Alanine aminotransferase (ALT) is an important marker of liver health. Keep reading to find out why this marker is often part of liver function tests–and when you should be concerned about your levels.
What is ALT?
Healthy liver cells store most of your ALT, but small amounts are also found in the muscles and blood. When liver cells are damaged due to illness, injury, or medication, they release ALT, increasing its blood levels .
Therefore, ALT blood levels are a marker of liver health: low levels typically indicate a healthy liver, while high levels suggest liver damage .
Before we talk about the ALT blood test, let’s take a look at what ALT normally does in the body.
ALT is involved in the body’s main energy-producing pathway (called the Cahill cycle). Muscles use amino acids for energy, but they produce waste in the process. ALT helps transport these amino acid byproducts to the liver, where they are recycled [5, 6, 7, 8].
In muscle cells, glutamate is a byproduct of burning proteins for energy, while pyruvate is a byproduct of burning glucose. ALT helps turn these into L-alanine and alpha-ketoglutarate, which are transported to the liver [8, 9].
In the liver, AST turns L-alanine into glucose for energy (via pyruvate), uses it to build new proteins (via L-glutamate), or eliminates it as waste (via L-glutamate). Alpha-ketoglutarate is either used to make urea or as a source of energy (in another chemical process known as the Krebs cycle) [8, 9, 10, 11].
ALT Blood Test
As Part of Liver Function Tests
- Assess liver health
- Investigate symptoms of liver disease, such as abnormally yellow skin or eyes (jaundice), or pain in the upper-right section of the abdomen
- Monitor progression of a liver disease
- Evaluate the effectiveness of a treatment for liver disease
- Determine if the liver is involved in or damaged by a health condition, such as diabetes or heart disease
You can request that your doctor test your ALT. Conventional doctors look at high or low ALT levels, often without mentioning anything. Sometimes, a lab result may be in the reference range, but not actually be in the optimal range. Reference ranges are not the same as optimal ranges. This is why even ALT in the ‘normal’ range can be unhealthy and indicate that certain processes in the body aren’t optimal.
Since ALT is an enzyme, its levels are typically determined by measuring its activity (the rate at which ALT transforms L-alanine and α-ketoglutarate into pyruvate and L-glutamate) .
ALT levels are often measured together with the liver enzyme aspartate transaminase (AST). The ratio AST/ALT is also used as a marker of liver health.
While ALT levels can signal the presence of liver damage, they cannot determine the type of damage, such as scarring, infection, or inflammation .
ALT is measured in units per liter of blood or U/L.
Until recently, 10 to 40 U/L was accepted as the normal range, and this was used in most experiments to define “normal” versus “high” .
However, the most recent comprehensive review recommended that these values be revised due to ALT variation between the genders .
These researchers recommended that the upper limit of normal should be :
- 31 U/L for women
- 42 U/L for men
What Affects Your Levels?
Various factors other than liver damage can affect your ALT levels.
In a study with over 44,000 participants, ALT levels rose until age 60 in women and around age 45 in men, and then decreased .
In another study, ALT levels peaked around ages 40 to 55 for both genders .
Adolescents are an exception, as they tend to have lower ALT levels than adults .
In a study of 371 school children, boys had higher ALT levels than girls starting at 7 years old, indicating that hormones could account for the difference .
Genetic variations between people of different ethnicities are most likely to influence ALT levels. However, in a study of 5,421 unrelated Dutch individuals, ALT levels were not significantly influenced by genetics, indicating that most genetic variation is likely between people of different ethnicities .
This conclusion is supported by research on Americans, who often have similar lifestyles and diets but come from a wide variety of ancestral backgrounds. For example, Mexican-Americans had higher levels of ALT (159%; 12%) than African- (8%; 6%) or white Americans (7.1%; 7.4%) in 2 studies over over 15,000 adults and 5,000 adolescents [29, 31, 29, 31].
