Obesity and diet affect glucose dynamics and insulin sensitivity in Thoroughbred geldings

A genome scan for positive selection in thoroughbred horses

Thoroughbred horses have been selected for exceptional racing performance resulting in system-wide structural and functional adaptations contributing to elite athletic phenotypes. Because selection has been recent and intense in a closed population that stems from a small number of founder animals Thoroughbreds represent a unique population within which to identify genomic contributions to exercise-related traits. Employing a population genetics-based hitchhiking mapping approach we performed a genome scan using 394 autosomal and X chromosome microsatellite loci and identified positively selected loci in the extreme tail-ends of the empirical distributions for (1) deviations from expected heterozygosity (Ewens-Watterson test) in Thoroughbred (n = 112) and (2) global differentiation among four geographically diverse horse populations (F(ST)). We found positively selected genomic regions in Thoroughbred enriched for phosphoinositide-mediated signalling (3.2-fold enrichment; P<0.01), insulin receptor signalling (5.0-fold enrichment; P<0.01) and lipid transport (2.2-fold enrichment; P<0.05) genes. We found a significant overrepresentation of sarcoglycan complex (11.1-fold enrichment; P<0.05) and focal adhesion pathway (1.9-fold enrichment; P<0.01) genes highlighting the role for muscle strength and integrity in the Thoroughbred athletic phenotype. We report for the first time candidate athletic-performance genes within regions targeted by selection in Thoroughbred horses that are principally responsible for fatty acid oxidation, increased insulin sensitivity and muscle strength: ACSS1 (acyl-CoA synthetase short-chain family member 1), ACTA1 (actin, alpha 1, skeletal muscle), ACTN2 (actinin, alpha 2), ADHFE1 (alcohol dehydrogenase, iron containing, 1), MTFR1 (mitochondrial fission regulator 1), PDK4 (pyruvate dehydrogenase kinase, isozyme 4) and TNC (tenascin C). Understanding the genetic basis for exercise adaptation will be crucial for the identification of genes within the complex molecular networks underlying obesity and its consequential pathologies, such as type 2 diabetes. Therefore, we propose Thoroughbred as a novel in vivo large animal model for understanding molecular protection against metabolic disease.

Medical implications of obesity in horses--lessons for human obesity

There is growing recognition that obesity is common and represents a significant detriment to the health of companion animals in a manner similar to that by which it is affecting the human population. As is the case for other species, obesity appears to promote insulin resistance in horses and it is through this pathophysiological process that many of the adverse medical consequences of obesity are being characterized. Equine medical conditions that have been described in the context of obesity and insulin resistance differ from those in humans. Chronic human conditions that have been attributed to obesity and insulin resistance, such as atherosclerosis and diabetes mellitus, are rarely described in obese horses. Significant current interest is centered on the recognition that insulin resistance plays a role in the pathogenesis of laminitis, a potentially severe and debilitating cause of lameness in the equine species. Other equine medical conditions that are more likely in obese, insulin-resistant individuals include hyperlipemia (hepatic lipidosis) and developmental orthopedic disease (osteochondrosis). Pituitary pars intermedia dysfunction (equine Cushing's syndrome) represents another common endocrinopathic condition of older horses associated with insulin resistance. This review presents an introductory overview of the present understanding of obesity and insulin resistance and how these conditions may be associated with disease conditions in horses.

Matrix metalloproteinases in inflammatory pathologies of the horse

The extracellular matrix (ECM) of connective tissue is constantly being remodelled to allow for growth and regeneration. Normal tissue maintenance requires the ECM components to be degraded and re-synthesised in relatively equal proportions. This degradation is facilitated by matrix metalloproteinases (MMPs) and their proteolytic action is controlled primarily by the tissue inhibitors of metalloproteinases (TIMPs). Both MMPs and TIMPs exist in a state of dynamic equilibrium, with a slight excess of one or the other depending on the need for either ECM breakdown or synthesis. Long-term disruption to this balance between MMPs and TIMPs will have pathological consequences. Matrix metalloproteinases are involved in a number of diseases in mammals, including the horse. Excess MMP activity can cause ECM destruction, as seen in the lamellar basement membrane in laminitis and the articular cartilage in osteoarthritis. Matrix metalloproteinase under-activity can potentially impede healing by preventing fibrinolysis in fibrotic conditions and the removal of scar tissue in wounds. Matrix metalloproteinases also degrade non-ECM proteins and regulate cell behaviour via the release of growth factors from the substrates they cleave, increasing the scope of their effects. This review looks at the involvement of MMPs in equine health and pathologies, whilst exploring the potential consequences of therapeutic intervention.

Short-term effects of energy changes on plasma leptin concentrations and glucose tolerance in healthy ponies

To determine whether plasma leptin concentrations and glucose tolerance are affected by changes in energy balance, nine healthy Shetland ponies were fed at 140% followed by 75% of their maintenance requirements for 13 days in each of the two periods. Bodyweight was recorded every three days. Blood samples were taken every two days and analysed for leptin and cortisol. An oral glucose tolerance test was performed on day 7 of each period. Serial blood samples were analysed for glucose and insulin. Although bodyweight was not affected, plasma leptin concentrations increased (P<0.001) initially during overfeeding, but returned to previous values after 7 days. During underfeeding, plasma leptin concentrations decreased (P<0.001). Underfeeding was associated with a higher AUC for plasma glucose (P=0.02) and plasma insulin (P=0.05) resulting in a decreased glucose tolerance (AUC glucose/AUC insulin; P=0.008), probably due to a plasma cortisol increase caused by the reduced feed intake. It is concluded that changes in energy balance, without altering bodyweight, can influence plasma leptin concentrations in ponies.

The effect of metformin on measurements of insulin sensitivity and beta cell response in 18 horses and ponies with insulin resistance

REASONS FOR PERFORMING STUDY

Laminitis in equids is a very common debilitating disease, and insulin resistance (IR) and hyperinsulinaemia are increasingly recognised as important predisposing factors. Pharmacological modification of IR and hyperinsulinaemia might reduce the risk of laminitis.

HYPOTHESIS

Metformin, a drug commonly prescribed for treatment of human IR, may also decrease IR in equids.

METHODS

Eighteen horses and ponies with IR and recurrent laminitis were treated with 15 mg/kg bwt metformin per os q. 12 h. Each animal served as its own control by comparing pre- and post treatment proxies for IR, insulin sensitivity (IS) and pancreatic beta cell function while controlling for possible dietary and managemental influences on IR.

RESULTS

Evidence of significantly improved IS and decreased pancreatic beta cell secretion was found following metformin treatment. The magnitude of effect was greater at earlier resampling (6-14 days) than at later times (23-220 days). Apparent subjective clinical benefits were good but less favourable than effects on IR.

