By Marielle St-Laurent, DVM

Years ago, veterinarians recognized that obese horses develop a different metabolism than healthy horses. They often find it difficult to lose weight, even when on a strict diet, and are prone to laminitis. Researchers began to investigate the cause of this altered metabolism, which was eventually attributed to equine metabolic syndrome (EMS). EMS is not a disease in itself. Rather, the term is used to describe a condition characterized by symptoms which may include insulin resistance, hypothyroidism, and obesity. The definition of EMS is continually changing as we learn more about the syndrome and its components. Although at this time there is no consensus among members of the scientific community as to a specific definition for EMS, it is clear that the syndrome is closely linked to, and may be caused partially, or even wholly, by obesity.

As with people, obesity in horses is a major health problem that can lead to several diseases. Being overweight can reduce the efficiency of reproductive and body temperature control, as well as placing stress on heart and lungs, and increasing the risk of laminitis and the severity of osteoarthritis. Obesity can also trigger insulin resistance and equine metabolic syndrome. Here is an overview of how your horse’s excess weight will affect his health.

Fat is not only underneath the skin

Fat is stored in the body in adipose cells, which, put together, constitute adipose tissue. Fat is not only underneath the skin; it surrounds each organ inside the body as well. By the time a horse is visually overweight, most of the internal organs, such as kidneys and heart, also have fat deposits. Most of the visceral fat (the fat in and around the organs) is stored in the abdomen in the omentum and the mesentery, which are two curtain like organs that are part of the gastro-intestinal system.

Fat tissue acts like a gland

The adipose cells, or fat cells, have a function just like the cells in other organs. Fat tissue, or adipose tissue, actually acts like a gland. It can secrete hormones and send and receive messages to and from the rest of the body via transmitters and receptors. There are over 100 substances called adipokines that are secreted by the adipose tissue. These adipokines play a role in inflammation, feeding behaviour, production of bone tissue, production of red blood cells, blood pressure, reproduction, immunity, and general metabolism. Obese individuals that have an excess of adipose cells release too much adipokines. Some of these adipokines are responsible for insulin resistance. Visceral fat (organ fat) rather than total fat is thought to release most of the adipokines responsible for insulin resistance.

Most people have heard the terms “insulin resistance” or even “glucose intolerance.” Insulin is a vital hormone produced by the pancreas that travels through the blood. It is secreted when the glucose (sugar) level in the blood is too high. Cells in the body need glucose to function, just like cars need fuel. The insulin’s job is to open the channel from the blood to the inside of the cells so that the glucose can pass from the blood to the cells. Insulin resistance refers to a condition in which the level of glucose in the blood needs to be higher than normal for the insulin to do its job and make the cells absorb the glucose. The blood sugar level remains elevated because the glucose is not reaching the cells, which so desperately need it. The excess glucose in the blood is then transformed into, and stored as, fat. Insulin resistance therefore promotes weight gain.Obesity leads to insulin resistance

We also know that obesity can cause insulin resistance. At this point we have too little information to confirm that the disease occurs in lean horses, but research might find otherwise in the future. On the same note, not all obese horses have the same degree of insulin resistance.

Once a horse is diagnosed with insulin resistance, it is crucial to work with both a veterinarian and a nutritionist to develop a proper and complete treatment plan for that horse. Insulin resistant horses do not have a normal metabolism and their cells do not respond to insulin like they should. It isn’t only about giving them less food, but also different types of food that are digested in such a way that the level of sugar in the blood does not spike rapidly but instead glucose is released into the bloodstream slowly.

Obesity is a condition that often contributes to such health problems as laminitis, osteoarthritis, and EMS.

Hypothyroidism leads to weight gain

The thyroid gland plays a big role in energy level and metabolism. An abnormal or malfunctioning thyroid gland can wreak havoc on the body. Although hypothyroidism is fairly common in dogs, little is known as to how much of the equine population is affected. The symptoms of hypothyroidism in the horse include weight gain, low exercise tolerance, lethargy, and poor coat condition. Affected horses have very low energy and a decreased need for calories, and their bodies are in full fat storage mode. Before confirming that a horse is overweight because of hypothyroidism, proper diagnostic testing should be performed since the symptoms are vague and their severity not necessarily proportionate to the severity of the disease itself.

Levothyroxine is a drug used to treat hypothyroidism. In horses that suffer from only insulin resistance and not hypothyroidism, this drug may still decrease body-fat mass and improve insulin sensitivity when combined with a carefully monitored diet and restricted access to pasture.

