Can an Obese Horse Lose Weight Too Fast?

Q. I recently purchased an obese pony, whom I estimate needs to lose more than 100 pounds. I know changes in weight should happen gradually, and I’ve heard rehabilitating starved horses needs to be done carefully to avoid refeeding syndrome. Are there similar concerns when rehabilitating an overweight horse to their ideal weight?

A. Good for you for recognizing your new pony is not at a healthy weight and needs a plan to get there. To help your pony lose weight, you need to put him in a state of negative energy balance where the energy consumed is less than the energy he uses each day. This will cause him to utilize stored energy reserves to make up the difference, resulting in weight loss.

The problem with being in negative energy balance is it can cause excessive mobilization of body fat into the bloodstream. The purpose of releasing these fatty acids is so the liver can metabolize them into glucose (gluconeogenesis) or package them into triglycerides for further transport in the blood. In and of itself this is not an issue; in fact, it’s a perfectly viable way for the body to use energy stores. The problem arises if too much fat is mobilized, especially in horses predisposed to hyperlipemia (the disease caused by hyperlipidemia, or the abnormally high concentration of fats or lipids in the blood).

This condition is the result of too much fat in the bloodstream. Normal triglyceride levels in the horse are below 100 mg/dl; however, during hyperlipidemia levels can be above 500 mg/dl. This fat buildup in the bloodstream can cause the liver to stop functioning properly and a condition known as hepatic lipidosis. Toxins then build up in the blood, and the horse might start feeling unwell, a side effect of which can be going off food. Of course, going off feed only acts to exacerbate the problem.

In the early stages of hepatic lipidosis, the clinical signs of lethargy and going off feed can be easy to miss. Often, owners wait to see whether the horse starts to eat again, not realizing what is going on. But this wait-and-see approach can be fatal for the horse. The condition can quickly progress to liver failure, jaundice, staggering, colic, and an inability to get up. Even when caught early, prognosis once hepatic lipidosis occurs can be poor, with 40 to 80% of horses dying as a result.

Therefore, careful weight loss is vital, and I recommend working with your vet to coordinate a plan. Regular bloodwork might be indicated to ensure blood triglyceride levels are not reaching dangerous levels. Additionally, horses that have insulin dysregulation issues tend to be predisposed to hyperlipemia and, in turn, hyperlipemia can exacerbate insulin resistance. With high circulating insulin being a potential trigger for laminitis, it’s important to monitor metabolic status, as well.  

Understand that this isn’t just a “fat pony” condition. It can occur any time a horse goes into negative energy balance for a prolonged period. This includes during pregnancy and lactation if the foal is drawing large amounts of energy off the mare and during severe illness that reduces or eliminates appetite, such as colic or fever. Any time a horse goes off feed, especially when combined with one of these scenarios, it is worth contacting your veterinarian.