Altered blood test results in winter

Many horse owners are familiar with the phenomenon: horses put on fat more quickly in winter. However, this “winter fat” is no coincidence, but a natural survival mechanism from the wild. A recent large-scale study now shows that this effect is also clearly reflected in horses’ metabolic and blood values.

Winter alters insulin metabolism

The study demonstrates that horses and donkeys show significantly higher basal insulin levels in winter than in other seasons. They also respond differently, depending on the season, to the oral sugar test (OST). The researchers suspect that this is due to evolutionary adaptations of the metabolism to environmental conditions.

Risk of ID and EMS misdiagnosis

This is particularly critical when diagnosing insulin dysregulation (ID) and Equine Metabolic Syndrome (EMS). If blood tests are carried out in winter, the elevated values can lead to misinterpretation – even if the horse is healthy.

What vets and horse owners should pay attention to

To obtain reliable results, the researchers (Lopes et al., 2025) recommend that the following factors must be taken into account when evaluating blood values:

Season (especially winter)
Sex
Breed

Only in this way can seasonal differences be correctly classified and unnecessary concerns or incorrect treatments avoided.

Conclusion

In winter, the horse’s metabolism demonstrably works differently. Anyone having blood tests carried out to clarify metabolic disorders such as ID or EMS should definitely take the season into account, as well as breed and sex. This keeps diagnostics reliable – and your horse optimally cared for.