Doctors use laboratory tests that measure blood enzymes known as aminotransferases to evaluate patients for liver disease. Also called transaminases, these enzymes -- aspartate aminotransferase (AST) and alanine aminotransferase (ALT) -- are typically elevated in people with certain disorders that affect the liver, such as hepatitis, cirrhosis and toxicity from certain drugs. However, a 2013 study reported in the Global Journal of Science, Engineering and Technology strongly suggests that elevations in these enzymes also occur in healthy individuals who engage in high-intensity exercise such as weightlifting and other extreme sports.
Liver Enzymes Explained
The liver contains a number of enzymes, including AST and ALT, that are responsible for chemical reactions that convert food into energy and remove harmful substances from the blood. Normally, these enzymes are contained within the liver cells where metabolism takes place. However, when the liver is inflamed or damaged, they leach out of the cells and into the circulating blood. What's more, these enzymes are not unique to liver cells. They are found in varying concentrations in the heart, liver, skeletal muscle, kidney, pancreas, spleen, lung, and red blood cells. Doctors use a number of parameters, such as the severity of the increase, the proportion of AST to ALT and clinical factors to determine whether liver damage has occurred.
Liver Enzymes and Liver Disease
Many conditions that affect the liver cause blood levels of liver enzymes to increase. When levels are very high, an acute infection such as hepatitis A or B or cellular injury due to a drug such as acetaminophen (Tylenol) is usually the cause. Toxins, such as industrial solvents and propellants, also may damage liver cells extensively, causing high levels of AST, ALT or both. More moderate increases usually are associated with fatty liver disease, which may be related to alcohol abuse or metabolic changes associated with obesity, high blood sugar and elevated cholesterol. Additionally, athletes who use anabolic steroids, particularly the oral variety, experience a high number of liver complications, including benign liver tumors and hepatocellular carcinoma, a malignant disease. Any of these complications may result in increased levels of AST and ALT.
Other Causes of High Liver Enzymes
Liver disease is not the only or even the most common reason for elevated liver enzymes. Because both AST and ALT are present in muscle tissue, patients who have had a heart attack, which causes inflammation and damage to the heart muscle, typically have elevated levels of both. Similarly, most patients who undergo surgery or sustain extensive muscle trauma have elevated blood levels of AST and ALT. Moreover, athletes have been shown to have elevated liver enzymes shortly after strenuous exercise. Scientists theorize that this is due to skeletal muscle inflammation or hemolysis, the breakdown of red blood cells. However, the results of a 1999 study reported by the British Journal of Sports Medicine, in which scientists evaluated changes in liver enzymes in ultra-marathon runners, suggests that damage to liver cells may be part of the cause.
Implications for Athletes and Their Physicians
Elevated liver enzymes in healthy-appearing individuals are difficult to evaluate. Many people with underlying liver disorders show no symptoms until their illness is well underway, which is why doctors are reluctant to ignore all but the mildest elevations in AST and ALT. However, in a study reported in the 2008 British Journal of Clinical Pharmacology, a group of 14 healthy, active young men experienced significant elevations in AST and ALT after one 60-minute weightlifting session. What's more, the increase lasted for at least seven days. This suggests that high levels of liver enzymes in healthy athletes with no history of alcohol or steroid use and no recent history of exposure to hepatitis are most likely benign. Furthermore, it provides a strong rationale for warning athletes to forgo weightlifting and other forms of strenuous exercise for at least a week before undergoing routine lab tests.