Elevated gamma-glutamyl transferase (GGT) is strongly associated with obesity and excess deposition of fat in the liver, which is thought to cause hepatic insulin resistance and to contribute to the development of systemic insulin resistance and hyperinsulinemia.
The normal range of GGT is 0-45 U/L. However, the level of GGT exceeds 20 U/L , then the risk for chronic disease increase significantly.
A key thing to remember is that high levels of GGT correlate proportionately with low levels of glutathione (GSH). Note that GGT is a significantly more powerful in predicting new cases of diabetes than obesity alone, even when BMI is above 30 or 35. Note also that elevated GGT increases the risk of diabetes in normal weight people by 3 fold.
In patients with raised alanine aminotransferase (ALT) or γ glutamyltransferase (GGT) levels or with hepatic steatosis noted on ultrasonography, non-alcoholic fatty liver disease (NAFLD) should be suspected in those with risk factors (increased body weight, raised fasting glucose or HbA1c, modestly raised triglycerides, low high density lipoprotein cholesterol, and AST:ALT ratio <0.8) and with daily alcohol intake <20 g in women and <30 g in men.
The GGT test is currently the most sensitive enzymatic indicator of liver damage and disease. This damage is often caused by heavy use of alcohol or other toxic substances, like drugs or poisons. GGT is used to screen for chronic alcohol abuse, because it will be elevated in about 75% of chronic drinkers.