GGT (gamma glutamyl transferase) elevation in racehorses is associated with a sudden onset of exaggerated poor performance.  Since affected horses outwardly appear healthy and normal, other than their inability to work more than½ or ⅝of a mile, this GGT-associated equine oxidative stress syndrome often goes unrecognized and unmanaged.

The correlation of poor performance to elevated GGT levels is beginning to be recognized by the Thoroughbred racing industry.  Traditionally, GGT has been used as an indicator of liver disease (biliary stasis, cancer).If an abnormal level of the enzyme were found in a horse’s blood work, it was dismissed as insignificant by diagnosticians. In recent years however, several high-profile racehorses have been plagued by inexplicable bouts of poor performance, and were found to have elevated GGT. Given the mounting evidence, the veterinary community has begun to investigate and conduct research studies on this formerly dismissed enzyme elevation.

It is now clear that GGT is an indicator and participant in equine oxidative stress syndrome.

Relevant Questions

What is GGT?

GGT (gamma glutamyl transferase) is a cell surface enzyme distributed throughout the body.  It has historically been measured as an indicator of liver disease, (bile stasis in particular) but it is found in all somatic cells.  GGT transfers amino acids across the cell membrane and plays a major role in maintaining glutathione, the body’s master antioxidant. When GGT levels are abnormally high, the enzyme degrades glutathione, the body’s primary antioxidant, causing a depletion of this critical peptide. Without its key antioxidant to protect it, the body undergoes oxidative stress.

What is oxidative stress and why does it affect performance?

Oxidation is a necessary biochemical process that provides energy for cellular function, but it also forms a small amount of destructive reactive species as byproducts. Reactive oxygen and nitrogen species, or “free radicals” as they are commonly known,are unstable and cause cell damage.Once formed, they need to be transformed as quickly as possible to prevent degenerative changes. Antioxidants, or “free radical scavengers,” are the primary defenders which curb free radicals, neutralizing the threat they pose to cells and tissues. When the body has enough antioxidants to stabilize the free radicals produced by oxidation, it is balanced. When the body is lacking in antioxidant levels, it suffers from oxidative stress.

When a horse endures oxidative stress, it is experiencing a biochemical attack on its cells, lipids, proteins—even its DNA! Chronic oxidative stress has been linked to numerous health issues. Even in its early stages, when only fatigue is present, it is an imbalance that needs to be corrected as quickly as possible for the overall health and longevity of the horse.

How do horses get elevated GGT?

The exact cause of equine oxidative stress syndrome is still unclear. A group of talented veterinarians and scientists are currently working on some fascinating research regarding GGT. Hopefully in the coming few years, these studies should provide veterinarians and horse owners with new insights.  Just remember that with any new area of exploration we will likely get a lot more questions before we get all the answers.

Some current theories as to why horses develop high levels of GGT include: 

Viruses: Though initially suspected, viral involvement has largely been ruled out. Extensive diagnostic efforts have failed to demonstrate a consistent viral agent in horses with EOSS.

Over training:  While overtraining was once proposed as a possible factor, it fails to explain the variability in GGT levels among horses on identical training regimens. Horses with longstanding, intense training histories may suddenly develop elevated GGT without changes in workload.

Environmental factors:  GGT is an inducible enzyme, meaning that its expression can increase in response to metabolic, nutritional, or oxidative stress. Certain dietary factors or environmental toxins may upregulate GGT expression. This area remains an active field of investigation.

Glutathione depletion: Based on mammalian studies, elevated serum GGT is inversely correlated with intracellular GSH levels. This relationship offers a compelling explanation for the link between GGT elevation and performance decline.

How can it be prevented?

Monitor training loads: Avoid overloading the horse’s oxidative capacity during intense conditioning.

Optimize nutrition: Provide balanced diets with adequate antioxidant nutrients.

Use hygiene protocols: Although viruses are not implicated, good biosecurity remains standard practice to reduce all types of infectious and inflammatory stress.

Track biomarkers: Regular monitoring of GGT and related biomarkers can identify emerging cases before clinical signs appear.

Maintaining detailed records on training, diet, environment, and performance can aid in early detection and guide treatment decisions.

How is it treated?

Currently, management of EOSS focuses on reducing oxidative load and allowing for cellular recovery:

1. Antioxidant supplementation: Horses with elevated GGT often show dramatic improvement in exercise tolerance and recovery following supplementation with targeted antioxidants. These formulations support glutathione synthesis, reduce free radical burden, and help restore cellular redox balance.

2. Modified training regimens: A reduction in workload—particularly less galloping and more jogging—helps reduce oxidative demands. Many horses return to near-peak performance within weeks when training is scaled back and antioxidant therapy is introduced.

While antioxidant supplementation is not a cure, it represents the most effective and practical management strategy currently available. In contrast to previous years—when few horses with elevated GGT could race competitively—supplemented horses now regularly return to successful performance.

Conclusion

Equine Oxidative Stress Syndrome is a common but often overlooked cause of poor performance in racehorses. While traditionally associated with liver disease, elevated GGT in these cases more accurately reflects oxidative stress due to glutathione depletion. Misdiagnosis remains common, but when correctly identified, the condition responds well to antioxidant therapy and adjusted training. Awareness and early intervention are key to restoring performance and protecting long-term equine health.