Tests for gamma-glutamyl transferase (GGT) are important in the diagnosis of liver/biliary tract disease and useful in identifying alcohol abuse. Read on to learn more about the function of GGT, its health effects, and ways to lower GGT levels.
What is GGT?
Gamma-glutamyl transferase (GGT) is a cell surface enzyme. It is located throughout the human body with the highest levels being in the kidneys, intestines, liver, prostate gland, and gallbladder. Smaller amounts are found in the pancreas, lungs, testis, and thyroid gland. The liver produces most of the GGT in the blood [1, 2, 3].
GGT is elevated in liver/biliary tract disease or as a result of moderate to excessive alcohol consumption.
Higher GGT levels also increase the risk of several diseases and may point to health problems. This article will cover the functions of GGT, its role in diagnosing or predicting disease, and ways to lower GGT levels in the body.
The primary function of GGT is the breakdown and recycling of glutathione, the most important antioxidant in the human body [4, 5].
It increases the number of available amino acids (especially cysteine), that are used to make glutathione inside the cell .
GGT is also involved in:
- Breaking down drugs and toxins
- Forming amino acids important for the brain and heart (gamma-glutamyl-taurine to taurine)
- Transforming inflammatory molecules (leukotriene C4 to leukotriene D4) [4, 7]
Normal GGT Levels
Tests for GGT measure the level of the enzyme in the blood. Age, gender, exercise, weight, pregnancy, diet, and medications can all influence GGT levels in the body [8, 9, 10].
The reference ranges for GGT levels vary between sources and laboratories. Normal levels are dependent on age and sex. Generally, they are higher in adult males than in adult females. The reference ranges based on 1,160 alcohol abstainers are organized below (one retrospective meta-analysis and one clinical study) [11, 12]:
The risk for heart disease increases around the middle of what is considered a “normal” range for both males and females (prospective clinical studies). Therefore, optimal levels of GGT are below the reference range midpoint (based on age and gender).
High GGT Levels
Elevated levels of GGT point to high glutathione use in the body and oxidative stress. Maintaining normal/optimal levels of glutathione in the body has a wide range of health benefits [13, 14, 15].
GGT is useful in 3 ways:
- Diagnose disease: Used to screen for liver/biliary tract disease, and to identify alcohol abuse [16, 17, 18, 19, 20, 21].
- Assess disease risk: Elevated levels of GGT are linked to increased risk of disease and disease-related death [22, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34].
- All-cause mortality: Clinical studies show a relationship between higher levels of GGT and increased the chance of death from any cause [35, 36, 37, 38, 24, 39].
GGT alone does not directly cause a particular disease or disorder. However, high levels of GGT may be a sign of systemic cellular damage.
GGT may increase oxidative stress, starting with the breakdown of glutathione (and production of cysteinylglycine). Other toxic molecules are then formed, leading to tissue, cellular, and DNA damage [13, 15].
GGT is present in plaques because it attaches itself to circulating fats (LDL). Once in the plaque, GGT can become pro-oxidant, injuring blood vessels (via oxidative stress), and contribute to heart disease [40, 29, 41].
Diseases Linked to High Levels
1) Liver Disease
GGT is increased in patients with liver diseases, such as cirrhosis, hepatitis, and fatty liver. These conditions are caused by a number of factors, including drug and alcohol abuse, toxins, or viruses [42, 43, 44, 45, 46, 47].
GGT levels are used with other liver enzyme tests to diagnose liver disease. For example, GGT levels are measured when alkaline phosphatase (ALP) is high. Elevated levels of both enzymes point to a liver or biliary tract disease. Normal levels of GGT in the presence of increased alkaline phosphatase (ALP) point to bone disease .
2) Biliary Tract Disease
High levels of GGT are also found in patients with biliary tract disease (cholestasis). This is usually caused by bile duct obstructions from one or more conditions including gallstones, inflammation, injury, infection, cysts, and/or carcinomas (cancer) [16, 48, 49].
Like with liver disease, GGT is used in combination with additional tests, such as alkaline phosphatase (ALP), for diagnosing biliary tract disease .
3) Alcohol Consumption and Cigarette Smoking
Prolonged alcohol use raises GGT levels. In some cases, additional tests may be conducted to identify alcohol abuse [17, 18, 19].
Increased levels of GGT in combination with another marker (CDT) were better at predicting alcohol abuse than either test alone .
Cigarette smoking has also been linked to elevated GGT levels. One clinical study conducted in 46k men found that regular alcohol consumption and daily cigarette smoking increased levels of GGT greater than each activity alone. Those people who drank and/or smoked more had higher levels of GGT [20, 21].
