Variation of Insulin Sensitivity Estimates in Horses

Evaluation of fasting plasma insulin and proxy measurements to assess insulin sensitivity in horses

Background

Proxies are mathematical calculations based on fasting glucose and/or insulin concentrations developed to allow prediction of insulin sensitivity (IS) and β-cell response. These proxies have not been evaluated in horses with insulin dysregulation. The first objective of this study was to evaluate how fasting insulin (FI) and proxies for IS (1/Insulin, reciprocal of the square root of insulin (RISQI) and the quantitative insulin sensitivity check index (QUICKI)) and β-cell response (the modified insulin-to-glucose ratio (MIRG) and the homeostatic model assessment of β-cell function (HOMA-β)) were correlated to measures of IS (M index) using the euglycemic hyperinsulinemic clamp (EHC) in horses with insulin resistance (IR) and normal IS. A second objective was to evaluate the repeatability of FI and proxies in horses based on sampling on consecutive days. The last objective was to investigate the most appropriate cut-off value for the proxies and FI.

Results

Thirty-four horses were categorized as IR and 26 as IS based on the M index. The proxies and FI had coefficients of variation (CVs) ≤ 25.3 % and very good reliability (intraclass correlation coefficients ≥ 0.89). All proxies and FI were good predictors of the M index (r = 0.76–0.85; P < 0.001). The proxies for IS had a positive linear relationship with the M index whereas proxies for β-cell response and FI had an inverse relationship with the M index. Cut-off values to distinguish horses with IR from horses with normal IS based on the M index were established for all proxies and FI using receiver operating characteristic curves, with sensitivity between 79 % and 91 % and specificity between 85 % and 96 %. The cut-off values to predict IR were < 0.32 (RISQI), < 0.33 (QUICKI) and > 9.5 µIU/mL for FI.

Conclusions

All proxies and FI provided repeatable estimates of horses’ IS. However, there is no advantage of using proxies instead of FI to estimate IR in the horse. Due to the heteroscedasticity of the data, proxies and FI in general are more suitable for epidemiological studies and larger clinical studies than as a diagnostic tool for measurement of IR in individual horses.

Effects of Diet Versus Exercise on Morphometric Measurements, Blood Hormone Concentrations, and Oral Sugar Test Response in Obese Horses

Weight loss plans in horses typically use dietary restriction, but exercise may have additional benefits. This study aimed to compare the effects of a diet or exercise protocol resulting in comparable caloric restriction in obese horses. Ten obese horses were paired according to sex, age, and breed or breed type. One horse from each pair was randomly assigned to either diet (DIET)-intake restricted to approximately 85% of digestible energy requirements or exercise (EX)-exercised to expend approximately 15% of digestible energy requirements, resulting in 85% of requirements available, for 4 weeks. Body weight (BW), heart girth (HG), girth-to-height ratio (G:H), neck circumference (NC), neck circumference-to-height ratio (NC:H), belly girth (BG), body condition score (BCS), cresty neck score (CNS), serum insulin, plasma leptin, and plasma ghrelin concentrations were measured weekly. An oral sugar test was conducted on days 0 and 28 to measure insulin to glucose ratio (Ins:glc) and 60-minute insulin sensitivity index (ISI₆₀). Results were analyzed using repeated measures. A P ≤ .05 was considered significant, and .05 <P ≤ .10 was considered a tendency. Both groups showed similar decreases over time in BW, HG, G:H, BG, BCS, and CNS, but the EX group showed significantly greater losses in NC (and NC:H). The EX group showed significant improvements in (log) Ins:glc, as well as a tendency for improvements in (log) ISI₆₀ and plasma leptin concentrations, while the DIET group showed no change. Horses showed no changes in (log) plasma ghrelin with weight loss.

Retrospective analysis of insulin responses to standard dosed oral glucose tests (OGTs) via naso-gastric tubing towards definition of an objective cut-off value

Background

Insulin dysregulation (ID) with basal or postprandial hyperinsulinemia is one of the key findings in horses and ponies suffering from the equine metabolic syndrome (EMS). Assessment of ID can easily be performed in clinical settings by the use of oral glucose challenge tests. Oral glucose test (OGT) performed with 1 g/kg bodyweight (BW) glucose administered via naso-gastric tube allows the exact administration of a defined glucose dosage in a short time. However, reliable cut-off values have not been available so far. Therefore, the aim of the study was to describe variations in insulin response to OGT via naso-gastric tubing and to provide a clinical useful cut-off value for ID when using the insulin quantification performed with an equine-optimized insulin enzyme-linked immunosorbent assay.

Results

Data visualization revealed no clear separation in the serum insulin concentration of insulin sensitive and insulin dysregulated horses during OGT. Therefore, a model based clustering method was used to circumvent the use of an arbitrary limit for categorization. This method considered all data-points for the classification, taking into account the individual insulin trajectory during the OGT. With this method two clusters were differentiated, one with low and one with high insulin responses during OGT. The cluster of individuals with low insulin response was consistently detected, independently of the initialization parameters of the algorithm. In this cluster the 97.5% quantile of insulin is 110 µLU/mL at 120 min. We suggest using this insulin concentration of 110 µLU/mL as a cut-off value for samples obtained at 120 min in OGT.