CONCLUSIONS

Metformin is safe and appears to increase IS in equids.

POTENTIAL RELEVANCE

Metformin may be indicated as a treatment for IR in equids. Further studies are required to define appropriate selection of subjects warranting therapy, dosing schedule and pharmacokinetics.

Dietary energy source affects glucose kinetics in trained Arabian geldings at rest and during endurance exercise

Advances in modeling and tracer techniques provide new perspective into glucose utilization and potential consequences to health or exercise performance. This study used stable isotope and compartmental modeling to evaluate how adaptation to a feed high in sugar and starch (SS) compared with a feed high in fat and fiber (FF) affects glucose kinetics at rest and during exercise in horses. Six trained Arabians adapted to each feed underwent similar tests at rest and while running approximately 4 m/s on a treadmill. For both tests, horses received 100 micromol/kg body weight [6,6-(2)H]glucose through a venous catheter. Circulating tracer glucose was described for 150 min by exponential decay curves and compartmental analysis. All parameters of glucose transfer increased with exercise (P < or = 0.004). Compared with FF horses, SS horses had higher circulating glucose (P = 0.022) and fractional glucose transfer rates (min(-1)) at rest (P = 0.055). Exercise increased glucose irreversible loss (mmol/min) more in SS horses (P = 0.037). Total glucose transfer during exercise tended to be greater in SS horses (0.027 +/- 0.002 mmol/min) compared with FF horses (0.023 +/- 0.002 mmol/min) (P = 0.109). This study characterized the effect of diet on glucose kinetics in resting and exercising horses using new modeling methods. Horses adapted to a fat-supplemented feed utilized less glucose during low-intensity exercise. Fat supplementation in horses may therefore promote greater flexibility in the selection of substrate to meet energy demands for optimal health and performance.

Effects of long-term oral administration of levothyroxine sodium on glucose dynamics in healthy adult horses

OBJECTIVE

To determine the effects of long-term oral administration of levothyroxine sodium (L-T(4)) on glucose dynamics in adult euthyroid horses.

ANIMALS

6 healthy adult mares.

PROCEDURES

Horses received L-T(4) (48 mg/d) orally for 48 weeks. Frequently sampled IV glucose tolerance test procedures were performed on 3 occasions (24-hour intervals) before and at 16, 32, and 48 weeks during the treatment period. Data were assessed via minimal model analysis. The repeatability of measurements was evaluated.

RESULTS

During treatment, body weight decreased significantly from the pretreatment value; mean +/- SD weight was 49 +/- 14 kg, 43 +/- 7 kg, and 25 +/- 18 kg less than the pretreatment value at weeks 16, 32, and 48, respectively. Compared with pretreatment findings, 1.8-, 2.4-, and 1.9-fold increases in mean insulin sensitivity (SI) were detected at weeks 16, 32, and 48, respectively; SI was negatively correlated with body weight (r = -0.42; P < 0.001). During treatment, glucose effectiveness increased and the acute insulin response to glucose decreased. Overall mean within-horse coefficients of variation were 5% and 29% for plasma glucose and serum insulin concentrations, respectively, and 33%, 26%, and 23% for SI, glucose effectiveness, and the acute insulin response to glucose, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Long-term administration of L-T(4) was associated with weight loss and increased SI in adult euthyroid horses, although other factors may have confounded results. Levothyroxine sodium may be useful for the treatment of obesity and insulin resistance in horses, but further studies are required.

Effects of an intravenous endotoxin challenge on glucose and insulin dynamics in horses

OBJECTIVE

To evaluate the effects of endotoxin administered IV on glucose and insulin dynamics in horses.

ANIMALS

16 healthy adult mares.

PROCEDURES

Each week of a 2-week randomized crossover study, each horse received an IV injection (duration, 30 minutes) of Escherichia coli O55:B5 lipopolysaccharide (LPS) in 60 mL of sterile saline (0.9% NaCl) solution (20 ng/kg) or sterile saline solution alone (control treatment). Frequently sampled IV glucose tolerance test procedures were performed at 24 hours before (baseline) and 24 and 48 hours after injection; glucose and insulin dynamics were assessed via minimal model analysis.

RESULTS

13 of 16 horses had a clinical response to LPS, which was characterized by mild colic and leukopenia. Before treatment, mean +/- SD insulin sensitivity was 2.9 +/- 1.9 x 10(4) L x min(1) x mU(1); this significantly decreased to 0.9 +/- 0.9 x 10(4) L x min(1) x mU(1) 24 hours after treatment (69% reduction) and was 1.5 +/- 0.9 x 10(4) L x min(1) x mU(1) 48 hours after treatment. At baseline, mean +/- SD acute insulin response to glucose was 520 +/- 196 mU x min x L(1); this significantly increased to 938 +/- 620 mU x min x L(1) (80% increase) and 755 +/- 400 mU x min x L(1) (45% increase) at 24 and 48 hours after LPS treatment, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Compared with baseline values, insulin sensitivity was decreased for 24 hours after IV injection of LPS, and affected horses had a compensatory pancreatic response. These disturbances in glucose and insulin dynamics may contribute to development of laminitis in horses.

Factors affecting clinical assessment of insulin sensitivity in horses

Insulin resistance is thought to be involved in the pathogenesis of many equine conditions such as pars intermedia dysfunction, equine metabolic syndrome, diabetes mellitus, hyperlipaemia, laminitis, endotoxaemia and osteochondrosis dissecans (OCD); whereas polysaccharide storage myopathy in Quarter Horses and equine motor neuron disease (EMD) have been associated with increased insulin sensitivity. However, it is clear that there is not one ideal test, in terms of both practicality and accuracy, for evaluating insulin sensitivity in horses and improved diagnostic techniques are required. This review sets out the background to the subject and identifies current knowledge regarding the measurement of insulin sensitivity by tolerance testing and clamping techniques. Factors affecting insulin sensitivity, such as breed, pregnancy, lactation, obesity and nutritional factors are discussed. In addition, the relationship with training, nutritional supplementation and drug administration are considered.

Effects of systemic inflammation on insulin sensitivity in horses and inflammatory cytokine expression in adipose tissue

Objective—To determine whether an inflammatory challenge induces insulin resistance in horses and examine possible contributions of adipose tissue to inflammatory cytokine production.

Animals—15 adult mares.

Procedures—Lipopolysaccharide (0.045 μg/kg, IV) or saline solution was administered, and insulin sensitivity was determined by means of the hyperinsulinemic, euglycemic clamp procedure or an adipose tissue biopsy was performed. Adipose tissue samples were collected, and mature adipocytes were obtained. Mature adipocytes were stimulated with lipopolysaccharide or dedifferentiated into preadipocytes and then stimulated with lipopolysaccharide. Interleukin-1, interleukin-6, and tumor necrosis factor A expression in blood, adipose tissue, and adipocytes was quantified with a real-time, reverse transcriptase– PCR assay.

