Increased obesity in horses certainly raises the risk of equine metabolic syndrome and laminitis, but researchers say it’s important to remember that excessive adipose tissue (body fat) also has negative impacts on performance. Obese horses must carry more weight, causing an inflammatory response and increased work effort, and excessive loads likely contribute to the deterioration of limb health.

In a recent study, researchers reviewed previous work on the effects of equine obesity on the overall health and performance abilities of sport horses. They described obesity as a condition where the horse has excessive body fat and is overweight. Defining a horse as obese requires quantifying excessive body fat, as well as establishing which body condition scores have negative health consequences.

Study co-author Shannon Pratt Phillips, PhD, professor of equine nutrition and physiology at  North Carolina State University, in Raleigh, noted that researchers have a better understanding of equine obesity than they did historically, leading to increased research on the topic. Because we know what an overweight horse looks like, it’s easier for more people to identify, she added.

Researchers attribute the high prevalence of equine obesity on lower workloads and increased access to high-quality feeds, especially in the U.S., as well as an overall lack of understanding of how to assess an overweight horse. “The availability of excellent equine feeds and supplements has increased dramatically, along with good marketing,” said Pratt Phillips. “Many horses are likely being fed at a higher plane of nutrition than what they might need.”

Study results have also revealed that owners tend to have a skewed perception of adiposity and often do not recognize when their horse is overweight or even obese. “Sadly, in some disciplines, the ‘squishy’ look is rewarded,” she added. “When an overweight horse wins, it perpetuates the problem because owners will see that as the ‘ideal’ for that sport and try to copy it.”

Overweight horses are not only at risk for conditions such as equine metabolic syndrome and insulin dysregulation but also more likely to develop laminitis and not survive it. Obesity can cause adipose tissue accumulation around internal organs, which can interfere with their function and affect fertility, particularly in mares.

Obese sport horses must work harder due to the extra weight they’re carrying, creating an overall welfare concern. Horses with adiposity have higher body temperatures during work, have a reduced ability to regain a normal heart rate after exercise, and are more likely to show gait asymmetry.

Overweight horses’ risk of heat stress increases because body fat acts as an insulator, making it difficult for them to dissipate heat, especially in hot or humid climates. During muscle contraction, up to 60% of the energy a horse metabolizes is lost as heat; therefore, overweight or obese horses can be limited in their performance abilities due to heat stress.

Researchers studying riding school horses in Sweden found an extra 18 kilograms (a little under 40 pounds) of weight during jumping exercises altered the horses’ landing behaviors and overall jumping style. Horses carrying additional weight also experienced increased ground reaction forces during both flatwork and jumping, leading to altered movement and increased injury risk.

“I think the most important point is that an overweight horse or pony is not only at risk of metabolic problems but is also not going to be able to perform at its best (due to carrying more weight) and is likely to develop over-use types of disorders (e.g., arthritis or tendinitis) at an earlier time in their careers,” said Pratt Phillips.

“Owners with obese/overweight horses should work with nutritionists who are specially trained to help develop a diet for their animals that is lower in calories but still meets the animal’s other nutrient requirements,” she added. Nutritionists can also help owners develop practical management plans, such as using hay-nets or finding less energy-dense forages, as well as work with veterinarians to ensure any medical problems are managed, she said.