Carbohydrate digestion in the stomach of horses grazed on pasture, fed hay or hay and oats

Concentrations of starch, mono- and disaccharides, fructans, hemicellulose and cellulose were analysed in feed and gastric digesta of horses in relation to acid insoluble ash as a marker indigestible in the stomach. Twenty-four horses were allocated to pasture 24 h/d (PST; n = 4), hay ad libitum (HAY; n = 8), hay ad lib. and oats at 1 g starch/kg body weight (BWT)/meal (OS1; n = 6) and hay ad lib. and oats at 2 g starch/kg BWT/meal (OS2; n = 5). One horse was excluded from the analysis. The horses were fed the ration a minimum of 34 days. Following euthanasia and dissection, digesta was sampled from Pars nonglandularis (PNG) and Pars glandularis (PG). Oat starch concentration in gastric digesta decreased from 309 to 174 g/kg dry matter (DM) in OS1 (44 %-reduction) and from 367 to 261 g/kg DM in OS2 (29 %-reduction) (P < 0.001). Glucose, fructose and sucrose disappeared from gastric digesta distinctly more in PST, HAY and OS1 than in OS2. In PST and HAY, sucrose concentration was completely cleared (P < 0.001). The concentration of fructans was reduced predominantly in PST (84 %-reduction) and HAY (54 %-reduction), mainly in the PNG (P < 0.05). Fructan degradation did not occur in the high-starch diet (OS2). Some evidence for fibre degradation was observed in PST (P < 0.01). Soluble carbohydrates disappear from the stomach dependent on the type of ration, which may lead to changes in the composition of the gastric microbial community and the endogenous response.

Time budgets differ in horses during continuous and space-restricted rotational grazing

Horses can become obese and develop related health issues such as laminitis from excessive grazing on high-quality pasture grass; limiting pasture intake can allow weight loss to occur. The objective of this study was to determine the effect of space-restricted rotational grazing on body weight (BW) and time budgets in horses. Eight mature geldings and mares with maintenance-only requirements were randomly assigned to either a space-restricted rotational grazing group (SRG; BW 512 ± 6 kg; n = 4) or a continuous grazing group (CG; BW 517 ± 49 kg; n = 4) for 42 d SRG horses grazed an area with dimensions to provide 80-90 % of mean digestible energy requirement for the 4 horses over a 7-d grazing period; whereas, the CG horses continuously grazed similar non-toxic endophyte-infected tall fescue pasture providing greater than maintenance requirements for the 42 d Horses in the SRG group were moved to a new area every 7 d for 6 weeks. On d 7 at 1600 h of each week, horses were brought inside, and feed was withheld overnight. At 0700 h the next day, BWs were recorded prior to turnout. Observers recorded behaviors simultaneously on SRG and CG horses every six minutes throughout the day three days per week according to an ethogram. This included 30 s scans of all horses. Proportion of grazing and standing had an inverse relationship. Proportion of grazing was affected by the treatment by time interaction, which grazing was displayed more in SRG than CG during weeks 2 and 3, and then reversed weeks 4, 5 and 6.

Investigation of glucagon-like peptide-1 response to six oral carbohydrates in ponies

Glucagon-like peptide-1 (GLP-1), the principal incretin in horses, may play a role in the pathophysiology of insulin dysregulation (ID). This study aimed to describe its concentration in response to three preserved forages and four dynamic tests for ID in ponies. Twelve adult ponies of mixed ID status were given a meal of hay, soaked hay or haylage, an in-feed oral glucose test (OGT), oral sugar test (OST), an oral test using a proprietary breakfast cereal (WEET) or a combined glucose-insulin tolerance test (CGIT) weekly in a randomised cross-over study. Glucose, insulin and GLP-1 concentrations were measured before and following each intervention. Ponies were designated ID or non-ID and insulin resistant (IR) or non-IR according to OGT and CGIT results, respectively. All interventions apart from the CGIT provoked a GLP-1 response within 30 min. The OGT and WEET interventions, (containing the greatest dose of non-structural carbohydrate, 1.06 and 1 g/kg BW, respectively), resulted in a greater area under the curve (AUC) for GLP-1 compared to all other interventions (P < 0.001). No difference in GLP-1 response was detected according to ID or IR status, despite there being strong positive correlations (rs [95 % CI]) between GLP-1 and insulin concentrations measured at individual time points (0.67 [0.62 - 0.71]; P < 0.001) and as AUC (0.66 [0.49-0.79], P < 0.001). These data do not support of the use of GLP-1 as an adjunctive diagnostic test for ID or IR, as defined by conventional intravenous or oral dynamic tests.

