An elevated GGT by itself really doesn't tell us very much.
Frequently an elevated Alkaline Phosphatase together with an elevated GGT suggests a liver or bile duct problem.
If AST and ALT were also elevated it would strongly suggest a liver disorder.
An elevated GGT may suggest a liver problem but it can also be seen with pancreas and gallbladder issues.
There is also evidence that an elevated GGT can be present with increased cardiovascular risk. Because of his running you might dismiss this possibility but if there are no other signs of liver disease you should consider asking his doctor about it.
Keep in mind that I am not in the medical field in any capacity. I am merely a patient and my post should be read in that context.
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