4) Coronary Heart Disease (CHD)
CHD is a condition caused by the narrowing of the coronary arteries, potentially leading to a heart attack. Elevated GGT levels are associated with an increased risk for coronary heart disease [29, 22].
Further studies also linked high GGT to death from coronary heart disease [23, 24].
The largest meta-analysis to date (10 studies) found that higher levels of GGT increased the risk of stroke (independent of excessive alcohol consumption), but only in men .
Arteriosclerosis is a narrowing and clogging of the arteries, usually due to plaques. GGT is present in human plaques, and levels are higher in patients with plaques, especially in those with more severely clogged arteries [40, 51].
7) Heart Failure
Heart failure is a condition where the heart is unable to supply enough blood to meet bodily demand. Clinical studies found an association between elevated GGT levels and an increased risk of heart failure, even when values fell within the “normal” range [26, 27, 28].
8) High Blood Pressure
In-depth meta-analyses found people with elevated levels of GGT also have an increased risk of high blood pressure. As levels of GGT went up, so did the risk of developing hypertension [52, 53].
9) Cardiac Arrhythmias
Atrial fibrillation causes rapid irregular heart beating. Higher levels of GGT were linked to an increased risk for atrial fibrillation in several studies.
Increased levels of GGT were also correlated with increased risk of sudden cardiac death/ventricular fibrillation [54, 55, 56].
Elevated levels of GGT may increase the risk of type 2 diabetes. However, a recent meta-analysis found no link between the two. Further studies looking at this relationship would be beneficial [57, 23, 58, 59].
11) Metabolic Syndrome
Metabolic syndrome is 2 or more of the following conditions including insulin resistance, high blood pressure, blood fat abnormalities (dyslipidemia), and obesity. Independent of other risk factors, a number of clinical studies have found that elevated levels of GGT increased the risk of metabolic syndrome [29, 30, 31].
GGT may be useful at predicting cancer risk and disease outcome. According to several clinical studies, elevated GGT levels increase overall cancer risk [32, 60].
In a few early studies, elevated GGT levels have been associated with some types of cancer [61, 62]:
- Breast cancer: Those with higher GGT have a greater risk of developing several subtypes of breast cancer, according to a study in 231k cancer-free women .
- Esophageal cancer: A study of 8.3 million Koreans found an increased risk for esophageal cancer in those with levels of GGT > 18 U/L, a value considered “normal”. As the level of GGT rose, so did the risk of esophageal cancer .
- Liver cancer: Higher GGT levels were associated with a worse prognosis in liver cancer [65, 66].
- Endometrial cancer: Elevated levels of GGT were associated with decreased survival rates for women with endometrial and cervical cancer [67, 68].
13) Kidney Disease
One study in 10k Korean men found that the risk of chronic kidney disease was greater in patients with elevated GGT levels. Another study of 9.5k Americans found no such link [69, 70].
Elevated levels of GGT may be associated with an increased risk of chronic kidney disease in specific populations.
14) Alzheimer’s Disease and Dementia
GGT can be elevated in patients with Alzheimer’s disease. However, a genetic study looking at 26 GGT-related gene variations (single nucleotide polymorphisms (SNPs)) and the risk of Alzheimer’s disease found no correlation between the two [71, 72].
One clinical study in 2.4k men found elevated GGT levels to be associated with an increased risk of dementia .
15) Thyroid Disease
Diseases of the thyroid include hyperthyroidism (higher T3/T4, usually with lower TSH levels) and hypothyroidism (lower T3/T4, usually with higher TSH levels).
In a study of 110 people, GGT was elevated in patients with hyperthyroidism. In a larger study of 1.5k subjects with hyperthyroidism, GGT levels were normal [74, 75].
GGT was slightly increased in patients with hypothyroidism in one study and decreased in another study. Additional research is needed to clarify the relationship between GGT and thyroid disease [76, 77].
16) Decreased Bone Mineral Density
Bone mineral density (BMD) represents overall bone health and strength. One clinical study in 462 people found people with high GGT levels have low BMD (independent of alcohol consumption) .
Factors That Increase GGT Levels
High levels of GGT usually indicate present disease or a higher risk of disease. Here we give a summary of the factors that increase levels of GGT.
1) Medications/Prescription Drugs and Supplements
Several medications increase GGT levels and must be taken into consideration when interpreting test results. These include older anti-seizure medications (such as phenytoin, phenobarbital, and carbamazepine) [79, 80, 81, 82].
