Liver Enzymes for Lifters: Why ALT and AST Mislead

The most common lab misunderstanding in strength sport is the assumption that ALT and AST are liver enzymes. They are not. They are enzymes, they are abundant in liver, and they are also abundant in skeletal muscle, cardiac muscle and several other tissues. The name aminotransferase describes a chemical function, not an organ of origin. Once that is clear, a great deal of unnecessary alarm resolves, and a smaller amount of genuinely necessary alarm becomes easier to identify.

What the Enzymes Are and Where They Live

Alanine aminotransferase and aspartate aminotransferase catalyse the transfer of amino groups between amino acids and keto acids, connecting protein metabolism to the citric acid cycle. AST is widely distributed across liver, skeletal muscle, cardiac muscle, kidney, brain and red blood cells. ALT is more liver-enriched, which is why it is generally treated as the more liver-specific of the pair, but it is still present in skeletal muscle in meaningful quantities.

When muscle fibres are damaged, which happens routinely and by design during heavy training, their contents leak into circulation. Creatine kinase leaks, myoglobin leaks, and so do the aminotransferases. The result is a lab report that pattern-matches to liver disease in a person whose liver is fine.

Pettersson and colleagues tested this directly, putting healthy men through a bout of heavy resistance training and following liver chemistries afterwards. AST and ALT rose markedly, in some subjects to levels that would prompt a hepatology referral, and remained elevated for days. The paper is titled, with admirable directness, around the observation that muscular exercise can cause highly pathological liver function tests in healthy men. It is the single most useful citation in this entire area and almost nobody in the training world has read it.

The Enzymes That Sort It Out

The way to resolve the ambiguity is to measure things that are not in muscle.

The practical algorithm is straightforward. Raised AST and ALT, normal GGT, high CK, normal bilirubin, in someone who trained heavily in the preceding week, is a muscle picture. Retest after seven to ten days without heavy training, particularly without novel eccentric work, and the values typically fall substantially. Kwo and colleagues, in the American College of Gastroenterology guideline on evaluating abnormal liver chemistries, explicitly address non-hepatic sources including muscle and recommend considering them before extensive hepatic workup.

When It Is Actually the Liver

Having made the case for calm, the opposite case needs making just as firmly, because the population reading this has genuine and well-documented hepatotoxic exposures.

Oral 17-alpha-alkylated androgens are structurally modified to survive first-pass metabolism, and that same modification makes them hepatotoxic. The characteristic injury is cholestatic: bilirubin rises, itching develops, urine darkens, stools lighten, and jaundice follows, sometimes with only modest aminotransferase elevation. Stolz and colleagues described severe and protracted cholestasis in 44 young men taking bodybuilding supplements, a series that illustrates how prolonged and debilitating this can be even when it eventually resolves. Peliosis hepatis and hepatic adenoma have been reported with long-term high-dose use.

Selective androgen receptor modulators have accumulated their own case literature. Mohideen and colleagues described SARMs as an emerging liver toxin, and Nash and colleagues reported on drug-induced liver injury from selective androgen receptor modulators and anabolic-androgenic steroids in a clinical series. These are marketed as research chemicals with a safety profile superior to traditional androgens, and the hepatic case reports are a direct challenge to that marketing. They are not approved for human use anywhere, the products sold are frequently mislabelled in composition and dose, and the injury reported is sometimes severe.

Beyond compounds, the most common liver disease in this population is not exotic. It is metabolic dysfunction-associated steatotic liver disease, driven by adiposity and insulin resistance, and it is common enough that it should be considered in anyone with persistently raised ALT and a large waist, regardless of what else they are taking. Alcohol remains the other major contributor and is systematically under-reported.

Symptoms That Override Any Lab Reassurance

No interpretive framework survives contact with the following: yellowing of the skin or the whites of the eyes, dark urine with pale stools, persistent itching without a rash, right upper quadrant pain, unexplained nausea and appetite loss, or easy bruising. These indicate hepatic dysfunction rather than enzyme leak, and they require prompt medical assessment rather than a repeat test in ten days.

Testing Properly

If the goal is to know what your liver is doing, control the variable that confounds the measurement. Avoid heavy or novel training for at least three to five days before the draw, ideally seven. Avoid alcohol for the same period. Stay hydrated. Draw a full panel including GGT, ALP, bilirubin and albumin rather than a two-enzyme screen, and add CK so you can interpret the aminotransferases in context. Establish this baseline when you are healthy, because a single abnormal result is far easier to interpret when there is a normal one to compare it against.

Under the Tony Huge Laws of Biochemistry Physics, this is chain optimisation. A measurement is only as useful as your understanding of what feeds into it. AST and ALT have multiple inputs; interpreting them as though they have one is how people either panic unnecessarily or, far worse, dismiss a genuine cholestatic injury as gym enzymes and keep taking the compound causing it. The point of testing is to distinguish those two situations, and that requires the full panel and honest disclosure to whoever is reading it.

Where This Fits in the Bigger Picture

Monitoring is the part of enhancement that nobody posts about, and it is the part that decides how long you get to keep doing this. Testing gives you a feedback loop; without one you are guessing with your organs. Start with the Enhanced Athlete Protocol bloodwork guide, then work through the full protocol hub to see how testing, training, nutrition and recovery connect. If you are early in this, the beginners page is the right entry point.

Related: the supplements pillar covers hepatic considerations for common supplements, and the hormone pillar discusses oral versus injectable routes.

Frequently Asked Questions

Can working out raise liver enzymes?

Yes. AST and ALT are present in skeletal muscle as well as liver, and heavy resistance training, particularly unfamiliar or eccentric-heavy work, releases them into the bloodstream. Published research has shown these enzymes rising into ranges suggestive of liver disease in healthy men after a single heavy training session, with elevations lasting a week or more.

How can I tell if high liver enzymes are from muscle or my liver?

The most useful discriminator is GGT, which is present in hepatobiliary tissue but not skeletal muscle. A raised AST and ALT with a normal GGT, a high creatine kinase and a normal bilirubin, in someone who has trained hard recently, suggests a muscle source. Retesting after seven to ten days without heavy training usually clarifies the picture. Persistent elevation, raised GGT, or any rise in bilirubin points toward the liver and needs medical evaluation.

Are oral steroids bad for the liver?

Oral 17-alpha-alkylated androgens are modified to survive metabolism in the liver, and that modification is associated with hepatotoxicity. The typical pattern is cholestatic injury with rising bilirubin, itching and jaundice, and published case series document prolonged and sometimes severe illness. Rarer complications including peliosis hepatis and hepatic tumours have been reported with long-term high-dose use.

Do SARMs damage the liver?

Case reports and clinical series have documented drug-induced liver injury associated with selective androgen receptor modulators, and reviews have described them as an emerging cause of liver toxicity. These compounds are not approved for human use, products sold online frequently differ from their labelled contents, and the reported injuries have in some cases been severe. The claim that they are hepatically safer than traditional androgens is not well supported.

What should I do before a liver blood test?

To get a result that reflects your liver rather than your training, avoid heavy or unfamiliar exercise for several days beforehand, avoid alcohol over the same period, and stay well hydrated. Ask for a full panel including GGT, alkaline phosphatase, bilirubin and albumin rather than only the aminotransferases, and consider having creatine kinase measured at the same time so the results can be interpreted together.