Obesity is widely believed to cause insulin resistance in people but that’s not necessarily the case in horses.
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.
Banse et al [2016] 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 [2016] 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 [2016] 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 in normal horses.]
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 probably 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. To achieve weight loss, change the diet to 2% of ideal weight or 1.5% of current weight (whichever is larger) in grass hay, no pasture, no grain and meet mineral and vitamin needs with a supplement rather than a “balancer” which adds calories.
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
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Dr. K's Horse Sense 
“Insulin, produced in the pancreas, is critical to your overall health and survival. Problems with insulin production or function can result in hypoglycemia (low blood sugars), hyperglycemia (high blood sugars), and diabetes.”
But what causes a malfuntion of the pancreas and insulin production?
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The problem in metabolic syndrome isn’t a malfunction of the pancreas. It’s a failure of the cells to respond normally to insulin so it takes more of it to get the job of keeping blood glucose normal done. There is strong evidence this is genetic and we see it in breeds whose ancestors survived with scant food. Their metabolism is geared to working or traveling many miles a day and a diet low in sugar and starch.
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So if ‘balancers’ are “likely to add calories” to an EMS horse’s diet I presume the ‘vitamin & mineral supplements’ should be based on hay analysis of the current hay being fed?? Which may end up being more than one supplement as opposed to a ‘carte Blanche’ approach balancer. Am I understanding this correctly please??
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Yes, that’s right, although I can usually use a well formulated vitamin supplement with no added iron, low manganese and fine tune it to the hay analysis. That is then fed in a small amount of rinsed/soaked/rinsed beet pulp or another known safe carrier.
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Dr. Kellon,
What amount would you consider to be low manganese? My focus has been on the ratio with iron/copper/zinc over the mg. Thank you!
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Manganese is absorbed by the same metal transporter as the other trace minerals so you can’t ignore it. The vast majority of hays are adequate to excessive in manganese so you want to start with a supplement that doesn’t make that worse.
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