GGT levels are elevated following prolonged alcohol use. As drinking increases, the level of GGT in the blood also rises [17, 18, 19].
3) Red Meat
Red meat intake over 10 years increased GGT levels .
One pilot study of 62 healthy adults found that GGT levels were increased by about 40% in those who drank kava .
5) Thyroid Hormone
Several clinical studies have found that elevated levels of thyroid hormone increased levels of GGT in patients with hyperthyroidism [84, 74].
How to Reduce GGT Levels
1) Treat the Underlying Condition
GGT is ultimately just a biomarker that indicates an underlying problem. Talk to your doctor to identify any underlying health conditions and determine the best strategies for treating those.
2) Decrease Alcohol Consumption
Alcohol abuse increases GGT levels. GGT is only slightly elevated in moderate drinkers, but the increase is relevant. The best way to lower GGT is to simply abstain from alcohol use. Levels should return to “normal” after 2 to 6 weeks [17, 18].
3) Eat More Fruits and Vegetables
Many fruits and vegetables contain moderate to high amounts of natural antioxidants. A study of 3k people found that eating fruits and/or vegetables 10-11 times per week, or drinking fruit juice 6-7 times a week for 10 years lowered GGT levels .
4) Drink Coffee
Drinking coffee decreased GGT levels in 1,300 Japanese men (independent of other risk factors) .
Two other clinical studies, conducted in a total of 31k participants, found that coffee consumption only lowered GGT levels in alcohol-drinking males [86, 87].
5) Eat Less Red Meat
Red meat contains large amounts of heme iron, the most readily absorbed form of iron. The same study that showed fruits and vegetables reduced GGT found that eating red meat over 10 years increased GGT levels [88, 10].
Note: Vegetarian sources do contain varying levels of iron, but in the form that’s harder to absorb (non-heme iron). Vegetarian iron contributes to overall iron levels in the body much less than meat sources. This is one reason that vegetarians sometimes suffer from iron deficiency or anemia [88, 89].
6) Moderate Exercise
One study of 10k participants found that moderate exercise (walking) was associated with decreased GGT levels in men, but not women .
7) Fish Oil
High doses of fish oil (4 grams/day) for 3 months significantly lowered GGT levels in 36 participants who had characteristics of non-alcoholic fatty liver disease .
8) Avoid Pollutants
Certain environmental pollutants such as lead, cadmium, dioxin, and organochlorine-containing pesticides increase GGT levels .
Six weeks of magnesium supplementation decreased levels of GGT in a group of chronic alcoholics. However, this was a small study and further research is needed .
Limitations and Caveats
The full range of functions for GGT is not known. It is a topic of continued scientific and clinical study.
There are disease states such as diabetes, chronic kidney disease, and hypothyroidism where clinical studies show conflicting conclusions regarding risk and elevated GGT levels. Further studies need to be done to determine if high GGT levels increase the risk of these diseases.
Clinical studies show elevated GGT levels to be associated with the development of diseases even within what would be considered “normal” ranges. This is mostly the case with GGT and the risk of cardiovascular diseases. Future clinical studies need to address the risk of other disease states within “normal” GGT ranges.
Conditions Associated with Low GGT Levels
Rarely are low levels of GGT dangerous. However, a few conditions are associated with abnormally low levels of GGT. Whether this is cause for concern depends on other factors.
1) Inherited Disorders
Familial Intrahepatic Cholestasis (FIC) includes inherited (genetic) disorders, usually observed in infants and children. It is characterized by the decreased flow of bile from the liver. It can be:
Mutations in the ATP8B1 gene are related to both the mild and severe forms of the disease. This gene controls the bile salt export pump, which drives bile flow .
2) Mid to Late Pregnancy
Women in their second and third trimesters may have lower levels of GGT in comparison to non-pregnant women. There is no indication that this is a cause for concern .
Genetics of GGT
GGT levels are influenced by your genes.
Many genetic and environmental factors can influence GGT levels. Genetic factors affect GGT levels, dependent on age, and contribute to about 1/3 to 2/3 of GGT levels [96, 97, 98].
The GGT gene family consists of 13 members, with 6 considered to be “active,” The gene responsible for most of the GGT throughout the body is GGT1 .
Early animal and cell-based studies suggest fullerene (buckminsterfullerene or C60) may be an antioxidant that removes toxic metabolic waste. According to these early studies, C60 may promote longevity and reduce oxidative stress. However, it showed the potential to damage DNA. Additionally, fullerene has not yet been studied in humans. Thus, its short- and long-term risk and safety profile remain unknown. We recommend against taking this compound until proper clinical trials are completed.