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Obesity and diet affect glucose dynamics and insulin sensitivity in Thoroughbred geldings R. M. Hoffman, R. C. Boston, D. Stefanovski, D. S. Kronfeld and P. A. Harris J ANIM SCI 2003, 81:2333-2342.

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Obesity and diet affect glucose dynamics and insulin sensitivity in Thoroughbred geldings1
R. M. Hoffman*2, R. C. Boston†, D. Stefanovski†, D. S. Kronfeld*, and P. A. Harris‡
*Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061-0306; †Department of ClinicalStudies, University of Pennsylvania, New Bolton Center, Kennett Square 19348; and ‡Equine Studies Group, Waltham Centre for Pet Nutrition, Melton-Mowbray, U.K.

ABSTRACT: Insulin resistance is considered a risk factor in obesity, laminitis, exertional rhabdomyolysis, and osteochondrosis. The objective was to use the minimal model to estimate glucose effectiveness (Sg) and insulin sensitivity (Si) innonobese to obese horses initially adapted to forage only, then adapted to forage plus supplements rich in starch and sugar (SS) or fiber and fat (FF). Ten Thoroughbred geldings, with BCS of 5 (nonobese), 6 (moderately obese), and 7 to 8 (obese), were adapted to pasture and hay, allocated to two groups, and fed SS or FF in a switch-back design with 8 wk of adaptation. Modified frequent-sampling i.v.glucose tolerance tests were applied after adaptation to forage, SS, and FF. For the tolerance tests, horses were kept in stalls overnight and provided hay, and venous catheters were placed the next morning. Baseline samples were collected, 0.3 g of glucose/kg of BW was given i.v., and blood was sampled at 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16, and 19 min. At 20 min, 30 mU of insulin/kg of BW wasgiven, followed by sampling at 22, 23, 24, 25, 27, 30, 35, 40, 50, 60, 70, 80, 90, 100, 120, 150, and 180 min. Plasma was analyzed for glucose and insulin, and Si, Sg, acute insulin response to glucose, and the

disposition index were calculated. Normality was tested using the Shapiro-Wilk statistic. Body condition effects were analyzed using a mixed model with repeated measures. Diet effectswere analyzed using a Wilcoxon signed rank test. The Sg was higher in obese than nonobese (P = 0.003) and moderately obese (P = 0.007) horses; Si was lower in obese than nonobese (P = 0.008) horses, and acute insulin response to glucose was higher in obese than nonobese (P = 0.039) horses. Effects of diet were likely confounded by body condition, but horses had lower Si (P = 0.066) when fed SScompared with FF, especially when nonobese. In conclusion, the minimal model effectively estimated Sg, Si, acute insulin response to glucose, and disposition index in horses. Obese geldings were insulin-resistant and seemed to rely primarily on Sg for glucose disposal. Feeding a diet rich in sugar and starch decreased insulin sensitivity of horses. Maintenance of body condition and avoidance ofgrain-based meals rich in sugar and starch would be beneficial to decrease the risk of developing insulin resistance and associated metabolic syndromes in horses, especially for horses at risk for these syndromes.

Key Words: Glucose, Glucose Tolerance, Horses, Insulin, Obesity
2003 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2003. 81:2333–2342

Introduction
Insulinresistance has been generally defined as a state in which normal concentrations of insulin fail to elicit a normal physiological response (Kahn, 1978). In-

We appreciate the support of the late P. Mellon, Upperville, VA, and the Waltham Centre for Pet Nutrition, Melton-Mowbray, U.K. The technical assistance and encouragement of the staff and students at the Virginia Tech Middleburg Agricultural...