The insulin-related health risks of feeding a high-energy diet to Shetland ponies have been demonstrated in a Dutch study.
Nicky d’Fonseca and her Utrecht University colleagues set out to learn about the effect of long-term overfeeding of a high-energy diet on glucose tolerance in Shetland pony mares.
Overfeeding and obesity are considered factors in the development or worsening of insulin dysregulation in horses.
Insulin dysregulation is a central feature of equine metabolic syndrome, with sustained high levels of insulin considered a trigger for endocrinopathic laminitis.
However, the relationship between chronic overfeeding and the development of insulin dysregulation and obesity is not completely understood.
Current knowledge about the development of insulin dysregulation in horses is based exclusively on short-term follow-up studies that explored the effects of overfeeding, changes in diet, or both, on insulin levels.
The researchers enlisted 11 pony mares for the two-year study, four of whom were controls. All had moderate body condition scores at the start of the study.
The study design involved three phases. The high-energy group of seven ponies was fed 200% of their net energy requirements for 24 weeks. Their diet comprised hay and a concentrated feed made up of 36% sugar and starch, and 13% fat. This was followed by 17 weeks on a hay-only diet before four of the animals resumed the high-energy diet for a further 29 weeks.
The mares were weighed weekly. Oral glucose tolerance tests were performed 3 to 4 times per dietary period, and the results compared with the control group, which received 100% of their net energy requirements each day.
The study team found there was a gradual but continuous increase in body weight in the ponies on the high-energy diet, with total weight gain reaching 27% of starting weight, during both overfeeding periods (years 1 and 2).
The high-energy diet led to more efficient glucose metabolism within five weeks, followed by significant hyperinsulinemia (excessive levels of insulin in the blood) after meals, and obesity.
“Hyperinsulinemic status was reversed during 17 weeks of hay-only feeding, regardless of body condition, but returned rapidly after restarting the high-energy diet,” the researchers reported in the Journal of Veterinary Internal Medicine.
The 17-week hay-only period did not result in significant weight loss, despite insulin levels being brought under control.
They reported that while insulin dysregulation status was reversed during a 17-week hay-only period, regardless of body condition, its rapid return after resuming the high-energy diet indicated the existence of more deeply integrated changes that take longer to resolve.
“Feeding a diet rich in sugar and starch over a long period of time is not recommended,” the researchers said.
They noted that the diet was tolerated well by all the overfed ponies, who consumed their full ration of concentrates. Some hay was left on occasion.
“After two years of consuming the high-energy diet, all overfed ponies moved slightly stiffly and, on a firm floor, walked with short strides, increasing suspicion of subclinical laminitis.” This was investigated further in another study.
The study team comprised d’Fonseca, Charlotte Gibson, David van Doorn, Marta de Ruijter‐Villani, Tom Stout and Ellen Roelfsema, all affiliated with Utrecht University.
Effect of long-term overfeeding of a high-energy diet on glucose tolerance in Shetland pony mares
Nicky M. M. d’ Fonseca, Charlotte M.E. Gibson, David A. van Doorn, Marta de Ruijter-Villani, Tom A.E. Stout and Ellen Roelfsema.
Journal of Veterinary Internal Medicine, First published May 6, 2020, https://doi.org/10.1111/jvim.15788