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

Miranda Dosi, Bruce McGorum, Roxane D. Kirton, Eugenio Cillan-Garcia, Richard Mellanby, John Keen, Emma Hurst, Ruth Morgan

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

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 endocrine disease have not been explored.

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

Study design
Cross-sectional cohort study.
Plasma concentrations of 25-hydroxyvitamin D2 (25(OH)D2) and D3 (25(OH)D3) were measured by LC-MS/MS 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 (>12and<12h of light/day) to determine the effect of sunlight exposure. Additionally, 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 measurable level of 25(OH)D2 (median 10.7nmol/L) while 25(OH)D3 was only detected in Thoroughbreds receiving D3 supplementation. Thoroughbreds had lower concentrations of 25(OH)D2 than ponies (7.4nmol/L v 12.6nmol/L p<0.01). In grazing ponies 25(OH)D2 concentrations were significantly higher on long-days compared to short-days (14.4nmol/L v 8.7nmol/L p<0.01), while 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).

Main limitations
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.
Original languageEnglish
Pages (from-to)672-680
Number of pages8
JournalEquine Veterinary Journal
Issue number4
Early online date12 Aug 2022
Publication statusPublished - Jul 2023

Keywords / Materials (for Non-textual outputs)

  • adipose tissue
  • horse
  • insulin
  • obesity
  • season
  • vitamin D


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