Changes in insulin sensitivity and adiponectin concentrations associated with pasture-induced obesity in UK native-breed ponies

Background: Insulin dysregulation (ID) and hypoadiponectinemia are risk factors for laminitis. They are sometimes, but not always, associated with obesity. Objectives: To investigate the effects of pasture-induced weight gain and obesity on insulin sensitivity and circulating total [adiponectin] in insulin-sensitive ponies. Study design: Longitudinal study. Methods: Eight insulin-sensitive, native-breed ponies with body condition score (BCS) 4.5–5.5/9 were allowed to graze until they reached BCS 7/9. Ponies were then maintained at BCS 7/9 for 10 weeks. Morphometrics, metabolic parameters (oral sugar test [OST], tissue insulin sensitivity [insulin tolerance test, ITT], and plasma [adiponectin]), whole-blood expression of receptors for adiponectin, insulin, and insulin-like growth factor 1, and pasture conditions (height and vigour) were determined fortnightly. Results: Mean±SD BCS increased significantly (P<0.001) from 4.9±0.4 (week 0) to 6.9±0.6 (week 12) and 7.0±0.7 (week 22). Mean basal [insulin] did not change over the study but insulin response to a 60-min OST (T60 [insulin]) was significantly higher at weeks 14, 16, and 20 than at week 0 (P<0.05). Compared with week 0, tissue insulin sensitivity was significantly lower at weeks 2–6 and 12–20, and [adiponectin] was significantly lower at weeks 10–22 (P<0.05). Mean basal [insulin] was positively correlated with pasture scores (Pearson’s r=0.620, P=0.03). Pasture scores were a significant variable in repeated measures models of [adiponectin] and ITT results. Mean BCS and cresty neck scores were significantly negatively correlated with mean [adiponectin] (Spearman’s r=-0.9, P<0.001). Using repeated measures models, BCS was significantly associated with basal [insulin], T60 [insulin], ITT results, and [adiponectin]. Main limitations: Small sample size comprising only native UK ponies; no control group with maintenance of ideal BCS. Conclusions: Despite being insulin-sensitive at the start, all ponies developed hypoadiponectinaemia and ID during the study. Both short, stressed grass and long, lush grass were associated with decreased tissue insulin sensitivity.