Conclusion

OGT performed with 1 g/kg BW glucose and administration via naso-gastric tubing can easily be performed under clinical settings. Application of the cut-off value of 110 µLU/mL at 120 min allows assessment of ID in horses.

Prospective cohort study evaluating risk factors for the development of pasture-associated laminitis in the United Kingdom

BACKGROUND

Certain individuals appear to be predisposed to recurrent pasture-associated laminitis. Previous studies have predominantly investigated risk factors only after disease occurrence.

OBJECTIVES

To investigate risk factors for pasture-associated laminitis prior to disease occurrence.

STUDY DESIGN

Prospective cohort study.

METHODS

Non-laminitic ponies aged ≥7 years were recruited. Body condition score (BCS), height, weight, crest height and thickness were measured and an overnight dexamethasone suppression test performed. Plasma or serum adiponectin, leptin, triglyceride, basal insulin, insulin post-dexamethasone, insulin-like growth factor 1 (IGF-1), IGF binding protein 1 (IGFBP-1), IGFBP-3, C-reactive protein, von Willebrand's factor, soluble E-selectin and P-selectin concentrations were assayed. Follow-up data were obtained from owners annually for 3 years to ascertain occurrences of veterinarian-diagnosed pasture-associated laminitis. Data were analysed by multivariate logistic regression. Receiver operating characteristic (ROC) curves analysis was performed for significant risk factors and cut-off values determined.

RESULTS

A total of 446 animals with a median (interquartile range) age of 15 (10-20) years were recruited. Of these, 50.4% were mares and 49.6% were geldings. The most common breeds were Welsh (36.4%), Shetland (17.0%) and cob (9.4%). Overall, 72.2% of animals were overweight/obese (BCS 7-9/9), 27.3% were of ideal weight (BCS 4-6/9) and 0.5% were underweight (BCS 1-3/9). After 1, 2 and 3 years, respectively, 18 (4.0%), 30 (6.7%) and 44 (9.9%) animals were reported to have had laminitis. Plasma adiponectin, and serum basal (insulin) and (insulin) post-dexamethasone levels were significantly (P ≤ 0.05) associated with laminitis occurrence cumulatively after 1, 2 and 3 years. Use of the area under the ROC curves to distinguish animals that did and did not develop laminitis showed good (basal [insulin] after 1 year), fair (all others) or poor ([insulin] post-dexamethasone) levels of accuracy.

MAIN LIMITATIONS

Animals were evaluated at a single time point and biomarkers were assayed using single assays.

CONCLUSIONS

Risk factors for future laminitis prior to disease occurrence include low plasma adiponectin and high serum basal insulin or insulin post-dexamethasone concentrations.

The repeatability of an oral glucose test in ponies

REASONS FOR PERFORMING STUDY

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

OBJECTIVES

To determine the repeatability of an in-feed OGT in ponies and examine some factors affecting the palatability of the test meal.

STUDY DESIGN

A repeated measures, longitudinal study.

METHODS

An in-feed OGT was performed at 08.00 h on 3 consecutive occasions under controlled conditions in 8 mixed breed ponies. d-glucose (0.75 g/kg bwt) was dissolved in water and combined with wheat bran and lucerne chaff. Blood samples were taken before and 90, 120, 180 min and 24 h after d-glucose. The repeatability of the test was analysed with repeated measures ANOVA. Insulin and glucose responses to d-glucose were also compared to an equivalent dose of dietary carbohydrate provided with a commercial grain mixture.

RESULTS

The overall insulin responses to the OGTs did not differ between tests. Individual insulin responses were more variable (P<0.05) than glucose responses. There was no difference in insulin concentration in post d-glucose samples over time. Insulin and glucose responses to grain and d-Glucose were not different.

CONCLUSIONS

An OGT is reasonably repeatable in ponies. The currently recommended post-glucose sampling time point of 2 h is acceptable, with sampling at 90 min also likely to produce a consistent result. The use of an alternative carbohydrate source to d-glucose, such as a commercial grain-based product, may be a viable and more palatable option for the test.

