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Effects of Obesity on Insulin and Glucose Metabolism in Cyclic Heifers^1,2,

Cornell University⁴ Ithaca, NY 14853

Abstract

Effects of degree of obesity on basal concentrations of insulin, glucose, thyroxine (T₄), triiodothyronine (T₃), estradiol-17ß (E) and progesterone (P) were measured in serum from 50 estrous and 73 diestrous Holstein heifers and the insulin response to glucose infusion was assessed in diestrous obese (n = 7) and lean (n = 7) heifers. Basal concentrations of glucose, T₄, T₃, E and P were not correlated with degree of obesity, although concentrations of glucose, T₄ and T₃ were higher (P<.05) at estrus than diestrus. Basal concentrations of insulin at estrus and diestrus were positively correlated (r = .6; P<.001) with degree of obesity but this relationship was different (P<.001) between estrus and diestrus. Furthermore, there was interaction (P<.001) between body condition and stage of the estrous cycle only for basal concentrations (mean ± SE) of insulin, with the difference in insulin levels (U/ml) between 12 obese and 12 lean heifers at diestrus (11.7 ± 1.3 vs 6.7 ± .6; P<.05) increasing during estrus (21.9 ± 2.4 vs 10.8 ± 1.3; P<.001). Insulin response to glucose infusion was greater in obese than in lean heifers, whether determined as actual concentration (P<.01) or as insulin response areas (P<.05) above base-line concentrations. Obese heifers were less responsive to insulin since hyperinsulinemia and euglycemia coexisted, and because glucose fractional removal rates were similar in both groups after glucose infusion in spite of greater concentrations of insulin in obese heifers. Thus, obesity in heifers was associated with insulin resistance, basal hyperinsulinemia and greater glucose-induced secretion of insulin.

¹ The authors are grateful to Dr. J. R. Stouffer for determining depth of subcutaneous fat by ultrasound and Or. R. D. Smith for assisting with body condition scoring of the heifers.

² Part of this work was presented in 1982 as an invited paper at the 74th Annu. Meet. of the Amer. Soc. of Anim. Sci. at the Univ. of Guelph, Canada.

³ Address correspondence to Dr. Joseph P. McCann, Dept. of Physiol., New York State College of Vet. Med.

⁴ Diagnostic Endocrinol. Lab. New York State College of Vet. Med.