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

Abstract

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(25(OH)D) and D(25(OH)D) were measured by liquid chromatography-tandem mass spectrometry in 34 healthy unsupplemented grazing ponies and 22 stabled Thoroughbreds receiving supplementary vitamin Din feed. A nested group of 18 grazing ponies were sampled on long and short days (>12 and <12&#x2009;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(median 10.7&#xa0;nmol/L) whilst 25(OH)Dwas only detected in Thoroughbreds receiving Dsupplementation. Thoroughbreds had lower concentrations of 25(OH)Dthan ponies (7.4 vs. 12.6&#xa0;nmol/L, p&#xa0;<&#x2009;0.01). In grazing ponies, 25(OH)Dconcentrations were significantly higher on long days compared to short days (14.4 vs. 8.7&#xa0;nmol/L, p&#xa0;<&#x2009;0.01), whilst 25(OH)Dwas undetectable. Measures of increased adiposity, but not basal insulin, were associated with higher 25(OH)Dconcentrations, conversely to humans. Increasing ACTH was associated with lower 25(OH)D(p&#xa0;<&#x2009;0.01). MAIN LIMITATIONS: Vitamin Dconcentrations were not measured in grass or forage. CONCLUSIONS: In horses 25(OH)Dis the predominant vitamin D metabolite, and there is an apparent lack of endogenous vitamin Dproduction. The relationship between vitamin D and endocrine disorders in horses does not reflect that of other species and warrants further investigation.