Insulin resistance in horses on grass probed in Finnish study

The fattening associated with grazing horses on cultivated high-yielding pasture induced only moderate changes in glucose, insulin and fatty acids as measured in a glucose tolerance test, a Finnish study has shown.

The researchers said the moderate changes recorded did not compromise the ability of the horses to control plasma glucose concentrations or adipose tissue lipolysis – the breakdown of lipids into glycerol and free fatty acids.

“The body condition score and weight gain during the grazing season were not associated with increased insulin resistance,” Kari Elo and his colleagues reported in the peer-reviewed open-access journal, PLOS ONE.

They said their findings supported the hypothesis that obesity and severe hyperinsulinemia – too much insulin in the blood – are required to produce change in the insulin resistance status of horses.

The team used a group of 16 Finnhorse mares, a native cold-blooded breed, which was split between cultivated high-yielding pasture and semi-natural grassland. They assessed the animals in terms of their body-condition score, fat changes around the neck and tailhead, and their results in an intravenous glucose tolerance test.

The horses on the higher quality pasture certainly moved toward the overweight end of the spectrum during the grazing period, which ran from the end of May to the start of September, but that did not in itself have much influence on their pre-existing levels of insulin sensitivity, the researchers found.

“Our results indicate that grazing on cultivated high-yielding pasture does not increase the risk for metabolic diseases in Finnhorse mares that have a normal body condition score at the beginning of the grazing season,” they reported.

Obesity and insulin resistance are known to be risk factors for laminitis, they said, and are associated with equine metabolic syndrome.

The researchers set out to determine the effect of changes in body condition during the grazing season on insulin resistance and the changes induced in fat levels stored in the neck and tailhead.

Each horse had a glucose tolerance test, along with neck and tailhead fat measurements, taken in May and in September. Each animal was also assessed for body condition score on the nine-point scale.

Following the 98-day grazing spell, the horses on the higher quality grass had increased in body score to 7, which would be considered overweight. Their weight was found to have increased considerably more than the horses on the semi-natural grassland.

However, greater basal and peak insulin concentrations, and faster glucose clearance rate during the intravenous glucose tolerance tested were observed in the horses in September on the better pasture compared to those on the semi-natural grassland.

The increased glucose clearance rate found in the horses on the better quality pasture indicated that higher body weight and a gain in body condition score had not compromised the ability of these horses to control plasma glucose.

However, greater basal and peak insulin concentrations might indicate a compensatory insulin secretion in response to decreased insulin sensitivity or responsiveness of peripheral tissues in horses on the better pasture compared to the others.

“Pasture type had only moderate effects on responses during the intravenous glucose tolerance test,” they said, noting that the quality pasture grazed by half the mares had not triggered insulin resistance.

They said their findings suggested that, in non-obese horses, previous health status or nutritional history may be more important factors affecting insulin function than pasture feeding.

Elo was joined in the research by Shaimaa Selim, Seija Jaakkola, Ninja Karikoski, Ray Boston, Tiina Reilas, Susanna Särkijärvi, Markku Saastamoinen, and Tuomo Kokkonen.

Selim S, Elo K, Jaakkola S, Karikoski N, Boston R, Reilas T, et al. (2015) Relationships among Body Condition, Insulin Resistance and Subcutaneous Adipose Tissue Gene Expression during the Grazing Season in Mares. PLoS ONE 10(5): e0125968. doi:10.1371/journal.pone.0125968
The full study can be read here.