Use of proxy measurements of insulin sensitivity and insulin secretory response to distinguish between normal and previously laminitic ponies

Reasons for performing study: Insulin resistance may be a risk factor for pasture-associated laminitis. Diagnosis of insulin resistance could help identify individuals at increased risk of laminitis. Objective: To calculate proxy measurements of insulin sensitivity (reciprocal of the square root of insulin: RISQI and quantitative insulin sensitivity check index: QUICKI) and insulin secretory response (modified insulin-to-glucose ratio: MIRG) based on basal glucose and insulin concentrations in normal (NP) and previously laminitic (PLP) ponies. Methods: Proxies were calculated in 7 NP and 5 PLP from 20 separate measurements of insulin and glucose taken in spring, summer and winter when ponies were adapted to eating either pasture or hay. Proxies were RISQI: Insulin-0.5, QUICKI: 1/(log[fasting Insulin]+ log[fasting Glucose]) and MIRG: (800-0.3 x [Insulin-50]2)/[Glucose-30]. A modified insulin-to-glucose ratio for ponies (MIGRP) was investigated using: (3000-0.012 x[Insulin-500]2)/[Glucose-30]. Statistical analysis used linear mixed models. Results: Diet did not significantly affect measurements, so values were pooled for further analysis. RISQI (mean +/- s.d.) was lower in PLP (0.26 +/- 0.15 [mu/l]-0.5) than NP (0.29 +/- 0.12 [mu/l]-0.5; P = 0.05). QUICKI was lower in PLP (0.31 +/- 0.05) than NP (0.33 +/- 0.04; P = 0.047). There was no difference in MIRG between NP and PLP. MIGRP (median [interquartile range]) was greater in PLP (4.0 [7.9][muins]2/10 center dot l center dot mggluc) than NP (2.6 [3.2][muins]2/10 center dot l center dot mggluc; P = 0.022). In spring, NP had higher RISQI and QUICKI and lower MIGRP than PLP (P<0.001). In PLP, RISQI and QUICKI were higher in summer than spring (P<0.02) and MIGRP was lower in summer than other seasons (P<0.01). In NP, RISQI, QUICKI and MIGRP were each different between seasons (P<0.017). MIRG did not vary with season. Conclusions: RISQI, QUICKI and MIGRP, but not MIRG, differentiated between NP and PLP. None of the proxies accurately identified individual PLP. Seasonal changes in insulin sensitivity and insulin secretory response were apparent. Potential relevance: Current proxy measurements cannot determine an individual's laminitis susceptibility. MIGRP may be useful in hyperinsulinaemic animals.