Horses carrying a few extra pounds must put in more physical effort when they exercise—and it’s not only due to the added weight. According to a new study, body fat also affects performance on a chemical level.

Scientists in Sweden and Iceland have discovered that only a slight body fat increase can lead to undesirable changes in blood parameters during and after exercise, as well as movement asymmetry. It can also result in lower performance scores, said Anna Jansson, PhD, professor in the Department of Anatomy, Physiology, and Biochemistry at the Swedish University of Agricultural and Sciences, in Uppsala.

“(Our) results clearly show that an increase in body fat percentage, body weight, and body condition of 5 to 8 % lowers physiological and metabolic fitness … and impairs true performance,” Jansson and her colleagues stated in their recent study.

Investigating Fat’s Effects on Individual Horses

Jansson had long noticed that riders sometimes compete with horses that are plumper than they probably should be, she said. As a scientist this concerned her. “We see a lot of overweight horses, and often the owners are not aware of this,” said Jansson. “They think the horse is okay or even ‘muscled.’”

Jansson wondered to what extent the fat content itself would affect the way the body works to use energy on a molecular level. So she and her colleagues designed a unique study that allowed them to investigate how more or less body fat affects metabolism and performance in the same individual.

To do so, they worked with nine Icelandic geldings aged 6 to 8 years old, all with approximately the same size, weight (about 880 pounds), body condition score, and training level. Then, for 36 days, they either increased or decreased their regular calorie intake by about 20-25%. At the end of that 36-day period, they switched to the opposite feeding program—meaning the horses that had been overfed were now underfed for 36 days, and vice versa. Meanwhile, the horses maintained their regular competitive training program.

At the start of the study and end of each 36-day period, the horses underwent exercise tests in front of experienced judges who were unaware of the experiment, as well as on treadmills with standardized scientific procedures. The researchers collected data on the horses through blood tests, locomotion analysis sensors, and clinical evaluations.

Physiological and Metabolic Changes

On average, the horses were 42 pounds heavier at the end of the overfeeding period compared to the underfeeding period, Jansson said. When overfed, they had greater rump fat thickness, body fat percentage, fat mass, and body condition scores in most parts of the body. The horses didn’t show higher body condition scores in their backs or the circumference of their necks, and they didn’t show signs of a cresty neck. But when they exercised, motion sensors indicated they were slightly asymmetrical compared to when they were thinner.

When the horses were heavier, their plasma profiles showed poorer lactate removal and glucose availability, as well as a reduced anaerobic energy threshold, and they had a lower proportion of red blood cells circulating in the blood. They also took longer to get their normal respiratory rate back after exercise. All these factors suggest poorer metabolic and physiological fitness, she said.

“We wanted to investigate the physiological and metabolic background for the decrease in performance—in other words, to know if there was more to it than just carrying more weight, which of course limits performance,” Jansson told The Horse. “Our results show that there is.”

The relative drop in red blood cells—an unexpected finding, said Jansson—is one example. “This indicates substantial changes in tissue function, which limits performance,” she said. Reduced hematocrit levels (percentage of red blood cells in the blood) in response to changes in body fat have never been found in any species until now, she explained.

The study underlines the importance of keeping horses fit and recognizing when they’re getting overweight. “Learn how to score body condition and keep your pleasure and sport horses in a moderate body condition,” said Jansson. “It supports both health and performance.”

The study, “Increased body fat content in horses alters metabolic and physiological exercise response, decreases performance, and increases locomotion asymmetry,” was published in  June 2021 by Physiological Reports.