In a study of 1,532 Mexican-Americans, 2 linked single nucleotide polymorphisms (SNPs) in the PNPLA3 gene (rs738409 and rs2281135) affected ALT levels. The combination C/T raised ALT levels, with G/C lowered ALT levels, and C/C had no effect. However, how strongly these SNPs impacted ALT levels varied by age and gender .
Other genetic variations that affect ALT are rare. In one case study, a newborn had extreme ALT deficiency due to mutations of liver cells (in the endoplasmic reticulum and mitochondria). Both parents must carry the mutation and pass it on their child for it to cause this disorder .
In another case study, a woman had low ALT levels and hepatitis C, which typically presents with elevated ALT. Since her sons had low ALT levels as well, genetics were likely the cause .
These medications and treatments have been shown to increase ALT levels in humans:
- Anti-seizure medication vigabatrin [38, 39, 40]
- Chemotherapy (immune checkpoint inhibitors, tyrosine kinase inhibitors) [41, 42, 43]
- Anti-inflammatory medication diclofenac and rofecoxib [44, 45, 46]
- Cholesterol medication cholestyramine and rarely statins [47, 48, 49, 50, 51, 52, 53]
- Anti-obesity medication rimonabant 
- Parenteral nutrition (nutrients delivered directly into the bloodstream, bypassing the digestive system) 
- Antibiotics when multiple antibiotics are taken at once or they are taken for a long time [56, 57]
- Tylenol (acetaminophen) may cause a temporary ALT spike [58, 59, 60, 61]
- Asthma medication Zileuton in a small percentage of patients [62, 63]
- Antipsychotic medication risperidone in a very small percentage of patients [64, 65, 66]
These medications and treatments have been shown to reduce ALT levels in humans:
- Hepatitis medication (telbivudine, lamivudine, ursodeoxycholic acid, dimethyl dicarboxylate, and interferon) [67, 68, 69, 70]
- Hemodialysis [71, 72, 73]
5) Strenuous Exercise
In a study of 20 Thai youths (ages 14 to 17), ALT levels significantly increased after a boxing match, but not during “intensive” training. This indicates that muscle damage is likely the cause of this ALT elevation, versus other biological effects of exercise .
6) Time of the Day
In a study of 12 patients with liver damage (cirrhosis), ALT levels increased during the morning, peaked in the afternoon, and dropped to their lowest at night [77*].
However, in a study with 22,970 participants, ALT values were not significantly different between the morning and afternoon. But, this study did not separately measure ALT by the hour or during the night .
* This experiment was published as a “To the Editor” letter and therefore was not peer-reviewed with the same scrutiny as the other experiments referenced in this article.
When Should You Worry?
If your ALT levels are high, your liver may be damaged. Your doctor will look at other markers of your liver function, and, in some cases, redo the ALT test to confirm your values–especially if you have no symptoms of liver disease .
Since ALT levels can normally rise in healthy people, you may not need to worry .
On the other hand, ALT levels sometimes remain normal despite liver injury. Several tests pointing to high ALT levels or liver damage and symptoms of liver disease are a reason for concern. Your doctor should work on finding the underlying cause and treatment options .
Limitations and Caveats
Another literature review determined that ALT levels can differ by as much as 10 to 20% from the same sample, indicating that testing needs to be standardized .
Read through this post to learn more about the causes of high ALT and how to lower your levels naturally.
Irregular ALT Levels?
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ALT is an enzyme your body needs to turn food into energy. Most of it is stored in your liver. Your levels will usually remain stable and relatively low if your liver is healthy, while liver damage causes ALT to leak into the bloodstream in high amounts. Thus, doctors typically order ALT as part of liver function tests.
However, ALT is not a reliable marker. Various factors beyond liver health can affect your levels, including your gender, age, genetics, exercise routine, medication, and even the time of the day.
What’s more, your levels can remain normal even if your liver is damaged. That’s why ALT is useful only when analyzed alongside other markers of liver health like AST, ALP, and GGT.