Results—Lipopolysaccharide induced a transient increase in insulin sensitivity followed by a reduction in insulin sensitivity at 24 hours. Increased cytokine expression was observed in blood and adipose tissue following administration of lipopolysaccharide, and adipocytes and preadipocytes stimulated with lipopolysaccharide stained positive for tumor necrosis factor A. Expression of interleukin-1, interleukin-6, and tumor necrosis factor A was detected in preadipocytes stimulated with lipopolysaccharide, and interleukin-6 and tumor necrosis factor A were detected in mature adipocytes stimulated with lipopolysaccharide.

Conclusions and Clinical Relevance—Results indicated that insulin resistance develops following systemic inflammation in horses and suggested that adipose tissue may contribute to this inflammatory response. Methods to regulate insulin sensitivity may improve clinical outcome in critically ill patients.

Insulin-like growth factor I in growing thoroughbreds

The objective of this longitudinal study was to characterize growth and plasma insulin-like growth factor I (IGF-I) concentrations in pasture-raised thoroughbreds fed two sources of dietary energy. Mares and foals were randomly assigned to either a sugar and starch (SS) or fat and fibre (FF)-rich feed, and plasma IGF-I and growth were measured once a month from 1 to 16 months of age. These dependent variables were also compared with day length and ambient temperature. There was an association between plasma IGF-I concentration and average daily gain (ADG) (r = 0.32, p < 0.001). There were also clear seasonal patterns in both ADG and plasma IGF-I, with high values in June and May, and a low value in March. Plasma IGF-I and ADG were positively associated with day length and temperature. Plasma IGF-I was never higher (p > 0.10) in the FF group when compared with the SS group, and was higher in the SS group during a rapid growth phase in the spring of year 2 (p < 0.10). The results establish an association between ADG and IGF-I in the horse and indicate that environment and age may influence this relationship. In addition, plasma IGF-I is influenced by dietary energy source at particular times of year. This link has important implications in designing feeding management strategies that are aimed at addressing skeletal development.

The effects of diet on blood glucose, insulin, gastrin and the serum tryptophan: Large neutral amino acid ratio in foals

High carbohydrate diets can affect the health and behaviour of foals, but the mechanisms are not always fully understood. The objective of this study was to compare the effects of feeding a starch and sugar (SS), or a fat (oil) and fibre (FF) rich diet to two groups of eight foals. Diets were fed from 4 to 42 weeks of age, alongside ad libitum forage. Faecal pH levels did not differ significantly between groups and endoscopic examination showed that the gastric mucosa was healthy in both groups at 25 and 42 weeks of age. At 40 weeks of age, SS foals had significantly higher total blood glucose and lower total blood gastrin than FF foals during the 6h period following ingestion of their respective diets, but insulin levels did not differ significantly. The ratio between serum tryptophan and other large neutral amino acids showed a trend towards an interaction between diet and sampling time. The results provide preliminary information about the effects of diet on the physiology of young horses.

Effects of dexamethasone on glucose dynamics and insulin sensitivity in healthy horses

OBJECTIVE

To determine effects of dexamethasone on glucose dynamics and insulin sensitivity in healthy horses.

ANIMALS

6 adult Standardbreds.

PROCEDURES

In a balanced crossover study, horses received dexamethasone (0.08 mg/ kg, IV, q 48 h) or an equivalent volume of saline (0.9% NaCl) solution (control treatment) during a 21-day period. Horses underwent a 3-hour frequently sampled IV glucose tolerance test (FSIGT) 2 days after treatment. Minimal model analysis of glucose and insulin data from FSIGTs were used to estimate insulin sensitivity (Si), glucose effectiveness (Sg), acute insulin response to glucose (AIRg), and disposition index. Proxies for Si (reciprocal of the inverse square of basal insulin concentration [RISQI]) and beta-cell responsiveness (modified insulin-to-glucose ratio [MIRG]) were calculated from basal plasma glucose and serum insulin concentrations.

RESULTS

Mean serum insulin concentration was significantly higher in dexamethasone-treated horses than control horses on days 7, 14, and 21. Similarly, mean plasma glucose concentration was higher in dexamethasone-treated horses on days 7, 14, and 21; this value differed significantly on day 14 but not on days 7 or 21. Minimal model analysis of FSIGT data revealed a significant decrease in Si and a significant increase in AIRg after dexamethasone treatment, with no change in Sg or disposition index. Mean RISQI was significantly lower, whereas MIRG was higher, in dexamethasone-treated horses than control horses on days 7, 14, and 21.

CONCLUSIONS AND CLINICAL RELEVANCE

The study revealed marked insulin resistance in healthy horses after 21 days of dexamethasone administration. Because insulin resistance has been associated with a predisposition to laminitis, a glucocorticoid-induced decrease in insulin sensitivity may increase risk for development of laminitis in some horses and ponies.

Effects of dietary energy source and physical conditioning on insulin sensitivity and glucose tolerance in standardbred horses

REASONS FOR PERFORMING STUDY

There is evidence that adaptation to diets rich in nonstructural carbohydrates (NSC) contributes to the development of insulin resistance in horses. To date, however, no study in horses has examined the effects of physical conditioning on diet-associated alterations in insulin sensitivity and glucose tolerance.

OBJECTIVES

To examine the effects of adaptation to concentrate feeds rich in NSC or fat on insulin sensitivity and glucose tolerance in horses, both in the sedentary state and after a subsequent period of physical conditioning.

METHODS

Fourteen mature Standardbred horses underwent both a euglycaemic-hyperinsulinaemic clamp (EHC) and oral glucose tolerance test (OGTT) after each of the following phases: Baseline - fed only forage cubes for 3 weeks; Diet horses were randomly assigned to receive either a high NSC (S) concentrate or a high fat concentrate (F) with forage cubes for 6 weeks; and Diet x Exercise - horses remained on the assigned ration and underwent a 7 week period of physical conditioning. An incremental exercise test was performed before, and after, the Diet x Exercise phase for measurement of the peak rate of oxygen consumption (VO2peak).

RESULTS

In both diet groups, there was an approximately 10% increase in mean VO2peak after physical conditioning. The mean rate of glucose disposal (M) per unit of serum insulin (I) during the EHC [M/I ratio] in S horses was 30% lower (P<0.05) in the Diet phase when compared to Baseline, but not different from Baseline after physical conditioning. The S diet also resulted in a greater (P<0.05) OGTT insulin response (area under the insulin vs. time curve, AUC(INS)) in both Diet and Diet x Exercise phases when compared to Baseline. In F, insulin sensitivity (mean M/I ratio) and glucose tolerance were unchanged during the study.