Comparison of one novel and four established diagnostic tests for insulin dysregulation in ponies

Several tests have been advocated for diagnosis of insulin dysregulation (ID). Tests using simple sugars may not reflect the response to naturally ingested carbohydrates. This study aimed to evaluate agreement between the oral glucose test (OGT), the oral sugar test (OST), a novel oral test using a proprietary cereal (WEET), the IV combined glucose-insulin tolerance test (CGIT) and fasted basal insulin (FI) for diagnosis of ID. Each of the five tests above was performed on a group of six normal and six insulin dysregulated mixed-breed ponies in a randomised crossover study. Area under the curve (AUC) and maximum concentration of insulin from OGT, OST and WEET showed strong to very strong bivariate correlations (r = 0.85-0.94, and r = 0.87-0.92, respectively; P ≤ 0.001) and were significantly different between tests (associated with dose of carbohydrate) and between CGIT-positive and -negative ponies. Dichotomous results showed substantial agreement between OST and both WEET (κ = 0.65; P = 0.02) and OGT (κ = 0.67; P = 0.01) and between CGIT and both OST (κ = 0.63; P = 0.03) and OGT (κ = 0.67; P = 0.01), and no agreement between FI, which had low sensitivity, and all other tests (κ = 0.15 - 0.31; P > 0. 05). Palatability of WEET was variable, resulting in one pony being excluded for analysis of WEET data. Further work on development of an oral test using a more palatable feedstuff and appropriate cut-offs or diagnostic thresholds for tests of ID is warranted.

Development of a Web App to Convert Blood Insulin Concentrations among Various Immunoassays Used in Horses

The measurement of the blood insulin concentration, and comparison to cut-offs, is essential in diagnosing insulin dysregulation, a common equine endocrinopathy. However, different insulin assays provide disparate results. We aimed to ease comparison between assays by compiling original and published data into a web app to convert insulin measurements from one assay to another. Data were available for ADVIA Centaur insulin chemiluminescent immunoassay (CLIA), Beckman Coulter insulin radioimmunoassay (RIA), Immulite 1000 CLIA, Immulite 2000 CLIA, Immulite 2000 XPi CLIA, Mercodia equine insulin enzyme-linked immunosorbent assay (ELISA), and Millipore porcine insulin RIA. Linear models were fitted for 13 assay pairs using non-decreasing splines, and integrated into this app. Assay comparisons including data from several studies showed a lower performance. This indicates technical variation between laboratories, which has not been described before, but is relevant when diagnostic measurements and cut-offs are provided by different laboratories. Nevertheless, the models' overall high performance (median r2 = 0.94; range 0.57-1.00) supports their use to interpret results from diagnostic insulin measurements when the reference assay is unavailable, and to compare values obtained from different assays.

A review of cellular and molecular mechanisms in endocrinopathic, sepsis-related and supporting limb equine laminitis

Equine laminitis has both fascinated and frustrated veterinary researchers and clinicians for many years. The recognition that many ponies suffering from pasture-associated laminitis have an insulin-dysregulated phenotype (endocrinopathic laminitis, EL) and that prolonged insulin and glucose infusions can experimentally induce laminar pathology and functional failure are seminal discoveries in this field. Researchers have studied the molecular basis for disease pathogenesis in models of EL, sepsis-related laminitis and supporting limb laminitis and generated much data over the last 15 years. This review attempts to synthesise those data, drawing comparisons between models and naturally occurring laminitis. A hypothesis is proposed that the basal epithelial cell stress is a central event in each category of laminitis. Furthermore, in naturally occurring pasture-associated laminitis, pathways that predominate in each type of laminitis contribute to laminar lamellar pathology to varying extents. Based on the molecular mechanisms determined in experimental models, interactions between these pathways are identified.

Effects of Macleaya Cordata Extract on Performance, Nutrient Apparent Digestibilities, Milk Composition, and Plasma Metabolites of Dairy Goats

