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Hormonal Responses to High and Low Planes of Nutrition in Weanling Thoroughbreds^1,2,

University of Maryland^,3, College Park 20742 and Cornell University^,4, Ithaca, NY 14853

Abstract

Growth-related skeletal diseases in young horses have been associated with high planes of nutrition, although the mechanisms underlying such an association have not been determined. It is likely that nutrition-induced effects on growth rate or growth quality involve the endocrine system. Hormonal and metabolic responses to the ingestion of meals containing either 80% (diet A) or 160% (diet B) of National Research Council energy and protein recommendations were examined in eight Thorough-bred weanling horses after 3 wk of dietary adaptation. After 24 h fasts, prefeeding serum concentrations of thyroxine (T₄) and insulin and plasma concentrations of glucose, total protein, total triglycerides, creatinine, Ca and P were similar regardless of which diet the horses had been consuming. Serum cortisol concentrations were higher (P<.05) in horses when fasted from diet B. Thyroxine, insulin and glucose concentrations increased during ingestion of both meals. During the first hour after ingestion of diet A, T₄ concentrations increased to about 150% of fasting concentrations, whereas after diet B, T₄ concentrations decreased to about 85% of fasting concentrations. Concentrations of T₄ were not different from fasting concentrations 4 h after the ingestion of both diets. Glucose concentrations increased during the first hour postprandially, from about 105 mg/dl to about 165 mg/dl. However, glucose decreased more rapidly after ingestion of diet B. Serum insulin concentrations increased more rapidly after ingestion of diet B. Cortisol concentrations decreased after both meals, and the concentrations of the other metabolites measured were not affected by diet level or by meal consumption. These findings suggest that a more rapid pancreatic insulin secretory response was associated with the ingestion of larger amounts of energy and protein by young horses, and that a subsequent transient decrease in T₄ concentration may be implicated in the development of growth-related skeletal disease.

¹ Scientific Article No. 3535, Contribution No. 6610 of the Maryland Agr. Exp. Sta. This research was supported in part by Biomedical Research Support Grant No. RR-07042 to the Univ. of Maryland, from the Division of Research Resources, National Institutes of Health, Public Health Service.

² Appreciation is expressed to J. Soares for allowing access to analytical equipment, E. Russek and L. Douglass for assistance with the experimental design, to Ms. Margaret Kempf for manuscript preparation and to Ms. Barbara Wood for preparation of the figure.

³ Dept. of Anim. Sci.

⁴ Diagnostic Endocrinol. Lab.