Temporal variation in incretin and insulin secretion in ponies in association with dietary macronutrients

Equine insulin dysregulation (ID) is a common condition that predisposes to hyperinsulinaemia-associated laminitis. Improved pathophysiological understanding would enable better management of ID and reduce the risk of laminitis. The incretin peptides, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), augment the insulin response to dietary glucose and GLP-1 has been implicated in the pathophysiology of ID. However, their temporal variation in response to a low-energy diet has not been reported. This study determined the variation in GLP-1, GIP and insulin in eight healthy ponies, in relation to both the fed and unfed state, over a 24 h period. The ponies were fed a high-fibre ration plus two types of forage. Blood samples were analysed for blood glucose, total protein, serum triglycerides, which were used as proxy measures for the dietary macronutrients, as well as GLP-1, GIP and insulin concentrations. All analytes varied throughout the study (P < 0.05) with the principal pattern being a peak after eating compared to the fasted state. Blood glucose, GLP-1 and insulin were strongly positively (ρ close to 1) and synchronously (ρ peaked at time shift (τ) 0) correlated over time. Plasma GIP concentration also peaked in the unfed state (P = 0.01) and was potentially positively correlated (r = 0.39; P = 0.08) with triglyceride concentration. This relationship was confirmed in a larger cohort of 30 ponies using archived samples (P = 0.03). These data indicate that GIP likely has a physiological role in lipid clearance and/or fat metabolism in ponies, as shown in other species. As obesity is an important component of equine metabolic syndrome, further studies on the role of GIP in metabolic dysfunction are warranted.