In this study, we aimed to investigate the effects of Macleaya cordata extract (MCE) supplementation on performance, nutrient apparent digestibilities, plasma metabolites, and milk quality in dairy goats. Twenty-four lactating Guanzhong dairy goats (n = 24) were randomly divided into two groups (each containing 12 goats) in a 52-day trial: the CON group was fed a basal diet; the MCE group was fed a basal diet supplemented with 400 mg/kg MCE. The results indicated that the 4% fat corrected milk yield (4% FCM); uncorrected milk yield; milk-fat concentration; content of C4:0, C18:0, and C18:1n9c fatty acids in milk; and apparent digestibility of neutral detergent fiber (NDF) and acid detergent fiber (ADF) in the MCE group were significantly higher (p < 0.05). Furthermore, the lactoferrin (LTF), alpha-lactalbumin (α-La), and beta-lactoglobulin (β-Lg) of the milk and feed conversion rate (FCR) of the goats were significantly greater (p < 0.01) in the MCE group than in the CON group. In contrast, the somatic cell count (SCC) (p < 0.01), content of C14:0 fatty acids (p < 0.01) of milk, and blood urea nitrogen (BUN) concentrations (p < 0.05) were significantly lower in the in the MCE goats. These results show that the feeding of MCE can increase the performance and apparent nutrient digestibility of fiber in dairy goats, improving the quality of goat milk.

Photoperiod-dependent changes in oxidative stress markers in the blood of Shetland pony mares and stallions involved in recreational horseback riding

The objective of the current study was to determine the photoperiod-induced variations and the impact of exercise on oxidative stress biomarkers [2-thiobarbituric acid reactive substances (TBARS), aldehydic (AD) and ketonic (KD) derivatives of oxidatively modified proteins (OMP), total antioxidant capacity (TAC), and activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx)] and biomarkers of metabolic alterations [glucose, urea, and uric acid and the activity of lactate dehydrogenase (LDH)] in the blood of Shetland pony mares and stallions involved in recreational horseback riding. Twenty-one healthy adult Shetland ponies (11 mares and 10 stallions) aged 6.5 ± 1.4 years old from the central Pomeranian region in Poland were used in this study. Blood samples were taken once per season for one year: spring (3 April), summer (5 July), autumn (1 October) and winter (25 January). A MANOVA analysis revealed that the photoperiod factor had a leading role in alterations of these biomarkers, while the exercise and sex of the ponies exerted a lower impact. The lipid peroxidation biomarkers, for example, the plasma TBARS level, indicated the maximum adjusted coefficient of determination R²_ad = 0.77. Before exercise (at rest), the plasma of the stallions and mares exhibited minimum values of TBARS levels in the spring and summer photoperiods and maximum levels in autumn and winter. A statistically significant reduction in the levels of both aldehydic and ketonic derivatives of OMP in the blood of ponies was observed during the autumn and winter periods; additionally, the level of ketonic derivatives of OMP declined after exercise in spring. TAC was statistically significant in the spring and winter photoperiods both before and after exercise. SOD activity did not have a pronounced photoperiod-induced pattern but was dependent on the sex and exercise. CAT activity varied and was statistically significant only in the plasma of the mares after exercise in the spring, summer, and winter photoperiods. The minimum GPx activity in the blood of the mares before exercise (at rest) was observed in autumn, while the maximum was noted in winter and summer. Photoperiod- and exercise-induced alterations in markers of oxidative stress and antioxidant defences may contribute to the adaptation of animals to exercise, depending on sex. The seasonal variations in the antioxidant defences demonstrated in our study, as well as substrates of energy metabolism in the blood of mares and stallions, depending on exercise capacity, could be an important aspect in the ability of endogenous adaptive mechanisms of animals to react in advance to environmental changes associated with seasons.

Effect of Dose and Fasting on Oral Sugar Test Responses in Insulin Dysregulated Horses

The oral sugar test (OST) is frequently used to identify insulin dysregulated (ID) equines. The effect of fasting and varying sugar dose for the OST has been investigated in the pony but little work has been done in the horse. This study aimed to investigate (1) an OST response with access to forage continued until the time of the OST or prevented for 3 hours prior to the OST and (2) responses of ID and non-insulin dysregulated (NID) horses to two different OST doses. Twenty-one mixed-breed horses (14.8 ± 3.2 years; 574.3 ± 83.3 kg) were used in two randomized crossover studies. Seven ID and seven NID horses were used in study A, and eight ID and eight NID in study B. Study A horses underwent an OST (0.15 mL/kg BW) either after a fast (FA) or directly off pasture (FE). Study B horses received either a low (LD; 0.15 mL/kg BW) or high dose (HD; 0.45 mL/kg BW) OST on one occasion each. Blood was collected at basal (T0), and post-60 minute (T60) for later determination of insulin (RIA). Data were analyzed via ANOVA with repeated measures. ID horses had significantly (P < 0.05) greater insulin responses than NID for all OSTs. There was no statistical difference between LD versus HD mean insulin concentrations (T0, T60, delta insulin) for either ID or NID horses. ID had higher T0 (P < 0.01) for FE compared to FA; however, FE and FA did not significantly affect T60 or delta insulins (DI) concentrations.

