How Season Impacts Horses' Insulin Secretion, Metabolism
When it comes to pregnancy and metabolism, management can sometimes be a real balancing act. Austrian and German researchers have learned that season’s effect on horses just adds to the complications.

Horses, regardless of sex or pregnancy status, release more insulin in response to glucose in winter compared to summer. And pregnant mares become more insulin resistant from about eight months’ gestation onward—often right at the height of winter. In mares that are already at metabolic risk for laminitis, this can create significant management challenges, said Christine Aurich, DVM, PhD, head of the Graf Lehndorff Institute in Neustadt, Germany.

“Insulin resistance during pregnancy is normal and helps to direct nutrients toward the fetus,” Aurich said. “Via the endocrine mechanism of partial insulin resistance in pregnant mares, usually enough energy is provided for fetal growth. But in winter, the insulin release to glucose increases, directing more glucose to the mother and her insulin-sensitive tissues (in particular, the laminae).”

Essentially, during the last two-thirds of gestation, most of the cells in pregnant mares’ bodies respond less to insulin when they consume sugars than they normally would, making them temporarily insulin-resistant, she said. While that would cause metabolic issues in a nonpregnant horse, it’s normal—and useful—in pregnancy because the incoming nutrients and related hormones just “skip over” the mare and go directly to her growing fetus, Aurich said.

Where that could become a problem, though, is in the mare that already has metabolic issues before pregnancy. “In mares that already suffer from metabolic syndrome, the situation may exacerbate during the later stages of pregnancy,” she said. “These mares can develop laminitis, and the whole situation does endanger the pregnancy. On the one hand, intake of grain should be reduced in favor of hay, but on the other hand, energy requirements of the growing fetus must be met.”

But as their recent study indicates, horses generally become more sensitive to glucose in winter, Aurich said. This is the body’s natural way of making the most of whatever food is available.

In their study, Aurich and colleagues analyzed glucose and insulin responses to an oral glucose test in 12 pregnant mares at progressive stages of pregnancy and in 10 geldings. They found that the mares showed increasing levels of elevated blood glucose levels after glucose consumption as the pregnancy progressed. In geldings these levels did not increase over time, meaning pregnant mares developed partial insulin resistance because of pregnancy, regardless of the season.

But they also found that there was an increase in the insulin response to glucose and the amount of insulin released from June to December in both geldings and mares, she said. And this suggests horses manage glucose differently depending on the season.

“Mares of robust breeds kept outside in winter may then not be able to meet the combined challenges of pregnancy plus limited food supply,” said Aurich. “This can result in abortion or preterm birth of immature foals. In mares with evident metabolic syndrome before pregnancy, it is better to consider not breeding them.”

In some cases, however, breeders insist on breeding these mares for personal or professional reasons. “If that’s the case, provide as much energy as possible via roughage, consult with a farrier or vet on preventive shoeing, exercise the mares lightly, and ensure that signs of developing laminitis are treated rapidly,” she said.

The study, “Effects of sex, pregnancy and season on insulin secretion and carbohydrate metabolism in horses,” was published in Animal Reproduction Science.