Effects of pasture consumption and obesity on insulin dysregulation and adiponectin concentrations in UK native-breed ponies

Background: Insulin dysregulation (ID) and hypoadiponectinaemia (total [adiponectin] < 7.9 µg/mL) are risk factors for laminitis. They are sometimes, but not always, associated with obesity.

Objectives: To investigate the effects of pasture consumption and obesity on ID and circulating total [adiponectin] in ponies.

Study design: Longitudinal.

Methods: Seven native-breed ponies with normal basal and post-oral sugar test (OST) [insulin] and body condition score (BCS) 4.3–5.5/9 were allowed to graze until they reached BCS 7/9. Ponies were then maintained at BCS 7/9 until completion of the study (week 22). Morphometric measures, OST, insulin tolerance test (ITT), 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: Median (range) BCS increased significantly (P<0.001) from 5.0 (4.3–5.5; week 0) to 7.2 (5.7–7.5; week 22). Basal [insulin] did not change significantly over the study but median post-OST [insulin] was significantly higher (P<0.05) at week 14 (95.2 [17.9–114.0] µIU/mL), week 16 (103.0 [16.4–166.0] µIU/mL), and week 20 (93.6 [10.0–153.0] µIU/mL) than week 0 (25.0 [10.0–64.0] µIU/mL). Compared with week 0, ITT results were significantly lower at weeks 2–6 and 12–20, and [adiponectin] was significantly lower at weeks 10–22 (P<0.05). [Adiponectin] decreased in all ponies during the study. Both low (3/10) and high (8-9/10) pasture scores were significantly associated with low ITT results. Low pasture scores were associated with low [adiponectin]. BCS was significantly associated with basal [insulin], post-OST [insulin], ITT results, but not [adiponectin].

Main limitations: Small sample size comprising native UK ponies; no control group with maintenance of ideal BCS.

Conclusions: Six ponies developed hypoadiponectinaemia and all showed transient or consistent ID during the study. Both short, stressed grass and long, lush grass were associated with decreased tissue insulin sensitivity.