Blood analyses of Thoroughbred racehorses can reveal elevations in the enzyme gamma-glutamyl transferase (GGT). In what appear to be otherwise healthy horses, the significance of these high GGT values remains unclear. Using advanced metabolomic, viral and chemistry techniques, veterinarians suggest that “GGT syndrome” in fit racehorses could be related to oxidative stress.
“The horse's body produces excess GGT in the face of liver, pancreatic, and kidney disease, particularly injury to the bile duct. Concomitant increases in other liver enzymes, however, usually do not occur, suggesting that primary liver disease does not explain elevated GGT levels seen in racehorses,” explained Peter Huntington, B.V.Sc., M.A.C.V.Sc., director of nutrition, Kentucky Equine Research.
Elevated GGT levels may be caused by:
- Hypoxia (decreased oxygen reaching tissues);
- Infection;
- Toxicity (e.g., pyrrolizidine alkaloids from toxic plants);
- Liver injury;
- Hepatic glycogen depletion and repletion;
- Overtraining; and
- Oxidative stress associated with inadequate selenium or glutathione
Previous studies show that GGT levels tend to increase with racing frequency and cumulative training load and then decrease with recovery. This trend in GGT raises the possibility that oxidative stress and oxidative depletion play a role in the syndrome.
In a recent study, veterinarians collected blood samples from Thoroughbred racehorses.* GGT values were analyzed, as were other liver values, selenium levels, viral load, and metabolomics. Comparisons were made between horses that did and did not have elevated GGT.
“Many of the potential causes of GGT syndrome were ruled out based on this testing, including viral hepatitis. One particularly interesting finding was that while selenium concentrations in horses with high GGT levels were within normal limits, they were significantly lower than selenium levels in horses with normal GGT,” noted Huntington.
These low selenium levels were reported in earlier studies and may reflect reduced antioxidant capacity related to oxidative stress. That said, one investigation found that selenium supplementation did not prevent increased levels of GGT in racehorses in training. In contrast, supplementation with the powerful antioxidant coenzyme Q10, such as Nano-Q10, can increase serum coenzyme Q10 levels.
“Racehorses in training with higher coenzyme Q10 levels had significantly lower GGT levels in one study. In addition, supplementation with EO-3, a marine-derived omega-3 fatty acid supplement, was associated with a significant reduction in GGT levels in horses in training,” Huntington said.
Overall, GGT syndrome is likely multifactorial rather than a result of a single mechanism, including oxidative stress. But, according to Huntington, there are some nutritional strategies that may be used to manage it, such as supplementing the horse's diet with antioxidants.
Read more about Kentucky Equine Research's work on GGT: The Effect of Long-chain Omega-3 (EO-3) Supplementation on Blood Serum Gamma-glutamyltransferase(GGT) Levels and Inflammation Post-exercise in Thoroughbred Racehorses.
*Mann, S., J.D. Ramsay, J.J. Wakshlag, T. Stokol, S. Reed, and T.J. Divers. 2021. Investigating the pathogenesis of high-serum gamma-glutamyl transferase activity in Thoroughbred racehorses: A series of case-control studies. Equine Veterinary Journal:13435.
Reprinted courtesy of Kentucky Equine Research. Visit ker.com for the latest in equine nutrition and management, and subscribe to Equinews to receive these articles directly
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