The role of Insulin Resistance
Insulin resistance is a naturally occurring trait in many British native horses and ‘good doers’ and would historically have put these breeds at an evolutionary advantage.
In the summer when food is readily available, naturally thrifty horses and ponies become fat, leading to some degree of insulin resistance. This results in high levels of circulating insulin- telling the body to store energy as fat NOT break it down. This allows the horses to maintain fat deposits ready for the winter when food becomes scarce and they need to live off their reserves.
In the winter these horses gradually lose weight and their insulin and glucose metabolism return to normal.
Insulin is a hormone produced by the pancreas which helps to regulate glucose levels in the body. In a normal horse, insulin released from the pancreas causes glucose to be taken up from the bloodstream into cells where it is used for energy.
In a horse with Insulin Resistance the pancreas still releases insulin, but the cells of the body do not respond to it as well, and a high proportion of glucose remains in the bloodstream rather than being taken up into cells.
Since the insulin has not had the desired response, more of it is produced, and this results in high levels of insulin in the blood stream. This is termed hyperinsulinaemia, and if found on a blood test is considered to be an indicator of Insulin Resistance.
Following several key studies conducted by researchers in Australia1 it is well established that abnormally high insulin concentrations are harmful and will directly trigger laminitis in horses. Although there are several theories as to why hyperinsulinaemia may cause laminitis in horses, the exact mechanism has not yet been confirmed.
Insulin Resistance can therefore be thought of as a factor that lowers the threshold for laminitis (by causing the horse to have high baseline levels of insulin in the blood), whereas the disease itself is triggered by changes in the diet (i.e. intake of starches and sugars which increase the insulin levels even further and trigger laminitis).
- De Laat et al (2010) Equine Veterinary Journal 42 (2) 129-135