Insulin and incretin responses to grazing in insulin-dysregulated and healthy ponies.

Background: Supraphysiological insulin and incretin responses to a cereal-based diet have been described in horses and ponies with insulin dysregulation (ID). However, the hormonal responses to grazing have not yet been described.

Objectives: To determine if there is a difference in the insulin and incretin responses to grazing pasture between insulin-dysregulated and healthy ponies.

Animals: A cohort of 16 ponies comprising 5 with normal insulin regulation (NIR), 6 with moderate ID (MID), and 5 with severe ID (SID).

Methods: In this case-control study, an oral glucose test (OGT) was used to determine the insulin responsiveness of each pony to PO carbohydrate before grazing pasture (4 hours) for 3 consecutive days. Serial blood samples collected during grazing were analyzed for glucose, insulin, glucose-dependent insulinotropic peptide (GIP) and active glucagon-like peptide-1 (aGLP-1), and compared among pony groups and day of pasture access.

Results: The area under the insulin curve when grazing increased with ID severity (P < .03). The median (range) maximal insulin concentration was greater in the MID (72.5 [129] μIU/mL) and SID (255 [338.5] μIU/mL) groups, compared to the NIR (11.7 [24.9] μIU/mL) group (P < .03) and occurred within 2-4 hours of grazing. Postprandial OGT insulin concentration was positively correlated with 2 hours post-grazing insulin across all 3 grazing days (P ≤ .03). The aGLP-1 and GIP concentrations increased in response to grazing but did not differ among groups.

Conclusions And Clinical Importance: Grazing pasture provoked an increased insulin and incretin response in insulin-dysregulated ponies within 4 hours of grazing. The pasture and OGT insulin concentrations were correlated.