CONCLUSIONS AND POTENTIAL RELEVANCE

Feeding a diet rich in NSC for 6 weeks resulted in decreased insulin sensitivity and impaired glucose tolerance. Physical conditioning lessened the effects of the high NSC diet on insulin sensitivity, as evidenced by the return to baseline M/I, but did not mitigate the impaired glucose tolerance. Decreased insulin sensitivity has been implicated in the development of obesity and laminitis in horses and the present findings provide support for avoidance of concentrates with high NSC in the dietary management of horses at risk for the development of these conditions.

Digestive and metabolic effects of altering feeding frequency in athletic horses

The aim of this study was to investigate the effect of feeding frequency on total tract digestibility and plasma glucose, insulin, urea, gastrin and cortisol concentrations at rest and following exercise in seven Standardbred horses in race training. The horses were fed every 12 h (twice a day, 2TD) and every 4 h (six times a day, 6TD) for 25 days, in a cross-over design. The diet (64% grass hay, 36% concentrates on weight basis) was fed at twice the maintenance energy requirement. Blood samples were taken every hour for 24 h on day 17, total collection of urine and faeces was made on days 19–21 and an intensive exercise test was performed on day 25. Altering feeding frequency caused small variations in diurnal plasma glucose, urea, gastrin and cortisol concentrations and did not affect total tract nutrient digestibility. There was an increase in the mean diurnal plasma urea concentration on the 2TD regime and low levels of plasma insulin were observed 7 h after feeding 2TD. The response to intensive exercise on heart rate, plasma lactate and plasma glucose was similar on both treatments but the plasma insulin concentration was higher following exercise in the 2TD regime, indicating that post-exercise glucose metabolism was altered. In conclusion, this study shows that feeding athletic horses only 2TD caused metabolic signs resembling those observed during feed deprivation (low levels of plasma insulin and an increased diurnal plasma urea concentration) and an altered post-exercise glucose metabolism, but did not affect the digestive response.

Relationship of insulin sensitivity to aerobic capacity in Standardbred mares and geldings

The objective of this study was to determine the relationship between insulin sensitivity and aerobic capacity and serum adipocytokine (leptin, adiponectin) concentrations in 14 mature, unconditioned Standardbred horses (eight mares, six geldings). Each horse underwent a euglycaemic–hyperinsulinaemic clamp (EHC) and a frequently sampled intravenous glucose tolerance test (FSIGT) for assessment of insulin sensitivity. Aerobic capacity was determined by measurement of the peak rate of oxygen uptake (V˙O2peak) during an incremental exercise test (IET). Serum leptin and adiponectin concentrations were measured in baseline samples obtained before tests of insulin sensitivity. Mean body weight, condition score, V˙O2peak and run time during the IET did not differ between the sex groups. However, minimal model analysis of the FSIGT showed that insulin sensitivity (SI, ×10−4 l mU−1 min−1) was higher (P = 0.002) in geldings (4.21±0.78) than in mares (2.43±0.95), while the acute insulin response to glucose (AIRg) and glucose utilization independent of insulin (SG) were significantly higher in mares. Similarly, glucose uptake (M) per unit of serum insulin (I) during the EHC (M/I ratio) tended (P = 0.08) to be higher in geldings than in mares (×10−2 mg kg−1 min−1 per μU ml−1: 2.41±0.64 vs. 1.80±0.51). There was no significant relationship between V˙O2peak and measures of insulin sensitivity. Stepwise multiple linear regression modelling determined that sex (65%) and leptin concentrations (13.7%) accounted for 78.7% of the variance in SI, while 46% of the variance in M/I could be attributed to sex. It was concluded that aerobic capacity is not an important determinant of insulin-mediated glucose disposal in mature, untrained Standardbred horses. Further studies are needed to examine the influence of gender on insulin sensitivity in horses.

A pilot study investigating the relationship between perceived and actual workload and estimated energy intake in riding centre horses

Feeding regimens for horses are usually based on perceived workload (PW) together with body condition. This can lead to inappropriate energy intake and, in particular, excess weight. Therefore, a more detailed understanding of the factors influencing individual energy needs under practical field conditions would be valuable. The purpose of this study was to evaluate to what extent the variations in actual versus PW may influence the estimated energy intake required to maintain body weight (BW) in individual animals. Eight mature, experienced riding school horses, chosen at random and maintaining a constant BW, were observed in standard flat work lessons with four different instructors at an equine training college. Heart rate (HR) and the structure of the lessons were recorded. Relative workload (RW) was determined for each horse by dividing its average HR per lesson by its estimated maximum HR (HRmax). PW was estimated by each instructor for each horse using a scale of 1–5. One-way ANOVA and t-test were used to determine the differences and Spearman's and Pearson's correlations were used to determine the correlations. The mean estimated RW was 39.1% of the estimated HRmax ( ± 5.7%) and was not significantly different between instructors (P>0.05). Rider weight as a percentage of BW showed a weak but statistically significant positive correlation with mean HR (r2 = 0.14, P < 0.05). Estimated energy intake was negatively correlated with BCS (r2 = 0.65, P < 0.001) and differed significantly (P < 0.05) between light (BW = 455–532 kg) horses (mean 0.18 ± 0.04 MJ kg− 1 BW) and heavy (BW = 622–660 kg) horses (mean 0.15 ± 0.03 MJ kg− 1 BW). No difference was found in estimated energy intake between different workloads (flat work only or flat work and jumping) (P>0.05). When BW was compared with estimated energy intake, no relation was found either (P>0.05). These results suggest that the amount of energy needed to maintain BW in individual horses cannot necessarily be estimated simply on the basis of the intensity and the duration of structured exercise. Other factors including age, individual digestive and metabolic efficiency, body condition and possibly the energy utilized through non-structured activity (e.g. box walking) may need to be taken into account.

