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Background: Obesity is associated with insulin resistance, vascular dysfunction and altered cortisol metabolism both in humans and in horses. Objectives: Evaluate the effect of weight gain induced by a haylage diet low in nonstructural carbohydrates (NSC) on insulin sensitivity, blood pressure and serum cortisol concentrations. Study design: In vivo experiment. Methods: Nine adult Standardbred mares fed a fat supplemented haylage diet at 250% of the horses' daily metabolisable energy requirements for 22 weeks. Horses were then turned out on pasture for 4 weeks. Insulin sensitivity (SICLAMP) was measured before and after weight gain and after 4 weeks of pasture using the euglycemic hyperinsulinaemic clamp (EHC) method. Body condition score (BCS), blood pressure and serum cortisol were monitored throughout the study. All data were analysed using the linear mixed model procedure. Values of P < 0.05 were considered as statistically different. Results: All horses became obese during the weight gain period (BCS> 7). Mean arterial blood pressure (MAP) increased during the weight gain period and was significantly higher than initial values at the end of the weight gain period (78 ± 3 mm Hg vs 92 ± 3 mmHg). MAP remained increased on pasture (93 ± 3 mmHg). SICLAMP was unaffected by weight gain 0.9 ± 0.1 vs 1.0 ± 0.1 ([mg/kg/min × 103]/[µIU/mL × mmol/L])) but improved after pasture (1.6 ± 0.1 ([mg/kg/min × 103]/ [mU/L]). Serum cortisol concentrations increased during the weight gain period (80 ± 9 nmol/L vs 112 ± 9 nmol/L) and remained increased during pasture. Main limitations: Limited number of horses and no control group. Conclusions: Obesity was associated with a linear increase in blood pressure and an increase in serum cortisol that was not associated with insulin sensitivity. [ABSTRACT FROM AUTHOR]