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

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