Obese horses are prone to laminitis

Laminitis is another major health concern that obese horses are susceptible to. The mechanism is not yet fully understood but the prevalence of laminitis in obese horses is thought to be linked to insulin resistance. Either the cells attaching the hoof wall to the lamella are not getting enough glucose, or other adipokines secreted by the adipose cells trigger the laminitis. It has also been suggested that the extra weight exceeds the strength power of the lamella and detachment occurs like a tear. Most likely, all of these mechanisms are involved which makes laminitis a complex subject to study and understand.

Obese horses have a hard time losing weight

Insulin resistance and hypothyroidism usually result in weight gain, and the more obese the horse, the more insulin resistant they are likely to become. It’s a vicious cycle, and one that means it is very difficult for an obese horse with EMS to lose weight. This is why it is so important to prevent obesity in the first place, and to carefully monitor your horse’s weight.

The main cause of obesity

The number one cause of obesity in horses and ponies is overfeeding. Like humans, not all similarly sized horses need the same daily caloric intake, or the same type of feed. There are many other factors that determine how much feed a horse truly needs, including work load, reproductive status, age, breed, and individual variation.

Monitor your horse's body condition on a regular basis using a weight tape. Photo: Pam MacKenzie Photography

Find your horse’s body condition score

Before putting your horse on a diet to prevent diseases such as EMS or IR, first establish whether your horse is, in fact, overweight. A body condition chart, weight tape, and your veterinarian are all helpful tools in determining whether your horse is a good weight. The Henneke body condition chart, with ratings ranging from 1 (extremely emaciated) to 9 (extremely fat), with 5 (moderate) being the ideal weight for most horses, is very commonly used. This chart takes into consideration where fat deposits appear on a horse. If you look only at a horse’s ribs, he might appear skinny, when, in fact, most of the fat cells are located on the neck and on the croup. This also doesn’t take into account the fact that age, body type, and breed can lead to variations in fat distribution. A horse’s ideal weight will also vary depending on its reproductive state and fitness level. For example, racehorses are usually on the thinner side at a score of 4 out of 9, while halter horses are usually on the heavier side at a score of 6 out of 9. Obese horses score 9 out of 9.

The combination of emotional involvement and seeing their horse every day makes most owners and caretakers unable to fairly assess their horse’s true body condition. When in doubt, ask a third person to go over the Henneke chart and assign a number to your horse’s body condition, or else ask your veterinarian when he/she is next at the stable.
 
Tips and tricks to equine weight loss

Horses are natural grazers and it is better for them to be eating all day. So how do you restrict a horse’s intake while feeding him all the time? Some tricks include using double hay nets or bags with little openings, or grazing muzzles. It will take them more time to eat and therefore will not only keep them occupied longer but also diminish their feed intake. Frequent, smaller feedings are ideal but sometimes not practical. If horses are usually fed together, make sure that the one dieting cannot have access to another’s food. Some other tips to achieve a healthy weight loss include making any change gradually (10 percent over 10 days is usually the norm) with regard to both quantity and type of food. Vitamins, minerals and proteins are essential and it is imperative that horses receive their minimal requirement even though you are limiting feed. Follow the guidelines on the feed bags and use food weight instead of volume to determine how much to give. That being said, your horse is an individual and the quantity of food that he is receiving may be too much for him, even if that’s what it says on the bag. Feed companies and distributors often have animal nutritionists who are available to help you choose the right food for your horse.

Finally, exercise is extremely important in a weight loss program. It stimulates the metabolism of the body and you can build on time and intensity as fitness improves.

Monitor your horse’s progress with a weight tape and write it down in a journal so you can identify changes more quickly.

Adjust Feeding Often

It is important to remember that nutritional needs also change constantly and feed may need to be adjusted more than you realize. If your horse is being ridden five times a week for one hour during the spring, versus once a week during the fall, that’s a huge difference in energy requirement. We all know that life gets in the way sometimes and you might not go see your horse as much as you planned to for an extended period of time, so be sure to adjust your horse’s feed according to his changing nutritional requirements.

About the Author: Dr. Marielle St-Laurent is originally from the West Island of Montreal. She graduated from the Faculty of Veterinary Medicine at the University of Montreal in June 2010 and then joined Paton & Martin Veterinary Services near Vancouver, BC, for their internship program. She is now a full time associate veterinarian and in her free time trains to be a competitive dressage rider. Visit the Paton & Martin Veterinary Services website at www.pmvetservices.com for more information.

This article originally appeared in the March 2012 issue of Canadian Horse Journal.