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Relationship of Obesity and Diet to Insulin Resistance in HorsesBy Kentucky Equine Research Staff · October 24, 2012

A study conducted at the University of Melbourne was designed to investigate changes in insulin sensitivity when equines became obese.

Mature mixed-breed ponies and Standardbred and Andalusian horses were used in the study. All animals were in moderate body condition with scores of 4 or 5 on a scale where 1 indicates severe emaciation and 9 indicates extreme obesity.

Horses and ponies were assigned to one of three groups so that each group had two equines of each breed. All horses and ponies had free access to hay that was low in NSC, and all received a ration of soaked soy hull pellets, alfalfa (lucerne) chaff, and a vitamin and mineral supplement twice a day. In addition, animals in the FAT group were given granulated vegetable fat and canola oil to provide up to around 210% of their maintenance energy requirement. Animals in the FAS group received added glucose in the morning meal, causing a once-daily glycemic and insulinemic response. They also received enough vegetable fat in the evening meal to bring the diet to the same caloric level as the FAT group. Animals in the control group received only hay and the basal diet which was designed to meet maintenance requirements and maintain moderate body condition scores.

The study was conducted for 21 weeks. An insulin-modified frequently sampled intravenous glucose tolerance test was performed at the beginning of the study and after the horses had gained weight. Insulin sensitivity and percentage increase in body fat were determined. Body weight, body condition score, ultrasound fat measurements, and blood samples were taken each week throughout the study period.

Body fat increased from 9.3% to 15.1% in the FAT group and from 7.8% to 14.9% in the FAS group. Body condition score also increased significantly in horses in these groups, going from 4.9 to 7.3 in the FAT group and 4.8 to 7.7 in the FAS group. Fat percentage and condition score did not change significantly in the control horses. Insulin sensitivity improved significantly in the FAS group, improved slightly in the FAT group, and did not change in the control group.

These results indicate that increasing obesity alone did not trigger insulin resistance in equines that gained weight on a high-fat, low-glycemic diet. Horses that were on a diet including a single glucose meal had improved insulin sensitivity even though they became obese, indicating the short term glucose and insulin peaks may have led to an improvement in insulin effectiveness.

The authors of the report stated, “With careful dietary modification, it may be possible to reduce the likelihood of insulin resistance in animals with increased adiposity, thereby reducing one potential risk factor for laminitis, although obesity may remain a risk factor independent of insulin resistance.”