Hormones to Detect Overtraining in Horses

In an ongoing attempt to find easily detectable red flags of overtraining, researchers have learned that certain hormones wave their colors particularly well.

Cortisol, adrenocorticotropic hormone (ACTH), testosterone, leptin, insulin, and other hormones could soon become reliable biomarkers in blood tests to determine if a horse is overworked, according to Concetta Amato, PhD, researcher in the nutrition and endocrinology unit of the French National Veterinary School in Nantes, France.

“Regular, standardized testing of the hormone profile could provide crucial and precocious information about the physiological evolution of the horse’s condition during training, with any changes in that profile being a potential indicator of fatigue and the risk of overtraining,” Amato said during her presentation at the French Equine Research Day held Feb. 28 in Paris.

Amato and her fellow researchers followed 37 Lusitanos and purebred Spanish horses used in public performance shows in France over an 11-month period. They evaluated horses regularly for weight, body condition, and hormone concentrations in the blood via a blood test. Meanwhile, they recorded the horses’ exercise intensity for the period of each evaluation.

In the off-season (October to April) the horses were trained one or two hours a day for four to five days a week and walked or rested the other days. During the show season the horses took part in between nine (April to June) and 19 (July to September) performances per week; horses had no rest days in July or August. During performances horses were required to gallop, sprint, carry vaulters and riders, and execute many abrupt starts and stops, Amato said.

Upon reviewing their results, the researchers found that during the intense exercise period, 49% of the horses lost a significant amount of weight (up to 10%) and body condition score , even though their energy rations (feed) were increased 12%. The team also noted a marked increase in physiologic pathologies—such as infections, musculoskeletal disorders, injuries, and behavior problems—during the show season, which rose from an occurrence rate of 3.5% during off-season training to 47.8% during the high-season performance period.

Amato said the team also saw considerable changes in horses' hormonal profiles between the off season and the show season:

  • Testosterone increased with progressive training but decreased during the high performance season; this is also a phenomenon seen in humans in or near a state of overtraining.
  • ACTH secretion, which typically activates cortisol secretions, did not correspond to cortisol secretion during the intense period (generally, as ACTH secretion increases, so does cortisol secretion). When ACTH isn't properly regulated, certain diseases can ensue: Not enough ACTH causes secondary adrenal insufficience, whereas too much can cause primary adrenal insufficience (or a lack of the cortisol necessary to manage biological stress).
  • Prolactin, which reflects stress levels, decreased during training but increased at the end of the show season.
  • Leptin, which regulates the appetite, decreased drastically during the show period. A drop in leptin can reduce a horse's capacity to use the energy found in nutrients and affect his ability to exercise.
  • Thyroxin, which activates certain kinds of metabolism, decreased. In racehorses, low levels of thyroxin have been associated with poor performance and myopathies.
  • Insulin-like factor 1 (IGF1) increased during training and decreased in the high season. IGF1 concentration is influenced by the duration and intensity of exercise, energy imbalance, and inflammation.
  • Insulin, another growth factor, which reflects the horse's nutritional state, increased progressively during work season, reaching the maximum peak of insulinemia in the show season. “This implies incomplete recuperation and a progressive decrease in the sensitivity of tissues to insulin,” Amato said.

“Overtraining is one of the major risks of reduced performance capacity in sport horses, with multiple consequences, on both a technical and an economical level,” Amato said. “Previous studies have already shown that overtraining and all its clinical manifestations (behavior problems, loss of appetite, muscular difficulties, variations in the heart rate, etc.) can be linked to perturbations of the hypothalamus (the part of the brain that manages hormone secretion) and to the resulting neurohormonal secretions. Our study confirmed that all the hormones evaluated were significantly influenced by the increasingly heavy workload.”

In the future, Amato said, ACTH, cortisol, testosterone, leptin, IGF1, insulin, thyroxin and prolactin might be reliable biomarkers for evaluating the risk of overtraining in horses. Her research is ongoing.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.