Research continues to advance our understanding of metabolic problems in horses. Equine metabolic syndrome (EMS) is known for its multiple risk factors for laminitis due to insulin dysregulation, genetic predisposition and obesity.
A Scottish study looked at the response of adipose tissue to excessive calorie intake. Internal adipose tissue—visceral and retroperitoneal—appears to experience the strongest pathologic disruptions [Reynolds, A.; Keen, J.A.A.; Fordham, T.; and Morgan R.A. Adipose tissue dysfunction in obese horses with equine metabolic syndrome. Equine Veterinary Journal Nov 2019, vol 51 (6); pp. 760-766; doi: 10.1111/evj.13097].
EMS horses (n=6) used in the study were older than two years with a body condition score >4/5, fasting basal insulin >20 ml U/L, current or history of laminitis, plasma ACTH within the seasonal reference range and pituitary histologic score <3/5. Healthy horses (n=9) had body condition <4/5 and had no clinical or laboratory signs of insulin dysfunction nor had they been treated with glucocorticoids in the previous three months. All horses selected for the study, including the “healthy” controls, were destined for euthanasia. Samples of peri-renal and retroperitoneal fat were taken at necropsy. Due to the difficulty in obtaining a representative sample of subcutaneous nuchal crest and rump fat, the researchers on this project chose to evaluate visceral and retroperitoneal depots. Internal adipose deposits not only are consistently present in lean and fat horses but also expand rapidly in obese individuals.
Adipocyte size and hypertrophy occur with excess calorie intake and these characteristics are associated with insulin resistance and dyslipidemia in humans. EMS horses experienced marked adipocyte hypertrophy and subsequent inflammation and circulating cytokines. Marked leptin gene expression also occurs in EMS horses and is related to adipocyte volume.
The authors stated in the results: “Our data demonstrate that adipocyte hypertrophy, inflammation and increased CCL2 (cytokine) expression are features of obese adipose tissue in horses but fibrosis and altered adipose tissue expression of insulin-signaling genes are not consistent features. Visceral and retroperitoneal adipose tissue of horses with obesity and EMS is markedly dysfunctional with a hypertrophic-inflammatory phenotype.”
They encouraged owner education about the “unhealthy” nature of such obese adipose tissue and how it affects the rest of the body by “inducing or worsening whole body insulin dysregulation.”