Relationships among inflammatory cytokines, obesity, and insulin sensitivity in the horse

Recent studies associate obesity and insulin resistance in horses with development of abnormal reproductive function and debilitating laminitis. The factors contributing to insulin resistance in obese horses are unknown. However, human studies provide evidence that elevated inflammatory cytokines such as tumor necrosis factor alpha (TNFalpha), IL1, and IL6 play direct roles in development of obesity-associated insulin resistance. Thus, inflammation may be a key link between obesity and insulin resistance in horses. The aim of the current investigation was to examine possible relationships between obesity, inflammatory cytokines, and insulin sensitivity (IS) in the horse. Age was recorded and BCS and percent body fat (% FAT) were determined as measures of obesity in 60 mares. In addition, blood mRNA expression of IL1, IL6, and TNFalpha and circulating concentrations of TNFalpha protein (TNFp) were determined in each mare. Finally, fasted concentrations of insulin were determined, and IS was determined using the hyperinsulinemic, euglycemic clamp. Significant correlations between several variables provided evidence for the design of 4 population regression models to estimate relationships between measures of obesity, inflammatory factors, and IS in the sample population. The results of these analyses revealed that IS decreased as BCS and % FAT increased (P < 0.001) in the sample population. Additionally, increased IL1 (P < 0.05) and TNFp (P < 0.01) were associated with decreased IS. However, increased TNFalpha (P < 0.001) was associated with decreased IS only in mares 20 yr of age and older. Increased BCS and % FAT were associated with increased expression of TNFalpha (P = 0.053) and IL1 (P < 0.05), and increased TNFp (P < 0.05). Surprisingly, increased BCS and % FAT were associated with decreased IL6 expression (P = 0.05) in mares <20 yr of age. Finally, evaluation of the influence of obesity and inflammatory cytokines on IS within the same model suggested that BCS and % FAT (P < 0.001) with TNFalpha [mRNA (P = 0.07) and protein (P < 0.05)] are inversely associated with IS independently of one another. Combined, these results provide the first evidence associating obesity with increased inflammatory factors in the horse. Furthermore, the results suggest that an interrelationship exists among obesity, inflammatory cytokines, and IS in the horse and emphasize the need for further studies to elucidate the nature of these relationships.

Assessment and mathematical modeling of glucose turnover and insulin sensitivity in lean and obese cats

Insulin sensitivity (SI) of glucose disposal can be quantified with the euglycemic hyperinsulinemic clamp (EHC) with tracer glucose infusion. True steady state is, however, difficult to achieve, and non-steady state analysis of EHC data is preferred. This analysis requires information on glucose kinetics that can be obtained from bolus injection of cold and tracer glucose. The aim of this study was to assess glucose kinetics in cats. Mathematical modeling and non-steady state analysis was applied to assess effects of obesity on glucose turnover, glycolysis/glycogen synthesis, SI, and inhibition of endogenous glucose production (EGP) in lean cats (L) and obese cats (O). D-[3-(3)H]-glucose kinetics and 3H-H2O production were analyzed in 4 L and 4 O with three-compartment modeling. Frequently sampled insulin-modified intravenous glucose tolerance tests (FSIGT) with minimal model analysis were performed in 5L and 3 O to assess glucose kinetics and SI. EHC was performed in 10 L and 10 O with primed-constant infusion of 3H-glucose. Data were analyzed with a modified minimal model segregating suppression of EGP by insulin using a non-linear mixed-effects population approach. FSIGT provided estimates of SI, glucose effectiveness SG, and distribution volume. EHC provided estimates of SI, SG, glycolysis, and suprabasal insulin concentration for 50% EGP inhibition. Obesity appears to affect glucose distribution but not utilization at basal insulin, and reduces SI estimated by FSIGT and EHC. Differences in SI between FSIGT and EHC depend on different descriptions of EGP inhibition by insulin. Finally, glucose disposal at basal insulin appears to occur entirely through glycolysis, whereas significant amounts of glucose are sequestrated from oxidation during EHC.

Endocrine responses in mares undergoing abrupt changes in nutritional management

Leptin, a protein hormone secreted by adipocytes, plays an important role in energy homeostasis and regulation of body composition. We previously observed that acute feed restriction resulted in a rapid decline in concentrations of leptin in obese pony mares. This acute response prompted us to characterize the temporal changes in concentrations of leptin, GH, and insulin in obese pony mares during the transition between fed and feed-restricted conditions. Nine obese pony mares of mixed breed, previously maintained on fescue pasture, were randomly allotted to 2 groups. Treatments consisted of a 48-h feed restriction, a 48-h refeeding, and a 24-h feed restriction (RFR; n = 4), or 48 h of alfalfa hay ad libitum, a 48-h feed restriction, and a 24-h refeeding (FRF; n = 5). Blood samples were taken every 15 min during restriction and feeding transitions (0600 to 1400 on d 2 and 4), and every 30 min thereafter until 0830 of the following days (d 3 and 5). In the FRF treatment, plasma concentrations of leptin declined precipitously 6 h after the removal of feed (sample by treatment interaction; P < 0.01), and remained low and unresponsive to refeeding. Similarly, in the RFR group, plasma concentrations of leptin were initially low, and did not respond to feeding during the second (refeeding) sampling period. After feed restriction in each of the 2 treatment sequences, plasma insulin decreased and GH mean concentration, pulse frequency, pulse amplitude, and area under the curve increased (P < 0.05). Refeeding reversed these effects on insulin and GH. These data provide evidence that peripheral concentrations of insulin and GH are dynamically responsive to feed removal (decrease in insulin; increase in GH) and replacement (increase in insulin; decrease in GH), whereas leptin decreases in response to feed restriction but is slow to recover from a transient nutritional insult.

Reproduction in high body condition mares with high versus low leptin concentrations

Previous results from our laboratory indicated that a majority of mares with high body condition scores (BCS) displayed estrous cycles or had considerable follicular activity during the winter. Among these high BCS mares, about 35% of them exhibited a persistent hyperleptinemia and hyperinsulinemia. The current experiment was designed to compare the reproductive characteristics of high BCS mares with hyperleptinemia to those with normal (low) plasma concentrations of leptin during the winter and the first estrous cycle (or the first full cycle encountered for those already cycling). Light horse mares with high BCS (6-8.5) were assigned to groups based on leptin concentrations (8/group): low (<5 ng/mL) and high (>10 ng/mL). Beginning 7 January, mares were assessed every 3d for follicular activity and then daily once a follicle >25 mm was detected. Mares were subsequently monitored through their first and second ovulations. Leptin concentrations remained higher (P<0.001) in mares in the high leptin group over the duration of the experiment. Also, high leptin mares had greater (P<0.0001) insulin response to glucose infusion and a faster (P<0.05) rate of glucose clearance. One mare with high leptin and three mares with low leptin had progesterone concentrations indicative of the presence of a corpus luteum at the onset of the experiment. Plasma concentrations of LH, FSH, and progesterone did not differ between groups (P>0.1) during the first estrous cycle occurring after 7 January. Date of first ovulation after 7 January and interovulatory interval were similar (P>0.1) for the two groups, as were estimates of follicular numbers on the ovaries (small, medium, and large; P>0.1). It is concluded that the perturbations in leptin and insulin secretion observed in some high BCS mares are not associated with alterations in ovarian activity or the estrous cycle during winter and into the period of vernal transition.