Nutritional Considerations When Dealing with an Obese Adult Equine

Equine obesity is common, reducing quality of life and requiring dietary energy restriction. Equine obesity is identified using subjective body condition scoring. Considerations are given for life stage and health status when managing obese equines. Every effort should be made to maximize feeding duration, and minimize time spent without feed while meeting all essential nutrient requirements. Limiting total daily dry matter intake to 2% of current bodyweight per day of a low caloric, forage-based diet may result in adequate body weight loss. Weight loss and weight management plans should be monitored for success and potential gastrointestinal, metabolic, and/or behavioral complications.

Diagnostic Testing for Equine Endocrine Diseases: Confirmation Versus Confusion

Despite there being only 2 common endocrine diseases in horses, pituitary pars intermedia dysfunction (PPID) and equine metabolic syndrome (EMS), diagnosis is still confusing. Failing to consider horse factors and treating based on laboratory results only have caused many animals to receive lifelong drug treatment unnecessarily. Increased plasma ACTH, baseline or TRH stimulated, supports a diagnosis of PPID; however, breed, age, thriftiness, illness, coat color, geography, diet, and season also affect ACTH concentration. Insulin dysregulation, the hallmark of EMS, can result from insulin resistance or excessive postprandial insulin release. Each requires a different diagnostic test to reach a diagnosis.

Incidence and risk factors for recurrence of endocrinopathic laminitis in horses

Background

Endocrinopathic laminitis is common in horses and ponies, but the recurrence rate of the disease is poorly defined.

Objectives

To determine the incidence of, and risk factors for, the recurrence of endocrinopathic laminitis.

Animals

Privately owned horses and ponies with acute laminitis (n = 317, of which 276 cases with endocrinopathic laminitis were followed up to study completion).

Methods

This prospective cohort study collected data on veterinary‐diagnosed cases of acute laminitis for 2 years. Each case was classified on acceptance to the study as endocrinopathic or non‐endocrinopathic using data collected in a questionnaire completed by the animal's veterinarian. Follow‐up data were collected at regular intervals to determine whether the laminitis recurred in the 2‐year period after diagnosis.

Results

The recurrence rate for endocrinopathic laminitis was 34.1%. The risk of recurrence during the 2‐year study period increased with basal, fasted serum insulin concentration (P ≤ .05), with the probability of recurrence increasing markedly as the insulin concentration increased beyond the normal range (0‐20 μIU/mL) to over the threshold for normal (up to approximately 45 μIU/mL). Being previously diagnosed with laminitis (before the study; P = .05) was also a risk factor for recurrent laminitis. Cases with a higher Obel grade of laminitis were likely (P = .05) to recur sooner.

Conclusions and clinical importance

Knowing that hyperinsulinemia and being previously diagnosed with laminitis are significant risk factors for recurrence will enable clinicians to proactively address these factors, thereby potentially reducing the risk of recurrence of laminitis.

Phenotypic, hormonal, and clinical characteristics of equine endocrinopathic laminitis

Background

Equine endocrinopathic laminitis is common and can be associated with an underlying endocrinopathy, such as equine metabolic syndrome (EMS), pituitary pars intermedia dysfunction (PPID), pasture consumption, or any combination of these factors.

Objectives

The aim of the study was to improve the risk assessment capabilities of clinicians, and to inform management strategies, for acute endocrinopathic laminitis by prospectively examining the phenotypic, hormonal, and clinical characteristics of the disease in a large cohort.

Animals

Privately owned horses and ponies (n = 301) of any age, sex, or breed diagnosed with laminitis by a veterinarian. A history of laminitis was acceptable.

Methods

This was a prospective cohort study. Veterinarians provided information on each case via an online questionnaire after informed consent from the animal's owner, and all data were de‐identified before analysis. Serum insulin and plasma adrenocorticotrophic hormone concentrations were measured in each case.

Results

Most cases were recruited in spring (109/301; 36.2%). Concurrent EMS and PPID resulted in higher basal insulin concentrations (49 [21.5‐141]; P < .02) than if an animal had a single underlying cause for their laminitis. The insulin concentration was negatively correlated (r² = −0.38; P < .001) with the animal's height, being higher in ponies (33[10‐14]; P < .001) than horses (9.5 [3‐25.7]) and was positively correlated (r² = 0.12; P = .05) with their grade (severity) of laminitis.

