ALT

What ALT Measures

ALT is a liver enzyme released into circulation when hepatocytes are damaged or stressed. The most common cause of elevated ALT is non-alcoholic fatty liver disease (NAFLD/MASLD) - affects ~25% of US adults. Less common but important causes include alcohol use, hepatitis (viral or autoimmune), medications, and acute liver injury.

The "normal" lab range (<55) masks meaningful clinical signal. Optimal ALT for metabolic health is <25 U/L. Sustained ALT 25–55 in a non-drinker usually means hepatic fat accumulation - actionable via weight loss, alcohol elimination, and metabolic intervention.

What Affects This Biomarker

ALT is influenced by: hepatic fat content (the primary driver), alcohol intake, body fat (visceral specifically), insulin resistance, viral hepatitis, medications (statins occasionally raise; high-dose acetaminophen; many supplements), and acute illness. GLP-1s reliably lower ALT via hepatic fat reduction; tesamorelin specifically reduces visceral and hepatic fat.

In the Context of Peptide Protocols

Track at baseline + every 3–6 months on any protocol. On GLP-1s expect 30–50% ALT reduction at maintenance dose - often the most reliable hepatic improvement signal. On HGH/tesamorelin protocols, watch for any rise (rare but possible). On peptide protocols generally, ALT shouldn't move much. Sustained elevation despite weight loss warrants hepatology workup.

Deep Dive

What ALT actually measures

Alanine aminotransferase is the enzyme that transfers an amino group from the amino acid alanine to alpha-ketoglutarate, producing pyruvate and glutamate. That reaction sits at the center of the glucose-alanine (Cahill) cycle, the shuttle that lets muscle export nitrogen to the liver for gluconeogenesis. ALT lives almost entirely in the cytosol of hepatocytes, with only trace amounts in kidney and muscle, which is what makes it more liver-specific than its cousin AST. AST is split between cytosol and mitochondria and is abundant in muscle and heart, so it is a noisier liver signal.

Two consequences follow. First, a rising ALT is a **leak** signal, not a **function** signal. It tells you hepatocyte membranes are turning over faster than usual, but says nothing directly about whether the liver is doing its job. True synthetic function is read from albumin, bilirubin, and INR. Second, ALT clears slowly, with a plasma half-life near 47 hours. An enzyme released today is still measurable days later, so after an insult resolves the number drifts down over roughly a week rather than snapping back. ALT also depends on vitamin B6 (pyridoxal-5-phosphate) as a cofactor, a detail that matters for interpretation below.

Why the optimal range sits far below the lab range

The gap between a sub-25 target and lab ceilings in the 45 to 55 range exists for a specific historical reason. The classic reference intervals were built decades ago from populations that unknowingly included people with fatty liver and undiagnosed hepatitis C. In other words, "normal" was calibrated against a metabolically unhealthy public. When **Prati and colleagues** rescreened healthy blood donors who were negative for viral and metabolic liver disease (Annals of Internal Medicine, 2002), the true healthy thresholds fell to roughly **30 U/L for men and 19 U/L for women**. That analysis is the reason the lower female cutoff exists, and why many clinicians now treat an ALT in the 30s as worth watching rather than reassuring.

One caveat keeps this from becoming "lower is always better." The relationship between ALT and long-term risk is **U-shaped**. Higher-normal ALT tracks with hepatic fat and metabolic risk, but very low ALT in older adults often reflects low muscle mass, frailty, or genuine B6 deficiency rather than a pristine liver. Context, not just a small number, is what to track over time.

Reading the magnitude, and staging beyond the enzyme

The height of an ALT elevation narrows the differential considerably.

- **Mild elevations**, roughly up to three to five times the upper limit, are usually chronic and metabolic: hepatic fat (MASLD, the 2023 consensus rename of NAFLD), alcohol, or a medication or supplement. - **Marked elevations** into the many hundreds or thousands point to acute hepatocellular injury such as acetaminophen toxicity, ischemic (shock) liver, or acute viral hepatitis. That is a same-day conversation with a clinician, not a tracking exercise.

Because a mildly raised ALT alone cannot distinguish simple fat from scarring, hepatologists lean on non-invasive scores that combine it with other routine data. **FIB-4** is the most common: FIB-4 = (age × AST) / (platelet count × square root of ALT). A result under 1.3 makes advanced fibrosis unlikely, while a result over 2.67 raises concern and typically prompts imaging such as elastography (Sterling et al., 2006). Logging AST and a platelet count alongside ALT lets you compute this yourself and gives an isolated enzyme number real context.

Testing pitfalls and timing

A few things reliably distort an ALT draw:

- **Exercise.** Heavy resistance training and long endurance efforts can transiently raise ALT, not only AST, because muscle carries some of the enzyme. An athletic user can show a "liver" flag that resolves with rest. Drawing after three to five relatively easy days isolates the hepatic signal. - **Vitamin B6.** Since the assay depends on P5P, a deficiency can falsely lower measured ALT and mask a real signal. That is one more reason a good-looking number in a frail or poorly-nourished person deserves a second look. - **Biological variation.** ALT runs modestly higher in the afternoon and swings day to day, so a single value is a snapshot. Confirm an unexpected result with a repeat draw before drawing conclusions. - **Fasting** is not strictly required, but sampling under consistent conditions makes the trend line honest.

Peptide and protocol tracking context

Anything that shrinks visceral and hepatic fat tends to pull ALT down as a downstream effect, which is why the enzyme is one of the most useful markers to log on a metabolic protocol. In the phase 2 semaglutide steatohepatitis trial (Newsome et al., NEJM 2021), ALT fell substantially alongside disease resolution, and enzyme normalization is now used as an early efficacy read in metabolic-liver drug development. Tesamorelin, a GHRH analog, reduced liver fat fraction in randomized trials of HIV-associated fatty liver, where treated participants were also less likely to show fibrosis progression, which makes ALT a reasonable marker to log on that kind of protocol. The 2024 approval of resmetirom (Rezdiffra) for MASH, where improved liver enzymes including ALT were part of the supporting evidence, underscores that this enzyme is now a regulatory-grade endpoint rather than a screening afterthought.

Two cautions for this audience. Growth hormone secretagogues and HGH antagonize insulin and can nudge hepatic metabolism, so read ALT alongside fasting insulin rather than in isolation. And the fastest way to spike ALT in optimization circles is not a peptide at all but methylated oral compounds, which are directly hepatotoxic. If ALT jumps, review oral agents and supplements first. Persistent elevation despite fat loss is a conversation to have with a clinician, not a number to chase on your own.

Peptides That Commonly Move ALT

Conditions That Track ALT

Frequently Asked Questions

Where This Fits in Your Panel

ALT is one marker on a fuller panel. The Peptide Blood Work Checklist lays out the complete baseline panel, what to add by protocol type, and when to retest.

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