Alanine aminotransferase (ALT) is an enzyme involved in the body’s metabolism. It is highly concentrated in the liver, and to a lesser extent, in the muscles and kidneys. Because the liver is its primary location, ALT is routinely measured in blood tests as a marker of liver health.
When liver cells are damaged, they release ALT into the bloodstream, causing elevated levels that signal injury or inflammation. While high readings receive the most attention, a result showing low ALT levels, or levels falling below the established normal range, can also indicate underlying health conditions. Understanding a low reading requires looking beyond the typical focus on liver disease.
Understanding Normal ALT Reference Ranges
A “normal” range for ALT is established by laboratories, but a common reference range for adults falls between 7 to 56 units per liter (U/L) of blood. This range is not absolute and can differ based on the testing method, age, and sex. For instance, men typically have higher average ALT levels than women.
A truly low ALT reading occurs when the level drops below the established lower limit of the normal range, often defined as below 7 U/L. Some research suggests that even levels in the “low-normal” range, such as below 17 U/L, may carry clinical significance in certain populations. Results significantly below the reference range suggest a reduction in the enzyme’s activity or production.
Factors Contributing to Low ALT Readings
Vitamin B6 Deficiency
A well-established cause for a low ALT result is a deficiency in Vitamin B6 (pyridoxine). The ALT enzyme requires B6 to function as a co-factor. Without adequate levels of this vitamin, the enzyme cannot effectively perform its metabolic reaction, leading to a decreased measurable amount of active ALT in the blood. Supplementation with B6 can often correct the low reading.
Reduced Muscle Mass
Low ALT levels are strongly associated with a reduction in overall muscle mass, a condition known as sarcopenia or general frailty, particularly in the elderly. Since small amounts of ALT are produced in muscle tissue, a significant loss of muscle mass leads to less enzyme production throughout the body. This association is so pronounced that low ALT levels can serve as a biomarker for frailty and increased mortality risk in older adults.
Malnutrition and Chronic Kidney Disease
Nutritional status plays a direct role, as severe malnutrition or a low-protein diet can lead to low ALT. The enzyme is involved in amino acid metabolism, and a lack of protein building blocks limits the substrate available for the enzyme’s function. This effect is compounded by chronic kidney disease (CKD), where ALT levels tend to decrease as the disease progresses and kidney function declines.
Advanced Liver Disease
In a counter-intuitive finding, very low ALT levels can sometimes signal advanced liver disease, such as severe cirrhosis. When the liver is extensively scarred and its mass is severely compromised, the number of functioning liver cells capable of producing the enzyme is drastically reduced. The lack of healthy tissue means less ALT is available to leak into the bloodstream, resulting in a low reading despite profound liver damage.
Interpreting Results and Medical Follow-Up
For most healthy individuals, an ALT result only slightly below the reference range is not considered a major health concern and requires no specific intervention. When the reading is significantly low or accompanied by other abnormal test results, further investigation is warranted. Interpretation must always occur within the context of the patient’s overall health profile, including age, diet, and existing chronic conditions.
A doctor may order follow-up tests to explore the possibilities suggested by a very low ALT level. These tests often include a nutritional panel to check for protein deficiency or specific blood tests to measure Vitamin B6 levels. For patients with existing conditions, a low reading may prompt a deeper assessment of muscle mass or the stage of chronic kidney disease.
Low ALT levels have been independently associated with an increased long-term mortality risk in specific patient groups, such as those with stable coronary heart disease. This suggests that a low ALT level may serve as an indicator for underlying, severe conditions like frailty or advanced systemic disease. Any blood test result should be discussed with a healthcare provider who can provide a personalized interpretation based on a complete physical examination and medical history.