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

Miranda Carlotta Maria Dosi*, Bruce McGorum, Roxane Kirton, Eugenio Cillán-García, Richard J Mellanby, John Keen, Emma Hurst, RM Morgan

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

3 Citations (Scopus)
54 Downloads (Pure)


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.

Original languageEnglish
Pages (from-to)672-680
Number of pages9
JournalEquine Veterinary Journal
Issue number4
Early online date12 Aug 2022
Publication statusPrint publication - Jul 2023


  • adipose tissue
  • horse
  • insulin
  • obesity
  • season
  • vitamin D
  • Cross-Sectional Studies
  • Cholecalciferol
  • Humans
  • Insulins
  • Endocrine System Diseases/veterinary
  • Horse Diseases
  • Animals
  • Vitamin D
  • Horses
  • Seasons


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