Serum insulin concentration at 60 min (InsulinT60) during an oral sugar test (OST) indicates future laminitis risk and insulin dysregulation (ID). Associations between InsulinT60 and physical and owner-reported variables may help clinicians select individual ponies for ID testing. Associations between InsulinT60 and other metabolic markers may help elucidate ID pathophysiology. The aims of this prospective longitudinal study were to A) Describe associations between binary InsulinT60 status and season, and physically apparent and owner-reported data to inform strategies to target ID screening; and B) Describe more complex associations between continuous InsulinT60 and other metabolic markers, owner-reported data and physical features to indicate relationships relevant to ID pathophysiology.
Non-laminitic ponies were examined and OSTs (0.3 mL/kg Karo syrup) performed every 6 months (autumn and spring) for ≤4 years. Factors associated with InsulinT60 were determined using mixed-effects models with physical, owner-reported, season and serum/plasma markers as fixed effects and pony and premises identifiers as random effects.
One thousand seven hundred and sixty-three OSTs from 367 ponies were included. High-risk InsulinT60 (>153 μIU/mL) was independently associated with (odds ratio, 95% confidence interval [CI]): age (1.36, 1.16–1.60), body condition score (BCS) (2.38, 1.21–4.69), and bulging supraorbital fat pads (7.25, 2.1–24.98). However, the initial models provided little explanatory power (Nakagawa R2 = 0.1–0.27). InsulinT60 was independently associated with (effect estimate, 95% CI): age (0.02, 0.01–0.04), Welsh/Welsh X breed (0.22, 0.05, 0.39), sex (gelding = −0.2, −0.34 to 0.06), BCS (0.16, 0.08–0.23), plasma adiponectin (−0.02, −0.02 to 0.01) and basal insulin (0.01, 0.01–0.01) in spring, and: age (0.03, 0.02–0.04), BCS (0.17, 0.08–0.26), bulging supraorbital fat pads (0.37, 0.2–0.54), turnout score (0.05, 0.02–0.09), plasma adiponectin (−0.01, −0.02 to 0.01), ACTH (per 10 pg/mL) (0.01, 0.00–0.01), triglycerides (0.28, 0.07–0.49) and InsulinT0 (0.01, 0.01–0.01) in autumn.
Season, owner-reported and physical features only explained 10%–27% of the differences in InsulinT60 risk status in this population. Thus, while physical and owner-reported features can be used to suggest ponies with a higher risk of ID, veterinarians should not limit testing for ID to ponies in which these risk factors are present.