Repeatability of an in-feed sugar test in ponies

Reasons for performing study:

Insulin resistance (IR) can be difficult to diagnose from basal insulin and glucose concentrations, so a field-based oral sugar test (OST) is preferred. However, the repeatability of this test has not been reported.

Objectives:

To determine the repeatability of an in-feed OST in ponies.

Study design:

A repeated measures, longitudinal study.

Methods:

Eight mixed-breed ponies received an in-feed OST at 08.00 h, after an overnight fast, once weekly for 3 weeks. D-glucose powder (0.75 g/kg bwt), wheat bran (200 g) and water (500 ml) were combined, and mixed with lucerne chaff (0.3% bwt). Blood samples were taken before, and 90 and 180 min after, feeding. All meals were consumed. Blood glucose concentration was determined immediately and serum was obtained for insulin determination by equine-specific ELISA. Fasting glucose, insulin and glucose:insulin ratios were used to classify ponies as IR, compensated IR or normal on each occasion and the results were compared to the OST.

Results:

Diagnosis of IR from basal values was consistent in only 3/8 ponies, with a large mean coefficient of variation (CV) of 51% for baseline insulin, across tests. The OST results were repeatable in 7/8 ponies, with mean CV at 25% for the 90 min insulin sample (CV for 7 ponies: 19%). The OST was repeatable on 2/3 occasions in the eighth pony, however, resting hyperinsulinaemia meant that IR was diagnosed on 3/3 occasions using basal values in this pony. There was significant (P<0.05) variability in insulin concentration at 180 min, but not 90 min, across tests.

Conclusions:

An OST is more repeatable when assessing IR in ponies than basal values/ratios. Both resting hyperinsulinaemia and insulin responses to oral sugar can be assessed with the OST, which increases the likelihood of accurately diagnosing IR. Blood sampling at 90 min post feeding is recommended for more consistent results