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Gestational Diabetes in Mares: What's the Skinny?By Kentucky Equine Research Staff · December 26, 2017

A small handful of studies suggest that pregnant mares develop signs similar to gestational diabetes mellitus (GDM) in women. In these mares, blood glucose levels are higher than expected, potentially resulting in negative health effects of mares and their foals.

“In a normal, healthy pregnancy, mares and women both experience an increase in blood sugar because glucose is the main energy source of the fetus. Tissues in a pregnant body, therefore, become transiently intolerant to the effects of insulin, the hormone that usually stimulates cells to take up glucose from the blood, to ensure the fetus receives adequate glucose,” explained Kathleen Crandell, Ph.D., an equine nutritionist with Kentucky Equine Research (KER).

The difference between the normal increase in blood glucose and the accompanying, transient insulin resistance is magnitude. In pregnant women, the glucose levels and insulin resistance are excessive and persist even after parturition. In some cases, GDM recurs in subsequent pregnancies and can even result in the permanent development of type 2 diabetes mellitus in the nongestational state. Risk factors for GDM in humans include excess body weight and advancing age at the time of pregnancy.

Given the recent surge in information regarding insulin resistance in horses, but the lack of knowledge surrounding gestational diabetes in mares, researchers identified and recruited pregnant mares that had evidence of insulin intolerance in a previous pregnancy to better understand glucose and insulin dynamics.*

Mares were divided into two groups: those that had a high body condition score (BCS) in their previous pregnancy and those with a low BCS in their previous pregnancy. Both groups were managed and fed similarly throughout the study to maintain a BCS of 7-9 on the Henneke scale. On day 325 of gestation, all mares underwent an oral glucose tolerance test. Key findings included:

  • At the start of the test, basal glucose and insulin levels were similar between the two groups of mares;
  • Circulating levels of glucose increased in the first 90 minutes after oral administration of glucose, as expected;
  • After 90 minutes, circulating glucose levels decreased in the previously high-BCS mares but continued to increase in the previously low-BCS mares, peaking at 150 minutes; and
  • Insulin increased swiftly in the previously high-BCS mares, peaking 15 minutes after oral glucose administration. In contrast, insulin increased far slower in the previously low-BCS mares, peaking 210 minutes after oral glucose administration.

These data show that mares with a low BCS and evidence of insulin resistance in a previous pregnancy also had impaired glucose tolerance and abnormal insulin responses late in a subsequent gestation, similar to GDM in women.

“More information on this condition in mares is needed to identify appropriate management strategies, including diet plans, to minimize foal morbidity in the perinatal period,” concluded Crandell.

*Anderson, S.T., X. Song, A.J. Cawdell-Smith, et al. 2014. Pregnant mares with insulin resistance and beta cell failure continue to exhibit glucose intolerance and diminished insulin responses in a subsequent pregnancy. In: Proc. Australasian Equine Science Symposium, Vol. 5. Gold Coast, Queensland. p. 36.