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Reproduction, Fertility and Development Reproduction, Fertility and Development Society
Vertebrate reproductive science and technology
RESEARCH ARTICLE

Obesity is associated with altered metabolic and reproductive activity in the mare: effects of metformin on insulin sensitivity and reproductive cyclicity*

M. M. Vick A , D. R. Sessions A , B. A. Murphy A , E. L. Kennedy A , S. E. Reedy A and B. P. Fitzgerald A B
+ Author Affiliations
- Author Affiliations

A Department of Veterinary Science, Maxwell Gluck Equine Research Center, University of Kentucky, Lexington, KY 40546, USA.

B Corresponding author. Email: bfitz@uky.edu

Reproduction, Fertility and Development 18(6) 609-617 https://doi.org/10.1071/RD06016
Submitted: 28 February 2006  Accepted: 9 April 2006   Published: 9 June 2006

Abstract

In mares, obesity is associated with continuous reproductive activity during the non-breeding season. To investigate the effect of obesity and associated alterations in metabolic parameters on the oestrous cycle, two related studies were conducted. In Experiment 1, obese (body condition score > 7) mares were fed ad libitum or were moderately feed restricted during the late summer and autumn months. Feed restriction did not alter the proportion of mares entering seasonal anoestrus. However, obese mares exhibited a significantly longer duration of the oestrous cycle, significant increases in circulating concentrations of leptin and insulin, and decreased insulin sensitivity and concentrations of thyroxine compared with feed-restricted mares throughout the experiment. Experiment 2 was designed to investigate the effects of administration of the insulin-sensitising drug metformin hydrochloride on insulin sensitivity and the characteristics of the oestrous cycle in obese mares. In a dose–response trial, metformin increased insulin sensitivity after 30 days following administration of 3 g day–1, but not 6 or 9 g day–1, compared with controls receiving vehicle only. However, there were no differences in insulin sensitivity or oestrous cycle characteristics between control and metformin-treated groups when the 3 g day–1 dose was tested for a longer period of 2 months. These results demonstrate that obesity is associated with aberrations in the oestrous cycle and perturbations in several markers of metabolic status. The results also indicate that metformin is not an effective long-term monotherapy for increasing insulin sensitivity in horses at the doses tested. Additional studies are needed to examine possible effects of increasing insulin sensitivity on reproductive activity in obese mares.

Extra keywords: horse, insulin resistance, reproduction.


Acknowledgments

The research reported in this article (No. 04-14-043) is published in connection with a project of the Kentucky Agricultural Experiment Station and is supported by funding from the Kentucky Equine Research Foundation. The authors acknowledge the contribution of Brenda White from the Veterinary Science Department at the University of Kentucky (Lexington, KY, USA) to this body of work and would like to thank the farm crew of the University of Kentucky Main Chance farm who provided invaluable assistance during data collection. The authors also acknowledge Scott McClintock from the Statistics Department at the University of Kentucky for his assistance with statistical analysis.


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* M. M. Vick and B. P. Fitzgerald contributed equally to the body of work presented herein.