The study "Influence of dietary restriction and low-intensity exercise on weight loss and insulin sensitivity in obese equids" was carried out by Nick Bamford and colleagues at the University of Melbourne’s Faculty of Veterinary and Agricultural Science, in collaboration with Spillers.
A total of 24 obese horses and ponies were randomly divided into two groups; 'restricted diet only' or 'restricted diet plus exercise'. All horses and ponies were fed the same diet of restricted hay (no grazing), a small amount of alfalfa chaff and soya bean meal, and a vitamin and mineral supplement.
The exercise programme, which was designed following feedback from horse owners to help ensure it could be implemented relatively easily in the real world, consisted of 15 minutes of brisk trotting (with a five minute walk before and after) five days per week for 12 weeks.
The 'restricted diet only' group showed an overall reduction in body weight and body condition score. They had increased levels of adiponectin – a hormone produced by fat cells, low levels of which are a risk factor for laminitis. They had decreased baseline insulin, high levels of which have been linked to an increased risk of laminitis, and decreased leptin, high levels of which are associated with obesity.
Although exercise did not increase weight loss, it did produce additional benefits that were not seen in the 'restricted diet only' group, the most important one being improved insulin sensitivity. More insulin sensitive horses and ponies need to produce less insulin in order to control their blood sugar. This is important because high levels of insulin in the blood and/or reduced insulin sensitivity are risk factors for laminitis. Horses and ponies in the exercise group also had decreased levels of ‘serum amyloid A’, a protein that is a marker for inflammation.
Clare Barfoot RNutr, the research and development manager at Spillers said: "Reducing calorie intake and feeding a diet low in starch and sugar should be the priority for overweight horses and ponies. However, the results of this study suggest that exercise may offer additional health benefits for obese horses and ponies and/or those with ‘EMS’ that cannot be achieved by cutting calories and weight loss alone."
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Photo: Spillers/Steve Bardens
Their study, which was published in the research journal PeerJ at the end of last year1, investigated the effects of insulin on equine hoof lamellar cells in the laboratory.
The researchers say that high levels of insulin have been known for some time to cause acute endocrine laminitis, seen when ponies and certain types of horses graze lush pasture or consume a starch- or sugar-rich diet, or when they develop Cushing’s syndrome.
Many ponies and horses at risk of the condition produce large spikes of insulin in their blood stream after consuming meals high in non-structural carbohydrates (in particular starch and simple sugars) which indicates that this could be the way in which they succumb to acute endocrine laminitis.
The mechanism by which insulin can cause effects in the foot leading to laminitis has been the subject of much debate in recent years.
Hormones like insulin must interact with a specific receptor on the surface of cells in order to produce their effects. What has confused researchers in the past is that there appear to be no insulin receptors on the hoof lamellar cells (these cells give the lamellar tissues their strength and changes to their growth or structure weaken the bonds and cause laminitis).
The researchers considered the close similarities between insulin and a growth factor hormone called insulin-like growth factor -1 (IGF-1). There are receptors for IGF-1 on the lamellar cells, and the researchers speculated that high concentrations of insulin might be able to cross-stimulate these cells by activating the IGF-1 receptors. To investigate this possibility PhD student Courtnay Baskerville, together with her supervisor Professor Simon Bailey, developed a method for culturing lamellar epithelial cells in the laboratory, and incubated them with increasing concentrations of insulin.
They found that high concentrations of insulin stimulated the cells to proliferate. Furthermore, this effect could be prevented using an antibody that specifically blocks only the IGF-1 receptor. Further downstream effects within the cells were also shown. The changes seemed to occur mainly at very high concentrations of insulin. Similar concentration levels can be seen in ponies and horses with insulin dysregulation linked to Equine Metabolic Syndrome, but are not commonly seen in normal horses that are considered to be at lower risk of laminitis.
Further work is now ongoing to determine exactly how these cellular changes induced by sustained high insulin concentrations might cause laminitis. However, it seems that targeting the IGF-1 receptor may be an option for developing new drugs to prevent and treat laminitis.
Professor Simon Bailey said: "The information gathered in this study provides the equine industry worldwide with valuable insights into the causes of the common and serious condition of laminitis. The research group would like to acknowledge the support of the Waltham Centre for Pet Nutrition, in particular the involvement of Professor Pat Harris.”