Conclusions and Clinical Importance

Horses and ponies with concurrent endocrinopathies have more marked hyperinsulinemia. Higher basal insulin concentrations were associated with more severe lameness.

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.

The effect of insulin on equine lamellar basal epithelial cells mediated by the insulin-like growth factor-1 receptor

Background

In horses and ponies, insulin dysregulation leading to hyperinsulinemia may be associated with increased risk of laminitis, and prolonged infusion of insulin can induce the condition. It is unclear whether insulin may have a direct or indirect effect on the lamellar tissues. Insulin is structurally related to insulin-like growth factor (IGF-1), and can bind the IGF-1 receptor, albeit at a lower affinity than IGF-1.

Methods

Immunohistochemistry was performed on formalin-fixed lamellar tissue sections from six normal horses, euthanised for non-research purposes, using an anti-IGF-1 receptor antibody. In further studies, lamellar epithelial cells were obtained by collagenase digestion from the hooves of 18 normal horses, also euthanised for non-research purposes, and incubated for 48 h in the presence of insulin (0–2,000 m IU/ml). The increase in cell numbers was determined using a cell proliferation assay, and compared to the effect of zero insulin using one-way ANOVA.

Results

Immunohistochemistry demonstrated IGF-1 receptors on lamellar epidermal epithelial cells. With cultured cells, insulin caused a concentration-dependent increase in cell proliferation compared to untreated cells (maximal effect 63.3 ± 12.8% more cells after 48 h with 1,000 m IU/ml insulin; P < 0.01). Co-incubation with a blocking antibody against the IGF-1 receptor significantly inhibited the proliferative effect of insulin (P < 0.01).

Discussion

These results demonstrate that IGF-1 receptors are present on lamellar epithelial cells. At high physiological concentrations, insulin may activate these cells, by a mechanism involving IGF-1 receptors, resulting in a proliferative effect. This mechanism could help to explain the link between hyperinsulinemia and laminitis.

Postprandial glucose, insulin, and glucagon-like peptide-1 responses of different equine breeds adapted to meals containing micronized maize

The enteroinsular axis is a complex system that includes the release of incretin hormones from the gut to promote the absorption and utilization of glucose after a meal. The insulinogenic effect of incretin hormones such as glucagon-like peptide-1 (GLP-1) remains poorly characterized in the horse. The aim of this study was to compare postprandial glucose, insulin, and GLP-1 responses of different equine breeds adapted to twice-daily meals containing micronized maize. Four Standardbred horses, 4 mixed-breed ponies, and 4 Andalusian cross horses in moderate BCS (5.5 ± 0.2 out of 9) were fed meals at 0800 and 1600 h each day. The meals contained micronized maize (mixed with soaked soybean hulls and lucerne chaff), with the amount of maize gradually increased over 12 wk to reach a final quantity of 1.7 g/kg BW (1.1 g/kg BW starch) in each meal. Animals had ad libitum access to the same hay throughout. After 12 wk of acclimation, serial blood samples were collected from all animals over a 14-h period to measure concentrations of glucose, insulin, and GLP-1, with meals fed immediately after the 0 and 8 h samples. Glucose area under the curve (AUC) values were similar between breed groups (P = 0.41); however, ponies and Andalusian horses exhibited significantly higher insulin AUC values after both meals compared with Standardbred horses (both P < 0.005). Postprandial GLP-1 AUC values were also significantly higher in ponies and Andalusian horses compared with Standardbred horses (breed × time interaction; P < 0.001). Correlation analysis demonstrated a strong positive association between concentrations of insulin and GLP-1 over time (rs = 0.752; P < 0.001). The increased insulin concentrations in ponies and Andalusian horses may partly reflect lower insulin sensitivity but could also be attributed to increased GLP-1 release. Given that hyperinsulinemia is a recognized risk factor for the development of laminitis in domestic equids, this study provides evidence that the enteroinsular axis warrants further investigation.