Changes of blood biochemical values in ponies recovering from hyperlipemia in Japan

Hyperlipemia in horses is a disorder of lipid metabolism peculiar to ponies. This study reports changes of blood biochemical values from the acute to the postconvalescent phases in 3 Shetland ponies with hyperlipemia in Japan. Diseased ponies (all 7 to 9 years old, in late pregnancy, and obese) were fed in the same farm. The periods of their hospitalizations ranged from 30 to 45 days. Twelve well-conditioned ponies (3 to 13 years old) around parturition were used to establish baseline values for blood test results. Main clinical findings in the affected ponies were depression, dysphagia, anorexia, ventral edema and milky-appearing plasma. Hypertriglyceridemia (40- to 70-fold rise of controls) was found in the acute phase of the disease in the affected ponies, and was derived from increased very-low density lipoproteins. Aspartate transaminase and gamma-glutamyl transpeptidase activities, blood urea nitrogen, and creatinin concentrations were increased in acute ponies compared to controls, suggesting impairment of liver and kidney functions. However, these values gradually recovered until the end of postconvalescent phase. Hyperinsulinemia was observed in the acute phase of the hyperlipemia of all affected ponies. And an exaggerated insulin response to intravenous glucose was observed in the 2 ponies given intravenous glucose tolerance tests. These findings suggest decreased insulin sensitivity in hyperlipemic ponies.

Countermeasures for pasture-associated laminitis in ponies and horses

Laminitis occurs throughout the world in horses and ponies and has major welfare implications. It is obviously important to be able to recognize and treat the condition in its early stages so that pain and suffering are kept to a minimum. However, ideally it would be preferred to be able to recommend certain interventions/countermeasures that avoid or prevent the condition from occurring in the first place. Because pasture-associated laminitis occurs with grass consumption, one obvious way to avoid the condition is to prevent access to pasture and to feed forage alternatives that are known to be low in rapidly fermentable material. For the majority of horses, total restriction is not always a viable or desired option for financial, welfare, and health reasons. It also may not be necessary for those animals that are not predisposed to laminitis. This review discusses the possible countermeasures that could be considered now and in the future in the following 7 key areas: 1) Identifying animals predisposed to the condition; 2) Limiting development of insulin resistance; 3) Avoiding high intakes of rapidly fermentable material; 4) Preventing/reducing the formation and absorption of the various "triggering factors"; 5) Reducing/preventing oxidative damage; 6) Preventing/reducing matrix metalloproteinase activity; and 7) Preventing changes in blood flow. It is unfortunate that little or no hard data exist at present on effective countermeasures, only mechanistic evidence for avoiding risk factors. However, there is much to gain, and research in this area is urgently required.

Pasture nonstructural carbohydrates and equine laminitis

Fresh forages constitute a majority of the diet for many horses and ponies that graze on pastures during the growing season in many parts of the world. Grasses generally predominate in such pastures, with varying proportions of legumes. Nonstructural carbohydrates (NSC) (simple sugars, starch, and fructan) can induce laminitis experimentally, and NSC can accumulate to >400 g/kg of dry matter (DM) in pasture grasses. In this article we discuss the environmental factors affecting NSC accumulation in pastures and estimate the potential daily intakes of pasture NSC by grazing horses. We also discuss strategies for both reducing the NSC content of pastures and management practices that can help reduce intakes of pasture NSC by equines at risk of developing laminitis. This study reveals the importance of accurate forage analysis in the development of feeding regimens for equines at risk of laminitis.

Physical characteristics, blood hormone concentrations, and plasma lipid concentrations in obese horses with insulin resistance

OBJECTIVE

To compare obese horses with insulin resistance (IR) with nonobese horses and determine whether blood resting glucose, insulin, leptin, and lipid concentrations differed between groups and were correlated with combined glucose-insulin test (CGIT) results.

ANIMALS

7 obese adult horses with IR (OB-IR group) and 5 nonobese mares.

PROCEDURES

Physical measurements were taken, and blood samples were collected after horses had acclimated to the hospital for 3 days. Response to insulin was assessed by use of the CGIT, and maintenance of plasma glucose concentrations greater than the preinjection value for > or = 45 minutes was used to define IR. Area under the curve values for glucose (AUC(g)) and insulin (AUC(i)) concentrations were calculated.

RESULTS

Morgan, Paso Fino, Quarter Horse, and Tennessee Walking Horse breeds were represented in the OB-IR group. Mean neck circumference and BCS differed significantly between groups and were positively correlated with AUC values. Resting insulin and leptin concentrations were 6 and 14 times as high, respectively, in the OB-IR group, compared with the nonobese group, and were significantly correlated with AUC(g) and AUC(i). Plasma nonesterified fatty acid, very low-density lipoprotein, and high-density lipoprotein-cholesterol (HDL-C) concentrations were significantly higher (86%, 104%, and 29%, respectively) in OB-IR horses, and HDL-C concentrations were positively correlated with AUC values.

CONCLUSIONS AND CLINICAL RELEVANCE

Measurements of neck circumference and resting insulin and leptin concentrations can be used to screen obese horses for IR. Dyslipidemia is associated with IR in obese horses.

Evaluation of genetic and metabolic predispositions and nutritional risk factors for pasture-associated laminitis in ponies

OBJECTIVE

To evaluate genetic and metabolic predispositions and nutritional risk factors for development of pasture-associated laminitis in ponies.

DESIGN

Observational cohort study.

ANIMALS

160 ponies.

PROCEDURES

A previous diagnosis of laminitis was used to differentiate 54 ponies (PL group) from 106 nonlaminitic ponies (NL group). Pedigree analysis was used to determine a mode of inheritance for ponies with a previous diagnosis of laminitis. In early March, ponies were weighed and scored for body condition and basal venous blood samples were obtained. Plasma was analyzed for glucose, insulin, triglycerides, nonesterified fatty acids, and cortisol concentrations. Basal proxies for insulin sensitivity (reciprocal of the square root of insulin [RISQI]) and insulin secretory response (modified insulin-to-glucose ratio [MIRG]) were calculated. Observations were repeated in May, when some ponies had signs of clinical laminitis.

RESULTS

A previous diagnosis of laminitis was consistent with the expected inheritance of a dominant major gene or genes with reduced penetrance. A prelaminitic metabolic profile was defined on the basis of body condition, plasma triglyceride concentration, RISQI, and MIRG. Meeting > or = 3 of these criteria differentiated PL- from NL-group ponies with a total predictive power of 78%. Determination of prelaminitic metabolic syndrome in March predicted 11 of 13 cases of clinical laminitis observed in May when pasture starch concentration was high.

CONCLUSIONS AND CLINICAL RELEVANCE

Prelaminitic metabolic syndrome in apparently healthy ponies is comparable to metabolic syndromes in humans and is the first such set of risk factors to be supported by data in equids. Prelaminitic metabolic syndrome identifies ponies requiring special management, such as avoiding high starch intake that exacerbates insulin resistance.