Characterization of the intravenous glucose tolerance test and the combined glucose-insulin test in donkeys

Glucose-insulin dynamic challenges such as the intravenous glucose tolerance test (IVGTT) and combined glucose-insulin test (CGIT) have not been described in donkeys. The objectives of this study were (1) to characterize the IVGTT and CGIT in healthy adult donkeys, and (2) to establish normal glucose-insulin proxies. Sixteen donkeys were used and body morphometric variables obtained each. For the IVGTT, glucose (300 mg/kg) was given IV. For the CGIT, glucose (150 mg/kg) followed by recombinant insulin (0.1 IU/kg) were administered IV. Blood samples for glucose and insulin determinations were collected over 300 min. In the IVGTT the positive phase lasted 160.9 ± 13.3 min, glucose concentration peaked at 323.1 ± 9.2 mg/dL and declined at a rate of 1.28 ± 0.15 mg/dL/min. The glucose area under the curve (AUC) was 21.4 ± 1.9 × 10(3) mg/dL/min and the insulin AUC was 7.2 ± 0.9 × 10(3) µIU/mL/min. The positive phase of the CGIT curve lasted 44 ± 3 min, with a glucose clearance rate of 2.01 ± 0.18 mg/dL/min. The negative phase lasted 255.9 ± 3 min, decreasing glucose concentration at rate of -0.63 ± 0.06 mg/dL/min, and reaching a nadir (33.1 ± 3.6 mg/dL) at 118.3 ± 6.3 min. The glucose and insulin AUC values were 15.2 ± 0.9 × 10(3) mg/dL/min and 13.2 ± 0.9 × 10(3) µIU/mL/min. This is the first study characterizing CGIT and IVGTT, and glucose-insulin proxies in healthy adult donkeys. Distinct glucose dynamics, when compared with horses, support the use of species-specific protocols to assess endocrine function.

Recovery of insulin sensitivity in mature horses after a 3 week course of dexamethasone therapy

REASONS FOR PERFORMING THE STUDY

Dexamethasone is an anti-inflammatory drug commonly used in equine medicine. Insulin sensitivity decreases with prolonged dexamethasone administration, but little information is available about the duration of this side effect after long-term treatment ends.

OBJECTIVES

To determine how long it takes for blood glucose, insulin and markers of insulin sensitivity to return to normal ranges after extended dexamethasone treatment has ceased.

STUDY DESIGN

Experimental study.

METHODS

Eight healthy, mature, mixed-breed horses received 0.04 mg/kg bwt/day oral dexamethasone for 21 days. Blood samples were taken weekly during dexamethasone treatment (Days -21, -14 and -7). Following the final dose of dexamethasone on Day 0, blood samples were taken on Days 1-6, 8, 10, 12, 15 and 22. Day -21 represents baseline or normal blood predexamethasone.

RESULTS

On Day 1, plasma glucose and insulin concentrations and the modified insulin-to-glucose ratio (a proxy for pancreatic β cell responsiveness) were higher and the reciprocal of the square root of insulin (a proxy for the estimate of insulin sensitivity) was lower, in comparison with Day -21 values. Blood glucose concentrations dropped and returned to Day -21 values by Day 2. Insulin concentrations remained elevated until Day 3. Values for the modified insulin-to-glucose ratio decreased and returned to Day -21 concentrations by Day 4. Values for the reciprocal of the square root of insulin did not return to Day -21 values until Day 15.

CONCLUSIONS

These results indicate that, in contrast to blood glucose concentrations, which return to normal quickly (within 2 days after treatment ends), the pancreatic insulin-secreting response has a delayed recovery.

Seasonal and annual influence on insulin and cortisol results from overnight dexamethasone suppression tests in normal ponies and ponies predisposed to laminitis

REASONS FOR PERFORMING STUDY

A simple, accurate test for identifying individual animals at increased risk of laminitis would aid prevention. Laminitis-prone ponies have a greater serum insulin response to dexamethasone administration than normal ponies in the summer, but the response during different seasons is unknown.

OBJECTIVE

To test the hypothesis that previously laminitic ponies have a greater insulin response to dexamethasone than normal ponies, which is present during all seasons.

STUDY DESIGN

Prospective longitudinal study.

METHODS

Overnight dexamethasone suppression tests were performed on 7 normal ponies and 5 previously laminitic ponies in spring 2009 and 2010, summer 2008 and 2010, autumn 2009 and winter 2008, while the ponies were at pasture. In spring 2010, a dexamethasone suppression test was performed after the ponies had been fed only hay for 3 weeks. Serum cortisol and insulin concentrations pre- and post dexamethasone were measured. Linear mixed models were used to analyse the data.

RESULTS

Insulin concentrations pre- and post dexamethasone were significantly higher in previously laminitic ponies than in normal ponies during spring 2009 and summer 2008, but there was no difference between groups in spring 2010, summer 2010, autumn 2009 or winter 2008. Insulin concentration varied significantly with season. Diet had no apparent effect on insulin concentration pre- or post dexamethasone in spring 2010. Cortisol concentrations post dexamethasone were significantly higher in previously laminitic ponies than in normal ponies in autumn 2009, with concentrations above the reference range (<25 nmol/l) in both groups in summer 2008 and autumn 2009. Individual ponies had insufficient cortisol suppression in all seasons.

CONCLUSIONS

There were significant differences between groups in insulin and cortisol concentrations post dexamethasone during some seasons, but this was not present in all years. Wide interindividual variation in response limits the usefulness of a dexamethasone suppression test for predicting the susceptibility of an individual animal to laminitis.

POTENTIAL RELEVANCE

Abnormal insulin and cortisol responses to dexamethasone must be interpreted in the light of the individual animal, seasonal and annual variation reported here.