Equine hyperinsulinemia: investigation of the enteroinsular axis during insulin dysregulation

Compared with some other species, insulin dysregulation in equids is poorly understood. However, hyperinsulinemia causes laminitis, a significant and often lethal disease affecting the pedal bone/hoof wall attachment site. Until recently, hyperinsulinemia has been considered a counterregulatory response to insulin resistance (IR), but there is growing evidence to support a gastrointestinal etiology. Incretin hormones released from the proximal intestine, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide, augment insulin secretion in several species but require investigation in horses. This study investigated peripheral and gut-derived factors impacting insulin secretion by comparing the response to intravenous (iv) and oral d-glucose. Oral and iv tests were performed in 22 ponies previously shown to be insulin dysregulated, of which only 15 were classified as IR (iv test). In a more detailed study, nine different ponies received four treatments: d-glucose orally, d-glucose iv, oats, and commercial grain mix. Insulin, glucose, and incretin concentrations were measured before and after each treatment. All nine ponies showed similar iv responses, but five were markedly hyperresponsive to oral d-glucose and four were not. Insulin responsiveness to oral d-glucose was strongly associated with blood glucose concentrations and oral glucose bioavailability, presumably driven by glucose absorption/distribution, as there was no difference in glucose clearance rates. Insulin was also positively associated with the active amide of GLP-1 following d-glucose and grain. This study has confirmed a functional enteroinsular axis in ponies that likely contributes to insulin dysregulation that may predispose them to laminitis. Moreover, iv tests for IR are not reliable predictors of the oral response to dietary nonstructural carbohydrate.

Effect of increased adiposity on insulin sensitivity and adipokine concentrations in horses and ponies fed a high fat diet, with or without a once daily high glycaemic meal

REASONS FOR PERFORMING STUDY

The relative influences of obesity and adaptation to high glycaemic diets on the development of insulin dysregulation in equids are unclear.

OBJECTIVES

To determine whether increased adiposity per se is responsible for the decreased insulin sensitivity often observed in obese horses or whether a dietary glycaemic response is critically important.

STUDY DESIGN

Randomised controlled trial.

METHODS

Eighteen horses and ponies were studied over a 20-week period. They received ad libitum hay plus either a high fat (low glycaemic) diet (FAT; n = 6) or a similar (isocaloric) diet containing 1.5 g/kg bwt once daily glucose (GLU; n = 6) to induce obesity. A third group received a control ration (CON; n = 6). Adiposity was monitored using body condition score (BCS) and total body fat mass percentage (TBFM) determined using a deuterium oxide dilution technique. Insulin sensitivity was assessed using a frequently sampled i.v. glucose tolerance test. Plasma concentrations of glucose, insulin, leptin, adiponectin, tumour necrosis factor-α (TNF-α) and serum amyloid A (SAA) were measured.

RESULTS

The FAT and GLU groups became obese (BCS ≥7) whereas the CON group maintained moderate condition (BCS ≤6). Total body fat mass and leptin concentrations were increased in the FAT and GLU groups than in the CON group (P<0.001 and P = 0.003, respectively). Values for both insulin-dependent (SI) and insulin-independent (Sg) glucose disposal were higher in the GLU group compared with the FAT and CON groups (P = 0.006 and P = 0.03, respectively). There were no differences in adiponectin, TNF-α or SAA between groups (all P≥0.4).

CONCLUSIONS

Increased adiposity did not reduce insulin sensitivity in either the FAT or the GLU diet groups, suggesting that obesity per se might not be responsible for the lower SI values reported in previous studies. Contrary to expectations, once daily glucose appeared to increase insulin sensitivity. Further work is required into the dietary causes of insulin resistance in equids.

Assessing the seasonal prevalence and risk factors for nuchal crest adiposity in domestic horses and ponies using the Cresty Neck Score

BACKGROUND

Nuchal crest adiposity in horses and ponies has been associated with an enhanced risk of metabolic health problems. However, there is no current information on the prevalence of, and risk factors specific to, nuchal crest adiposity in horses and ponies. In addition, the cresty neck score has not previously been utilised across different seasons within a UK leisure population, it is not know whether nuchal crest adiposity shows the same seasonal trends as general obesity.

RESULTS

A Cresty Neck Score (CNS, 0-5) was given to 96 horses with access to pasture (>6 h per day) at the end of winter and at the end of summer in order to obtain two prevalence estimates. Risk factors were assessed using the single outcome cresty neck/no cresty neck in either season (binary), from owner questionnaires and analysed using a mixed effects logistic regression model (outcome variable CNS <3 or CNS ≥3/5). Agreement between winter and summer scores was assessed using weighted Kappa methods. Winter CNS values were significantly higher than summer CNS values (p = 0.002) indicating a systematic bias. The prevalence of a CNS ≥ 3/5 was 45.83% at the end of winter, falling to 33.33% at the end of summer and was higher in ponies (<14.2 hh) than horses (≥14.2 hh) in both seasons. This may reflect a real winter increase in regional fat deposition, or an increased difficulty in obtaining an accurate estimate of regional adiposity in winter months. Breed was the strongest risk factor for CNS ≥3/5 in both seasons, with native UK breeds appearing to be most at risk (p < 0.001). In a separate, small validation study, the CNS showed good inter-observer reliability.

