Ghrelin, leptin and adiponectin characterization in horses: The effects of various nutritional and physiological challenges

Maintaining energy balance is crucial for optimal health and performance of exercising horses. Ideally, energy intake and energy expenditure should be matched, so that horses avoid a detrimental positive or negative energy balance. Several studies were designed to characterize the hormones ghrelin, leptin and adiponectin that are involved in the regulation of energy intake and expenditure, in response to nutritional and physiological challenges in horses. A 24-h study determined that ghrelin and leptin may be entrained to meal feeding in horses, while adiponectin appears unaffected. There was also a diurnal variation (p < 0.05) in glucose and insulin sensitivity and elevated glucose and insulin from feeding may be responsible for increases in leptin. Additionally, both oral and intravenous glucose challenges were able to alter (p < 0.05) ghrelin and leptin. Furthermore, differences (p < 0.05) in ghrelin, leptin and adiponectin concentration were reflective of body composition and vary in fit vs. unfit horses. In response to short-term, high-intensity exercise, leptin and ghrelin concentration were altered (p < 0.05) post-exercise, while longer-term, high-intensity exercise was associated with decreases (p < 0.05) in feed intake and alterations (p < 0.05) of leptin and ghrelin during exercise. Training was associated with lower increases (p < 0.05) in plasma leptin compared to non-exercising horses, and was also related to an increased incidence of gastric ulceration. Overall, exercise training was unsuccessful in combating increases in body weight and body fat that were attributed to total mixed ration (TMR) feeding. TMR feeding was associated with increases (p < 0.05) in body weight, body fat and leptin concentration. Ghrelin infusion did not cause a change (p > 0.05) in feed intake or the number of eating episodes post-treatment in the horses as a group. An increase (p < 0.05) in feed intake was seen in horses that had the highest growth hormone response to ghrelin infusion, which may be due to differences in body composition. Overall, ghrelin, leptin and adiponectin do respond to different forms of energy intake and expenditure in horses. High-intensity exercise does decrease feed intake in horses, but the mechanism does not appear to involve alterations in ghrelin. Ghrelin administration may prove useful for increasing energy intake in inappetant horses.