L. Perry, J.D. Pagan, and L. Wood
Kentucky Equine Research, Versailles, Kentucky 40383, USA
Introduction
Insulin resistance is an associated risk factor in laminitis, metabolic syndrome, and equine Cushing's disease (Andrews and Frank, 2009), especially among aged horses. Management strategies to reduce the incidence and severity of these diseases include exercise, weight loss, and dietary changes. Diets high in nonstructural carbohydrates have been suggested to negatively affect insulin sensitivity in horses (Hoffman et al., 2003; Treiber et al., 2005), while it is unclear how high-fat diets affect insulin sensitivity. Therefore, the objective of this study was to examine whether short-term adaptation to a high-fat diet would affect insulin sensitivity in aged horses.
Materials and Methods
Three Thoroughbred geldings (18-24 yr; BCS 6.5-7; weight 591.0 ± 51.0 kg) were used in a three-period longitudinal study with each period lasting 35 days. During the first and third periods, the horses were fed 9.09 ± 1.90 kg/d of mixed grass/legume hay and 2.5 kg of an unfortified sweet feed (CHO 1 and 2) (Table 1). During the second period, the horses received the same amount of mixed grass/legume hay and 1.5 kg of a grass/legume hay cube and 600 ml of soybean oil (FAT) (Table 1). The CHO and FAT diets were formulated to be isocaloric. In the FAT ration, 21% of daily DE was supplied from fat and 10% of DE came from nonstructural carbohydrates (NSC). In the CHO ration, 9% of daily DE was from fat and 26% of DE was from NSC.
Table 1 Nutrient composition of experimental diets and hay.
| Nutrient % (DM basis) | CHO | FAT | Grass/Legume Hay |
| DM | 88.30 | 93.50 | 89.50 |
| CP | 10.75 | 15.60 | 22.45 |
| ADF | 5.20 | 19.10 | 37.45 |
| NDF | 13.90 | 32.90 | 51.00 |
| Fat | 5.40 | 25.0 | 1.20 |
| Starch | 55.20 | 1.10 | 0.60 |
| WSC | 6.70 | 5.00 | 6.00 |
Fasting blood samples were taken on d 0, 7, 14, 21, 28 and 35 of each period. An oral (OGTT) and intravenous (IVGTT) glucose tolerance test was conducted each period on d 28 and d 35, respectively. For the OGTT, a 50% dextrose solution was administered at a rate of 1 g glucose/kg BW via nasogastric tube. Blood samples were taken via jugular catheter immediately before (0 min) and at 30, 60, 90, 120, 180, 240, 300, and 360 min post administration. For the IVGTT, a 50% dextrose solution was administered intravenously at a rate of 0.5 g glucose/kg BW over 10 min and blood samples were collected immediately before (0 min) and at 5, 15, 30, 60, 90, 120, 180, 240, 300, and 360 min post administration. Weekly fasting blood samples were analyzed for glucose, insulin, and triglycerides. Samples taken during the OGTT and IVGTT were analyzed for insulin, glucose, and nonesterified fatty acids (NEFA).
Results and Discussion
During the OGTT, blood glucose was higher in the FAT group at 120 min (P ≤ 0.05) compared to CHO1 and was higher at 120 min (P ≤ 0.01) and 180 min compared to CHO2 (Figure1). Insulin during the OGTT was not different (Figure 2). During the IVGTT, the area under the curve (AUC) for glucose concentration vs. time was higher for the FAT group compared to both CHO1 (P ≤ 0.01) and CHO2 (P ≤ 0.001). Glucose during the IVGTT was higher (P ≤ 0.05) in the FAT group at 5, 30, 90, 120, 180, and 240 min compared to CHO1 and at 5, 15, 30, 60, 90, 120, 180, and 240 min compared to CHO2 (Figure3).
The AUC for insulin during the IVGTT was not different between treatments, but insulin in the FAT group was lower (P ≤ 0.05) at 5, 15, 30, 60, 90, and 120 min compared to CHO1 and at 5, 60, 90, and 120 min compared to CHO2 (Figure 4). The results of this study suggest that feeding a high-fat diet to aged horses reduces insulin sensitivity compared to a moderately high-carbohydrate diet. During the IVGTT, horses on the high-fat diet produced less insulin and took longer to clear glucose from their blood. Further research is needed to determine if these differences were due to high fat in the ration or a lack of readily digestible carbohydrate.
Figure 1. Glucose response during an oral glucose tolerance test (OGTT).
Figure 2. Insulin response during an oral glucose tolerance test (OGTT).
Figure 3. Glucose response during an intravenous glucose tolerance test (IGTT).
Figure 4. Insulin response during an intravenous glucose tolerance test (IGTT).
References
Andrews, F.M., and N. Frank. 2009. Pathology of metabolic-related conditions. In: J.D. Pagan (Ed.) Advances in Equine Nutrition IV. p. 277-292. Nottingham University Press, Nottingham, UK.
Hoffman, R.M., R.C. Boston, D. Stefanovski, D.S. Kronfeld, and P.A. Harris. 2003. Obesity and diet affect glucose dynamics and insulin sensitivity in Thoroughbred geldings. J. Anim. Sci. 81(9):2333-2342.
Treiber, K.H., R.C. Boston, D.S. Kronfeld, W.B. Staniar, and P.A. Harris. 2005. Insulin resistance and compensation in Thoroughbred weanlings adapted to high-glycemic meals.J. Anim. Sci. 83(10):2357-2364.