CONCLUSIONS

The prevalence of a CNS ≥3/5 was higher at the end of winter than at the end of summer, which was the opposite pattern seasonal variation to that observed for general obesity. Further studies are required to investigate the potential influence of time of year upon CNS interpretation and studies utilising the CNS should consider potential seasonal variability in nuchal crest adiposity.

Short-term effect of ovariectomy on measures of insulin sensitivity and response to dexamethasone administration in horses

OBJECTIVE

To evaluate the effect of ovariectomy on insulin sensitivity in horses and determine whether the effects of suppression of the hypothalamo-pituitary-adrenal axis differ before and after ovariectomy.

ANIMALS

6 healthy mares.

PROCEDURES

The horses underwent an IV glucose tolerance test (IVGTT), an insulin sensitivity test, and a dexamethasone suppression test before and 5 weeks after ovariectomy. Body weight, serum cortisol and plasma ACTH concentrations, serum insulin-to-blood glucose concentration ratios, and changes in blood glucose concentration with time after injection of glucose or insulin were compared before and after ovariectomy.

RESULTS

The dexamethasone injection resulted in a decrease in serum cortisol concentration before and after ovariectomy. In all horses, baseline plasma ACTH concentrations were within the reference range before and after ovariectomy. For each mare, results of an IVGTT before and after ovariectomy were considered normal. No significant differences in basal blood glucose concentration or time to reach baseline glucose concentration after an IVGTT were observed. Basal serum insulin concentration and serum insulin-to-blood glucose concentration ratios were not significantly different before or after ovariectomy, nor was the mean time to attain a 50% decrease in blood glucose concentration after insulin injection.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that ovariectomy does not appear to modify dexamethasone response in horses and that it does not modify short-term measures of insulin sensitivity. Findings suggested that horses undergoing ovariectomy are not at higher risk of developing equine metabolic syndrome or hypothalamo-pituitary-adrenal axis dysfunction and associated morbidity.

Breed differences in insulin sensitivity and insulinemic responses to oral glucose in horses and ponies of moderate body condition score

Breed-related differences may occur in the innate insulin sensitivity (SI) of horses and ponies, an important factor believed to be associated with the risk of laminitis. The aim of this study was to measure the glucose and insulin responses of different breeds of horses and ponies in moderate body condition to a glucose-containing meal and to compare these responses with the indices of SI as determined by a frequently sampled intravenous glucose tolerance test (FSIGT). Eight Standardbred horses, 8 mixed-breed ponies, and 7 Andalusian-cross horses with a mean ± SEM BCS 5.0 ± 0.3 of 9 were used in this study. Each animal underwent an oral glucose tolerance test (OGTT) in which they were fed a fiber-based ration (2.0 g/kg BW) containing 1.5 g/kg BW added glucose, as well as a standard FSIGT with minimal model analysis. The glucose response variables from the OGTT were similar between groups; however, the peak insulin concentration was higher in ponies (94.1 ± 29.1 μIU/mL; P = 0.003) and Andalusians (85.3 ± 18.6; P = 0.004) than in Standardbreds (21.2 ± 3.5). The insulin area under the curve was also higher in ponies (13.5 ± 3.6 IU · min · L(-1); P = 0.009) and Andalusians (15.0 ± 2.7; P = 0.004) than in Standardbreds (3.1 ± 0.6). Insulin sensitivity, as determined by the FSIGT, was lower in Andalusians (0.99 ± 0.18 × 10(-4)/[mIU · min]) than in Standardbreds (5.43 ± 0.94; P < 0.001) and in ponies (2.12 ± 0.44; P = 0.003) than in Standardbreds. Peak insulin concentrations from the OGTT were negatively correlated with SI (P < 0.001; rs = -0.75). These results indicate that there are clear breed-related differences in the insulin responses of horses and ponies to oral and intravenous glucose. All animals were in moderate body condition, indicating that breed-related differences in insulin dynamics occurred independent of obesity.

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.

