Diet and exercise are the best ways to control insulin resistance in horses and ponies.
A study into the pharmacological management of insulin resistance using metformin failed to demonstrate any benefit.
The research was conducted by the School of Animal and Veterinary Sciences at Charles Sturt University in New South Wales, Australia, and the Department of Clinical Studies, New Bolton Centre at the University of Pennsylvania, in collaboration with the Waltham Equine Studies Group.
In insulin resistance, insulin seems unable to exert its normal effect. This is particularly noticeable in relation to glucose metabolism.
As a result, the body releases increasing amounts of insulin, ending up with normal or raised blood glucose levels despite high concentrations of insulin.
Insulin resistance appears to be a significant risk factor for conditions such as laminitis.
Therefore, it is important to prevent insulin resistance from developing, or to manage it before it contributes in turn to the development or progression of other potentially life-threatening conditions.
While the correct management of energy intake and exercise levels is thought to be essential, in some cases medication is also considered, especially when increased exercise is not possible.
No licensed drugs are currently available for treating insulin resistance in horses and ponies.
Metformin has been suggested as a possible treatment for the condition. It appears to enhance insulin sensitivity of peripheral tissues without stimulating insulin secretion.
The research team hoped to confirm that metformin had a positive effect on insulin and glucose dynamics in insulin-resistant ponies, so that it could be used as a positive control in other studies.
Six ponies that were insulin-resistant, but not obese, took part in the study.
Three ponies were allocated to the treatment group, and they received metformin (at 15mg/kg bodyweight orally, twice daily) for 21 days. The control group received a placebo.
After a "wash out" period, the trial was repeated with the ponies being swapped between the treatment and placebo groups.
The response to metformin (and the placebo) was assessed using a frequently-sampled intravenous glucose tolerance test (FSIGT).
The researchers found no measurable benefits of metformin. No significant change was noted in any of the indices of insulin sensitivity. Neither was there any change in bodyweight, body condition score or cresty neck score.
Clare Barfoot, research and development manager at Spillers, commented: "Although clinical trials with metformin are ongoing, this trial was consistent with our previous work showing that the bioavailability of metformin in horses is poor, and chronic dosing at these levels may not achieve therapeutic blood concentrations."
She continued: "On this basis it would seem that a fibre-based diet, that is low in starch [found in cereals] as well as sugar and fructans [found in grass], in conjunction with regular exercise if possible, is still the safest and most effective way to manage insulin-resistant ponies.
"It is important to recognise that, even if other doses of metformin are shown to be beneficial in the future for certain sub-groups of animals, full consideration to diet and management should remain a key tactic."