The vast majority of us are well familiar with how easy it is to gain weight with the wrong foods or simply too much food and how hard it can be to lose it. The same is true for most horses.
Being overweight does not mean your horse has a metabolic issue
In people, it is generally accepted that weight gain can lead to the development of insulin resistance and type II diabetes but that’s where parallels with the equine situation end. As obesity develops in people, so does a clear picture of higher inflammatory cytokines circulating in their blood and increasing insulin levels reflecting insulin resistance. This does not hold true for horses.
Banase et al  found that markers of inflammation in skeletal muscle were actually lower in horses that were obese, and lowest of all in obese horses with elevated insulin – the exact opposite of what would be expected in a human.
What about obesity causing high insulin and equine metabolic syndrome? Nope. Lindase et al  induced obesity in horses that already had moderately elevated insulin by feeding additional fat but the resultant 10% weight gain did not worsen insulin resistance. Similarly, Bamford et al  studied normal horses and induced obesity by feeding excess fat with or without a once daily high simple carbohydrate meal. Again, the weight gain did not result in insulin resistance in either group. Contrary to expectations, the horses receiving the high carb meals actually had better insulin sensitivity. [This has also been reported in earlier studies and represents an adaptation to the higher carbohydrate feeding.]
If obesity doesn’t cause EMS, where did that idea come from? As above, there is a connection in people but it’s also true that you will find a higher percentage of obese horses with abnormally high insulin than in horses of normal weight. However, the reason for this is that horses with insulin resistance gain weight more easily, not because the weight gain causes EMS – a good reminder that association is not causation https://wp.me/p2WBdh-Ex .
This doesn’t mean you can just ignore it if you horse is overweight. Excess weight puts a lot of unnecessary strain on the heart and skeletal system, reduces exercise tolerance, makes it more difficult to breathe and interferes with temperature regulation in the heat or when exercised. These all improve with weight loss. The effects of weight loss on insulin sensitivity are not as clear.
Studies have shown reduction in baseline insulin and/or insulin resistance with weight loss induced by diet and/or exercise but they differ in regards to whether there is a difference in effectiveness between dieting versus exercise. Comparing the studies is complicated by different breeds, horse versus pony, different diets and different degrees of weight loss. Suffice it to say that weight loss won’t hurt the situation with EMS and might help, particularly if exercise is included.
Obesity per se does not cause laminitis either – high insulin does. Being normal weight won’t protect from laminitis or guarantee normal insulin, but a normal weight in conjunction with a physiologically sound trim will certainly help to mechanically reduce the damage it causes.
In the chicken or egg world of obesity and EMS, it’s the EMS that comes first.
Eleanor Kellon, VMD