Use of proxies and reference quintiles obtained from minimal model analysis for determination of insulin sensitivity and pancreatic beta-cell responsiveness in horses

OBJECTIVE

To develop proxies calculated from basal plasma glucose and insulin concentrations that predict insulin sensitivity (SI; L.min(-1) x mU(-1)) and beta-cell responsiveness (ie, acute insulin response to glucose [AIRg]; mU/L x min(-1)) and to determine reference quintiles for these and minimal model variables.

ANIMALS

1 laminitic pony and 46 healthy horses.

PROCEDURE

Basal plasma glucose (mg/dL) and insulin (mU/L) concentrations were determined from blood samples obtained between 8:00 AM and 9:00 AM. Minimal model results for 46 horses were compared by equivalence testing with proxies for screening SI and pancreatic beta-cell responsiveness in humans and with 2 new proxies for screening in horses (ie, reciprocal of the square root of insulin [RISQI] and modified insulin-to-glucose ratio [MIRG]).

RESULTS

Best predictors of SI and AIRg were RISQI (r = 0.77) and MIRG (r = 0.75) as follows: SI = 7.93(RISQI) - 1.03 and AIRg = 70.1(MIRG) - 13.8, where RISQI equals plasma insulin concentration(-0.5) and MIRG equals [800 - 0.30(plasma insulin concentration 50)(2)]/(plasma glucose concentration - 30). Total predictive powers were 78% and 80% for RISQI and MIRG, respectively. Reference ranges and quintiles for a population of healthy horses were calculated nonparametrically.

CONCLUSIONS AND CLINICAL RELEVANCE

Proxies for screening SI and pancreatic beta-cell responsiveness in horses from this study compared favorably with proxies used effectively for humans. Combined use of RISQI and MIRG will enable differentiation between compensated and uncompensated insulin resistance. The sample size of our study allowed for determination of sound reference range values and quintiles for healthy horses.

Effects of rice bran oil on plasma lipid concentrations, lipoprotein composition, and glucose dynamics in mares1

Plasma lipid concentrations, lipoprotein composition, and glucose dynamics were measured and compared between mares fed diets containing added water, corn oil (CO), refined rice bran oil (RR), or crude rice bran oil (CR) to test the hypothesis that rice bran oil lowers plasma lipid concentrations, alters lipoprotein composition, and improves insulin sensitivity in mares. Eight healthy adult mares received a basal diet fed at 1.5 times the DE requirement for maintenance and each of the four treatments according to a repeated 4 x 4 Latin square design consisting of four 5-wk feeding periods. Blood samples were collected for lipid analysis after mares were deprived of feed overnight at 0 and 5 wk. Glucose dynamics were assessed at 0 and 4 wk in fed mares by combined intravenous glucose-insulin tolerance tests. Plasma glucose and insulin concentrations were measured, and estimated values of insulin sensitivity (SI), glucose effectiveness, and net insulin response were obtained using the minimal model. Mean BW increased (P = 0.014) by 29 kg (range = 10 to 50 kg) over 5 wk. Mean plasma concentrations of NEFA, triglyceride (TG), and very low-density lipoprotein (VLDL) decreased (P < 0.001) by 55, 30, and 39%, respectively, and plasma high-density lipoprotein and total cholesterol (TC) concentrations increased (P < 0.001) by 15 and 12%, respectively, over 5 wk. Changes in plasma NEFA (r = 0.58; P < 0.001) and TC (r = 0.44; P = 0.013) concentrations were positively correlated with weight gain over 5 wk. Lipid components of VLDL decreased (P < 0.001) in abundance over 5 wk, whereas the relative protein content of VLDL increased by 39% (P < 0.001). Addition of oil to the basal diet instead of water lowered plasma NEFA and TG concentrations further (P = 0.002 and 0.020, respectively) and increased plasma TC concentrations by a greater magnitude (P = 0.072). However, only plasma TG concentrations and VLDL free cholesterol content were affected (P = 0.024 and 0.009, respectively) by the type of oil added to the diet. Mean plasma TG concentration decreased by 14.2 mg/dL over 5 wk in the CR group, which was a larger (P < 0.05) decrease than the one (-5.3 mg/dL) detected in mares that received water. Consumption of experimental diets lowered S(I), but glucose dynamics were not affected by oil supplementation. Addition of oil to the diet altered blood lipid concentrations, and supplementation with CR instead of water specifically affected plasma TG concentrations and VLDL free cholesterol content.

Insulin resistance and compensation in Thoroughbred weanlings adapted to high-glycemic meals1

Insulin resistance has been suggested to increase the risk of certain diseases, including osteochondrosis and laminitis. Our objective was to evaluate the effect of adaptation to high-glycemic meals on glucose-insulin regulation in healthy Thoroughbred weanlings. Twelve Thoroughbred foals were raised on pasture and supplemented twice daily with a feed high in either sugar and starch (SS; 49% nonstructural carbohydrates, 21% NDF, 3% crude fat on a DM basis) or fat and fiber (FF; 12% nonstructural carbohydrates, 44% NDF, 10% crude fat on a DM basis). As weanlings (age 199 +/- 5 d; BW 274 +/- 5 kg) the subjects underwent a modified frequently sampled i.v. glucose tolerance test. A series of 39 blood samples was collected from -60 to 360 min, with a glucose bolus of 300 mg/kg BW injected at 0 min and an insulin bolus of 1.5 mIU/kg BW at 20 min. All samples were analyzed for glucose and insulin, and basal samples also were analyzed for plasma cortisol, triglyceride, and IGF-I. The minimal model of glucose and insulin dynamics was used to determine insulin sensitivity (SI), glucose effectiveness, acute insulin response to glucose (AIRg), and disposition index (DI). Insulin sensitivity was 37% less (P = 0.007) in weanlings fed SS than in those fed FF; however, DI did not differ (P = 0.65) between diets because AIRg tended to be negatively correlated with SI (r = -0.55; P = 0.067). This finding indicates that the SI decrease was compensated by AIRg in the weanlings adapted to SS. This compensation was further demonstrated by greater insulin concentrations in SS-adapted weanlings compared with FF-adapted weanlings at 11 of 36 sample points (P < 0.055) and greater (P = 0.040) total area under the insulin curve in SS than in FF weanlings. Plasma cortisol and triglycerides did not differ between dietary groups, but IGF-I was greater (P = 0.001) in SS weanlings. Despite appearing healthy, horses adapted to high-glycemic feeds may exhibit changes in altered insulin sensitivity and compensation that increase the risk of diseases involving insulin resistance. These changes seem to be partially amenable to dietary management.

