The Disease of Obesity in Horses

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Equine obesity feeds several common maladies

Some people wonder, "Is obesity a disease?" The answer is it could be. Consider: In early March 2010, researchers at Emory University found that increased appetite and insulin resistance could be transferred from one mouse to another via intestinal bacteria, strengthening the case that intestinal bacteria--germs--can contribute to obesity and metabolic disease.

As the obesity/disease theory pertains to horses, Amanda A. Adams, PhD, of the University of Kentucky's Gluck Equine Research Center, says, "Obesity is a condition that is now described as a mild but chronic inflammatory state and thus is a key risk factor for developing a myriad of health conditions.

Let's take a closer look at how obesity feeds some common equine maladies.


"We have recently shown that obese old horses have more inflammation in their systems when compared to thin old horses," states Adams. "Termed 'inflamm-aging,' or increased inflammation with old age, age-related obesity contributes to changes in immune function. Although the association between inflamm-aging and disease in the aged horse remains unclear, many diseases of older horses are conditions associated with chronic inflammation such as insulin sensitivity; thus, obesity-associated inflammation in the old horse would likely exacerbate these conditions."

After gradually decreasing body weight and body fat in obese older horses, researchers found the measures of inflammation significantly decreased. "Dietary management modulates this chronic inflammatory state and improves obesity-related conditions," Adams reports.


Obese mares have an extended interval between successive ovulations, and they experience prolonged periods of elevated circulating concentrations of progesterone (which prevents estrus). According to studies conducted at Gluck Equine Research Center, obese mares displayed continuously elevated circulating concentrations of progesterone for 37-78 days compared to 22 days or less in nonobese mares.

Obesity also can reduce stallion fertility. Simply stated, an obese stallion has overly insulated testicles. "Testicles are outside the body cavity for a reason," says Sarah Ralston, VMD, PhD, Dipl. ACVN, associate professor in the Department of Animal Science at Rutgers University. "You don't get normal spermatogenesis (sperm production) at body temperature. If the testicles are protected from cooling--if they're at body temperature because they're pressed between fat thighs--the stallion will have reduced fertility."

Bone & Joint Stress

Excess pounds put additional forces on the joints, basically wearing them out more quickly and contributing to or exacerbating degenerative joint disease (arthritis). In turn, joint stiffness or pain causes reduced stamina and athletic performance.

"With an arthritic joint, even a hundred pounds of excess weight can really make a difference in the amount of stress on that joint," says Ralston.

Insulin Resistance and Laminitis

The link between insulin resistance (a reduction in sensitivity to insulin that decreases the ability of glucose--sugar from digestion of food--to be transported into the body's cells from the bloodstream) and obesity has been investigated for several years. "There is an underlying metabolic alteration going on there, but it's not fully understood," Ralston says.

What is understood is fat horses are at an increased risk for becoming insulin resistant, which results in the horse secreting abnormally high levels of insulin when he eats a meal of grain. "The high insulin response is associated with an increased risk of laminitis (inflammation of the sensitive laminae of the hoof, which serve as the connection between the coffin bone and the hoof wall) or founder (separation of the laminae and downward rotation of the pedal bone, or coffin bone, in the hoof--a sequela of laminitis)," explains Ralston.

As with arthritis, the excess physical weight exacerbates laminitis and founder. "Excess weight carried by obese horses further increases the risk of actual rotation of the pedal bone," Ralston says. "Extra weight pushing down on that hoof increases the likelihood of laminae tearing."


Obese horses tend to accumulate large amounts of fat in their abdominal cavities. Explains Ralston, "This excess abdominal fat can lead to the formation of lipomas--solid balls of fat encased in tissue that form a long stalk. This stalk can wrap around the intestines, cutting off blood circulation to the gut, causing a strangulation colic that requires surgery to correct."

Deal With It

Obesity is endemic in the United States. Various studies show that about 33% of American adults, 25-40% of American pets, and 19-32% of American horses and ponies are described as obese. Says Carey A. Williams, PhD, equine extension specialist at Rutgers University and associate director of outreach with the university's Equine Science Center, "Overfeeding of grain and/or supplements in horses is the most common practice I see as a nutritionist. Excess energy is by far the most common excess seen in horses' diets."

Generally, healthy horses should receive about 2% of their body weight in feed, including forage, per day (i.e., 20 pounds for a 1,000-pound horse), says Williams. However, since easy keepers are more metabolically efficient, they need fewer calories to maintain their body weight, so a scale or a body condition score are the best indicators of normal or abnormal weight. (For more information on body condition scores (BCS) see

"Many people feel horses must have grain in their diets," notes Williams. "But the addition of grain gives horses a much higher calorie content in their diets than is necessary. Most horses can maintain their body condition with good quality forage (pasture and/or hay). If a horse needs a little extra energy, add a small amount (1-4 pounds) of commercial grain product. The key here is the forage. Good-quality forage is sometimes hard to find in the winter months or in the spring before the new season's hay is cut. In these cases the horse's diet could include a vitamin and mineral supplement."

If your horse needs to shed pounds, Williams suggests restricting his feed to 1.5% of his body weight or even 1% if obesity persists. However, if severe feed restriction and weight loss are necessary, consult with an equine nutritionist to avoid creating any vitamin or mineral deficiencies.

Introduce feed and turnout changes slowly, over one or two weeks, as sudden changes can be stressful for a horse's GI tract. "Gradually reduce turnout time or increase the time the horse is muzzled," Williams says, referring to a grazing muzzle.

Additionally, during the course of the weight loss, monitor your horse's weight using a weight tape (available from any feed store) every other week. Says Williams, "This will let you know if the horse is losing the right amount of weight or if you need to decrease turnout time even more. Once the right amount of weight is lost, you can slowly increase pasture or feed intake until he starts to maintain his appropriate weight."

Increasing your horse's physical activity level will also help take off the weight. Exercise--riding, driving, longeing, even shooing your horse around the paddock--is a big calorie burner.

Take-Home Message

Excess weight is largely an avoidable condition because you control your horse's portions and make his dietary choices for him. The questions you need to ask are: Do I choose to put my horse at risk for the diseases of obesity, or do I choose to increase his odds for good health?

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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