Obesity and metabolic syndrome are highly prevalent in horses and ponies worldwide. Equine Metabolic Syndrome (EMS) shares much of the same pathophysiology with its human counterpart including adipose tissue dysfunction and insulin dysregulation. Vitamin D deficiency is strongly associated with obesity and insulin resistance in humans and supplementation is often recommended. In horses our understanding of vitamin D biology is limited and its association with EMS unknown. The limited data available suggest that horses have comparatively low circulating total 25OHD, and it is almost entirely 25OHD2 obtained through their diet. To further investigate vitamin D status in horses plasma concentrations of 25OHD2 and 25OHD3 were measured by UHPLC-MS in 22 racehorses receiving 6600-8800IU/day of D3 supplementation and 34 ponies kept at pasture and not supplemented. Grazing ponies (n = 18) were sampled in summer and winter to determine the effect of sunlight exposure. Regression analysis was used to determine the effect of age, breed, sex, morphometric scores, serum insulin on vitamin D status in horses/ponies with (n = 33) and without EMS (n = 74). Total plasma D concentrations were much lower in horses than in humans (< 25nM). 25OHD3 was detected only in supplemented horses (8.6±3.2nM) and was undetectable all year-round in non-supplemented horses (P < 0.01) indicating, horses do not synthesise D3 in the skin. 25OHD2 was detected in all horses (11.3±4.0nM) and at significantly higher levels in the grazing herd (P < 0.01) suggesting grass is the primary source of D2. In contrast to humans, increased adiposity was associated with higher 25OHD2(β=0.17, P < 0.01) and there was no association with serum insulin. These comparative data raise important questions on our understanding of vitamin D biology suggesting that unlike humans, horses do not synthesise 25OHD3 and vitamin D status is positively influenced by adiposity. Further investigation is warranted to understand this unusual observation.
08 Nov 2021 - 10 Nov 2021