Can normal ALT and AST miss functional hepatobiliary stress?

Normal aminotransferases do not reliably exclude clinically important liver or biliary dysfunction. The proposed network linking erythrocyte turnover, bilirubin handling, iron metabolism, and oxidative stress is biologically well grounded; methylation-related effects are credible but remain more indirect.

Clinical significance

• ALT and AST primarily reflect hepatocellular enzyme release, not bilirubin transport, bile flow, fibrosis burden, or overall hepatic clearance capacity. Biopsy-confirmed MASLD/NASH cohorts include people with normal aminotransferases despite NASH, advanced fibrosis, and cirrhosis.
• AASLD advises that aminotransferases alone should not be used to exclude significant NASH or advanced fibrosis. Proposed “true-normal” ALT ranges (about 29–33 U/L in men and 19–25 U/L in women) may also be below many laboratory upper limits.
• Cholestatic/hepatobiliary processes may be better reflected by alkaline phosphatase, GGT, bilirubin, and imaging; early or partial cholestasis can still have normal bilirubin.

Mechanistic basis

• Senescent red cells are processed by splenic and hepatic macrophages. Heme degradation generates bilirubin precursors and recycles iron; hemoglobin turnover supplies most bilirubin in healthy adults.
• Hepatic UGT1A1 conjugates unconjugated bilirubin, and MRP2 exports bilirubin glucuronides into bile. Iron is managed through ferritin, ferroportin, and hepcidin.
• Excess heme or unliganded iron can drive reactive-oxygen-species formation. Glutathione redox status may affect UDP-glucuronate availability and thus glucuronidation substrate supply; bilirubin–biliverdin cycling may provide antioxidant protection and support canalicular transport.
• One-carbon metabolism may influence this network through transsulfuration-derived cysteine for glutathione synthesis, although direct human evidence tying methylation capacity to bilirubin-clearance outcomes is limited.

Bottom line

• A normal ALT/AST pattern can coexist with functional hepatobiliary stress or significant liver disease. When clinical concern exists, interpretation should incorporate metabolic risk, broader liver tests, fibrosis assessment (for example FIB-4 and elastography when indicated), and targeted biliary imaging rather than aminotransferases alone.