A protein building block the body cannot make and must obtain from food; almost anyone eating adequate protein already takes in several times the amount needed. Controlled human testing shows one small effect — slight easing of involuntary muscle tightness after nerve damage, called too small to matter. Slower aging, gut and sleep claims rest on animals. Cheap and well tolerated. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Plasma threonine, fasting | 120–180 µmol/L | Confirms a shortfall exists before dosing |
| Aspartate aminotransferase (AST) | 15–25 U/L | Detects the enzyme rise reported at 9 g daily |
| Alanine aminotransferase (ALT) | 10–25 U/L | Separates liver from muscle as the source of an AST rise |
| Creatine kinase (CK) | 40–150 U/L | Identifies muscle as the source of an enzyme rise |
| Serum vitamin B12 | 500–900 pg/mL | Confirms the propionate route that clears threonine is supported |
| Methylmalonic acid (MMA) | Below 0.27 µmol/L | Functional test of the propionate disposal pathway |
| Estimated glomerular filtration rate (eGFR) | Above 90 mL/min/1.73 m² | Establishes whether a prescribed protein restriction governs the dose |
| High-sensitivity C-reactive protein | Below 1.0 mg/L | Flags the active inflammation during which threonine is deferred |
| Fecal calprotectin | Below 50 µg/g | Detects an active bowel flare |
Cadence: Baseline before starting; liver and muscle enzymes repeated at 8 weeks after reaching the target dose, then at 6 months, then annually while the dose is stable; plasma amino acids and methylmalonic acid repeated at 6 months and annually thereafter.