Weight loss is linearly associated with a reduction of the insulin response to an oral glucose test in Icelandic horses.  Delarocque, J., Frers, F., Huber, K. et al.  BMC Vet Res 16, 151 (2020)

Thanks to the authors for publishing on an open access site.  Full text available here:   https://bmcvetres.biomedcentral.com/articles/10.1186/s12917-020-02356-w

The aim of this study was to describe the relationship between weight variations and the insulin response to a glucose challenge.   Two groups of horses were kept on small pastures and fed hay.  Most horses lost weight over the course of the study.  The horses in the larger pasture had to walk 450 meters (.28 miles) to water and hay feeders.   Over 5 OST (oral sugar tests) the weight loss was closely correlated to the AUC (area under the curve) for insulin.  The AUC is created by plotting insulin response over time after a glucose challenge.  Both the magnitude and duration of insulin response are factors in AUC.  As insulin drives endocrinopathic laminitis, a greater insulin AUC indicates greater risk.  This study directly shows that weight loss decreases risk of laminitis and other insulin related commodities. 

Researchers at conferences have said for the last decade that empirical evidence suggests that a lean body condition is preventative for laminitis. In the wild, horses that get fat from eating cold stressed grass in the fall lose excess weight over winter from scarcity of feed and burning fat to stay warm.  They are then metabolically prepared for safer introduction to spring grass.  I have myself watched a horse that foundered while obese, yet after a bad rehoming that left him very thin, he was left on spring pasture 24/7 and gained needed weight without getting footsore. 

This paper describes why the term Insulin Dysregulation is more descriptive than the outdated term Insulin Resistance.  IR describes a condition where mechanisms that allow entry of insulin into cells is impaired.  Feedback signals do not function, causing increased signaling for more insulin leading to hyperinsulinemia.   ID includes those horses that have high insulin in spite of having insulin sensitive muscle tissues, encompassing dysfunction in glucose metabolism mechanisms in other parts of the body, such as in the gut and liver.  These other impairments are not fully understood, but at least better questions are being asked.   The acknowledgement of this broader definition of ID explains why the Oral Sugar Test (OST) should be carried out even when fasting insulin levels are within normal range.  Some horses have an abnormal response to feeding even while having normal insulin levels while in the fasted state.  When I observe people that are stuck on the older definition of IR, I wonder if they are also uninformed about the newest science and preferred tests.  I fear that people are too complacent when their fat horses test negative for IR using outdated testing protocols.  While not all fat horses are IR, the focus on IR ignores the fact that obesity itself is a serious issue that any responsible horse owner should address.  In a study on Standardbreds (1), a breed not genetically predisposed to IR, obesity was associated with a linear increase in blood pressure and an increase in serum cortisol that was not associated with insulin sensitivity.  Obesity causes systemic inflammation (2) with consequences throughout the body. 

Over twenty years of consulting, I have heard too many horse owners quick to blame others for their horse’s obesity; grass breeders, barn managers, or show judges that pin obese horses.  Others refuse to accept the seriousness of obesity to a horses health, or have such ingrained confirmation bias that they won’t even accept that their horse is obese.  There are so many obese horses that it has become easy to think this is the way horses are supposed to look.

Things we can do:  Grazing muzzles, larger grass free turnout areas, slow feeders, systems with small amounts of hay spread over distance, require horses to walk further to water, provide tested hay that is lower in Digestible Energy (.9  Mcal/pound or less), more exercise.  In some areas, hay with lower DE that allows horse’s adequate roughage without weight gain is difficult to find.  We must make our needs be known to hay growers and brokers.   Horse owners historically have only been concerned with palatability, color, and hygienic qualities of hay.  We need to let them know that we are now better educated about what we want.   Ask for a hay analysis before purchasing, and specify that you are looking for hay with lower DE. Horses with Insulin Dysregulation will also need hay that is less than 10% WSC + starch.  Offer to pay $5/ton more if they will test all lots to facilitate having the data available to make more appropriate choices.   Let them sell the rocket fuel to race horses and brood mares. It’s win-win for everyone. 

We can also take action to advocate for better standards of body condition that are healthier for horses.  Share this information with someone who has not been made aware of the risks of obesity for their horse.  Learn and teach people how to evaluate Body Condition Scoring paying special attention to those areas where horses with ID tend to develop abnormal fat deposits.  Horse show organizers should disqualify obese horses in halter classes to dispel the concept that fat is beautiful and desirable.  Breed organizations have the opportunity to identify and remove horses from their registry that have genetic predispositions to ID and laminitis if they have the courage and integrity to put the welfare of horses before profits.  Fat ponies in cartoons are meant to be cute, but after one has dealt with a foundered pony, the same cartoons only cause dismay.  We can do better.  We owe it to our horses to do better.

(1)    The effect of diet-induced obesity and pasture on blood pressure and serum cortisol in Standardbred mares K. Nostell S. Lindåse E. Winqvist J. Bröjer EVJ  https://beva.onlinelibrary.wiley.com/doi/abs/10.1111/evj.13294

(2)    Relationships among inflammatory cytokines, obesity, and insulin sensitivity in the horse M. M. Vick, A. A. Adams, B. A. Murphy, D. R. Sessions, D. W. Horohov, R. F. Cook, B. J. Shelton, and B. P. Fitzgerald J ANIM SCI 2007, 85:1144-1155 https://pdfs.semanticscholar.org/ec00/52dcf56f439b5bdfbc73b90c08431c42dc96.pdf