃ production. The relationship between vitamin D and endocrine disorders in horses does not reflect that of other species and warrants further investigation.