Insulin dysregulation

Abnormalities of insulin metabolism include hyperinsulinaemia and insulin resistance, and these problems are collectively referred to as insulin dysregulation in this review. Insulin dysregulation is a key component of equine metabolic syndrome: a collection of endocrine and metabolic abnormalities associated with the development of laminitis in horses, ponies and donkeys. Insulin dysregulation can also accompany prematurity and systemic illness in foals. Causes of insulin resistance are discussed, including pathological conditions of obesity, systemic inflammation and pituitary pars intermedia dysfunction, as well as the physiological responses to stress and pregnancy. Most of the discussion of insulin dysregulation to date has focused on insulin resistance, but there is increasing interest in hyperinsulinaemia itself and insulin responses to feeding. An oral sugar test or in-feed oral glucose tolerance test can be performed to assess insulin responses to dietary carbohydrates, and these tests are now recommended for use in clinical practice. Incretin hormones are likely to play an important role in postprandial hyperinsulinaemia and are the subject of current research. Insulin resistance exacerbates hyperinsulinaemia, and insulin sensitivity can be measured by performing a combined glucose-insulin test or i.v. insulin tolerance test. In both of these tests, exogenous insulin is administered and the rate of glucose uptake into tissues measured. Diagnosis and management of hyperinsulinaemia is recommended to reduce the risk of laminitis. The term insulin dysregulation is introduced here to refer collectively to excessive insulin responses to sugars, fasting hyperinsulinaemia and insulin resistance, which are all components of equine metabolic syndrome.

A review of recent advances and current hypotheses on the pathogenesis of acute laminitis

With the increasing number of studies being published on the different experimental models used to induce and study acute laminitis, the pathophysiological events associated with these various models (i.e. starch overload, oligofructose overload, black walnut extract and hyperinsulinaemia) can be compared more realistically. Within this review, the mechanisms for metabolic vs. inflammatory laminitis are discussed, and the question of how pasture laminitis may fit into any of the proposed mechanisms is addressed.

Metabolic responses of horses and ponies to high and low glycaemic feeds: implications for laminitis

Equine laminitis is the painful condition resulting from disruption of the laminar bonds within the foot. Understanding the dietary triggers may facilitate strategies to prevent laminitis in susceptible animals. The purpose of this review is to examine how dietary factors may lead to insulin resistance and/or excessive insulin production from the pancreas, and why certain breeds or types of horses are more predisposed to this form of laminitis than others. Understanding these relationships will be very important when considering appropriate feeds and the dietary countermeasures necessary for preventing this condition. It is important to note the breed type when considering the likely metabolic effects of dietary carbohydrate, because there are major differences between the Thoroughbred/Standardbred type and some other breeds of horses and ponies. Ponies and certain breeds of horses produce excessive amounts of insulin in response to dietary carbohydrates and this may lead to the development of the three main features of the equine metabolic syndrome, namely obesity, insulin resistance and laminitis. Relative glycaemic index or glycaemic load may be useful in predicting peak plasma insulin (with due consideration for breed type), but carbohydrates such as starch and fructans may have particularly marked effects on insulin sensitivity. Although it is normal for ponies and certain horse breeds to be relatively insulin resistant, it may be possible to reduce the likelihood of exacerbating insulin resistance in obese animals with careful dietary modification. This may help to some extent in reducing hyperinsulinaemia and thereby reducing the risk of laminitis.

Insulin response to feeding forage with varying crude protein and amino acid content in horses at rest and after exercise

This study assessed the insulin response to forage intake with varying crude protein (CP) content in horses at rest and after exercise. Six geldings were fed three grass haylage-only diets for 7 days according to a 3×3 Latin square design. On day 7, blood samples were collected before and for 120 min after feeding 15% of the daily allowance before exercise (feeding A) and after standardised exercise (feeding B). Feed samples were collected before each feeding. Dry matter (DM) and nutrient content varied (DM: 37-58%, water-soluble carbohydrates minus fructans (WSC-f): 3-12% of DM, CP: 10-15% of DM) which resulted in a variation in nutrient intake within haylage batches. Based on individual CP and WSC-f intakes, intake groups were therefore formed (low and high CP intake; ≤ and ≯180 g CP/100 kg body weight, respectively and low and high WSC-f intake; ≤ and ≯100 g/100 kg body weight, respectively). Amino acids were analysed and intakes were generally higher in the high CP group than in the low CP group. An ANOVA model including horse, CP group and WSC-f intake explained 95% of the variation in plasma insulin response compared to 87% using a model including horse and WSC-f group alone. The plasma insulin area under curve (AUC) following feeding A tended to be higher in the high CP group than in the low CP group (P=0.08), but there was no difference after feeding B. Plasma glucose AUC was not affected by CP group (P≯0.05). The study indicates that the post-prandial plasma insulin response in horses fed a forage-only diet is increased by high WSC-f intake but may also be increased by high CP intake, at least at rest. However, due to the low number of observations further studies are needed.