Diurnal variation of ghrelin, leptin, and adiponectin in Standardbred mares

Twelve Standardbred mares underwent blood sampling for 24 h to test the hypothesis that there is diurnal variation of humoral mediators of peripheral energy balance including active ghrelin, adiponectin, leptin, glucose, insulin, and cortisol. The experiment was conducted under acclimated conditions. Grass hay and pelleted grain were provided at 0730 and 1530. Plasma concentrations of active ghrelin and leptin concentrations both peaked (47.3 +/- 6.5 pg/ mL and 5.9 +/- 1.1 ng/mL, respectively; P < 0.05) at 1550, 20 min after feeding. Active ghrelin decreased (P < 0.05) to 28.9 +/- 4.5 pg/mL overnight. The nadir of leptin (4.6 +/- 0.9 ng/mL) occurred at 0650. Neither hormone showed variation (P > 0.05) after the morning feeding. Plasma glucose and insulin concentrations increased (P < 0.05) in response to feeding; however, the morning responses (glucose = 96.9 +/- 2.6 mg/dL; insulin = 40.6 +/- 7.3 uIU/mL) were greater (P < 0.05) than the afternoon responses (glucose = 89.9 +/- 1.8 mg/dL; insulin = 23.2 +/- 4.3 uIU/mL at 180 and 60 min after feeding, respectively). Cortisol concentrations increased (P < 0.05) during the morning hours, but did not respond to feeding, whereas adiponectin concentrations remained stable throughout the study. Hence, active ghrelin and leptin may be entrained to meal feeding in horses, whereas adiponectin seems unaffected. We concluded that there seems to be a diurnal variation in glucose and insulin response to a meal in horses. Furthermore, elevated glucose and insulin concentrations resulting from the morning feeding may be responsible for the increase in leptin concentration in the afternoon.

Physiologic assessment of blood glucose homeostasis via combined intravenous glucose and insulin testing in horses

OBJECTIVE

To characterize the physiologic response to i.v. bolus injection of glucose and insulin for development of a combined glucose-insulin test (CGIT) in horses.

ANIMALS

6 healthy mares and 1 mare each with pituitary adenoma and urolithiasis.

PROCEDURE

Horses were given a CGIT (glucose, 150 mg/kg; insulin, 0.1 U/kg); results were compared with a singular i.v. glucose tolerance test (GTT; 150 mg/kg) and a singular i.v. insulin sensitivity test (IST; 0.1 U/kg). Healthy horses were also given a CGIT after receiving xylazine and undergoing stress.

RESULTS

Physiologically, the CGIT resulted in a 2-phase curve with positive (hyperglycemic) and negative (hypoglycemic) portions; the positive phase came first (250% of baseline at 1 minute). The descending segment declined linearly to baseline by approximately 30 minutes and to a nadir at 58% of baseline by 75 minutes. After a 35-minute valley, a linear ascent to baseline began. Addition of insulin in the CGIT increased glucose utilization by approximately 4.5 times during the positive phase but not during the negative phase. The diseases' effects and experimental inhibition of insulin secretion with xylazine and stress were detectable by use of the 2 phases of the CGIT. Only a single positive phase resulted from the GTT and a single negative phase from the IST CONCLUSIONS AND CLINICAL RELEVANCE: The CGIT resulted in a consistent, well-defined glycemia profile, which can be disrupted experimentally or by a disease process. The CGIT has clinical potential because it provides integrated information and more information than either the singular GTT or IST.

Effects of oral administration of levothyroxine sodium on concentrations of plasma lipids, concentration and composition of very-low-density lipoproteins, and glucose dynamics in healthy adult mares

OBJECTIVE

To evaluate glucose and lipid metabolism in healthy adult horses administered levothyroxine sodium (L-T4).

ANIMALS

12 healthy adult mares.

PROCEDURE

8 horses received an incrementally increasing dosage of L-T4 (24, 48, 72, or 96 mg of L-T4/d) for weeks 1 to 8. Each dose was provide between 7 AM and 8 AM in the morning grain meal for 2 weeks. Four additional horses remained untreated. Serum concentrations of nonesterified fatty acids, triglyceride (TG), total cholesterol (TC), and very-low-density lipoprotein (VLDL) were measured and composition of VLDL examined in samples obtained between 8 AM and 9 AM at weeks 0, 2, 4, 6, and 8. Glucose dynamics were assessed by use of a combined IV glucose-insulin tolerance test (IVGITT) conducted before and at the end of the 8-week treatment period. Data for each combined IVGITT were interpreted by use of the minimal model.

RESULTS

Plasma TG, TC, and VLDL concentrations significantly decreased over time in treated horses. At the completion of the 8-week treatment period, mean plasma VLDL concentration was 46% of the mean value for week 0 in treated horses. Insulin sensitivity significantly increased (> 2-fold) in treated horses, but glucose effectiveness and net insulin response were not affected. Levothyroxine sodium significantly increased the rate of insulin disposal.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of L-T4 decreases blood lipid concentrations, improves insulin sensitivity, and increases insulin disposal in horses. Levothyroxine sodium may have potential as a treatment for horses with reduced insulin sensitivity.

Influence of starch intake on growth and skeletal development of weanling horses1,2

Forty-four weanling horses were used in two experiments to evaluate the effect of starch intake on growth and skeletal development. In Exp. 1, the weanlings were fed either a grain-based, high-starch (31.1%, DM basis) concentrate or a by-product-based, low-starch (0.0%) concentrate with coastal bermudagrass (Cynodon dactylon) hay. Corn oil was used to equalize the energy concentration of the concentrates. The concentrate:hay ratio was 64:36 (as-fed basis), and intake was the same for both diets. Body weight gains were greater by the weanlings consuming the high-starch concentrate (0.81 vs. 0.67 kg/d; P = 0.01). Total body length gain also was greater for the weanlings consuming the high-starch concentrate (15.5 vs. 13.2 cm; P = 0.045). Other body measurements and bone mineral deposition were not influenced by diet or gender. At the end of the experiment, postprandial blood glucose concentrations suggested that the horses on the low-starch diet were less efficient in metabolizing blood glucose than were those that had been consuming the high-starch diets. In Exp. 2, the weanlings were fed either a high-starch (34.7%) or medium-starch (17.0%) concentrate plus coastal bermudagrass hay. Corn oil again was used to equalize the energy content of the medium-starch concentrate to that of the high-starch concentrate. The concentrate:hay ratio was 64:36 (as-fed basis), and the intake was the same for both diets. The diets did not influence rate of gain (0.75 kg/d; P = 0.98), body measurements (P = 0.11 to 0.93), or bone mineral deposition (P = 0.66). Animals on the medium-starch diet tended to have blood glucose concentrations that peaked earlier and were lower at later times than those consuming the high-starch concentrate. Bone osteochondrotic lesions were not related to the diet and were found to decrease during the course of the experiment for both the high-starch and the medium-starch diets (P = 0.006 and 0.016, respectively).