It’s a common belief, but diet does not cause metabolic syndrome.
It’s easy to see where the idea came from. When a horse/pony/donkey has metabolic syndrome it is very important to limit the sugar and starch in the diet. Starch is digested to glucose before being absorbed. The higher the intake, the higher the animal’s insulin levels will go and the higher the laminitis risk.
However, these abnormal elevations don’t happen with every horse and evidence continues to grow that metabolic syndrome is genetic.
In 2006, Trieber et al published the results of a year-long mixed breed pony herd study following 160 ponies. There were 54 with a previous history of laminitis and 106 were never laminitic. Diet was pasture for all. There were exaggerated responses to spring pasture in the previously and currently laminitic group only and this was “consistent with the expected inheritance of a dominant gene or genes with reduced penetrance”.
Genetics as the root cause is entirely consistent with the observation that certain breeds, like Thoroughbreds, Standardbreds, full size Drafts and Warmbloods rarely, if ever, develop EMS unless they also have PPID. On the other hand, ponies, minis, donkeys, Arabians, Morgans and others are at high risk.
A study published in 2016 by Bamford et al attempted to induce EMS by making horses obese using a high fat diet or high fat + a high carbohydrate meal. They succeeded in making them obese, but not making them insulin resistant. In fact, the horses also fed high carbohydrate had better insulin sensitivity than those fed high fat alone. This adaptation to higher simple carbohydrate intake has been reported before in normal horses. Work is ongoing to identify the specific genetic components.
The most helpful way to think of reactions to sugar is that it is like a food allergy. The vast majority of people can eat peanuts but for some it triggers a severe reaction. It’s not that peanuts are inherently dangerous. It’s the individual sensitivity.
It’s actually not all that simple. For example, exercise can protect from elevated insulin reactions even in susceptible horses. The point is though that sugar/starch isn’t the villain here. The issue is the individual’s genetics. You cannot cause metabolic syndrome by diet.
Eleanor Kellon, VMD
Dr. K's Horse Sense 