L-Tyrosine

Cheap amino-acid precursor to dopamine, norepinephrine, and epinephrine that only works when those neurotransmitters are being burned faster than baseline synthesis can replace them — i.e., under acute stress, sleep deprivation, cold, or sustained high cognitive load. At baseline, in well-rested unstressed adults, it does essentially nothing. Best human evidence: combat-training cadets, military cold/sleep-deprivation paradigms, demanding multitasking. The marketing claim that NALT is "more bioavailable" is wrong — oral L-tyrosine raises plasma tyrosine 130-276%, while NALT raises it only 0-25% because deacetylation is inefficient. For Dylan: PRN tool only — pre-sparring days, pre-heavy-cognitive-load days, modafinil-washout days. Not daily-core.

The catecholamine pathway in plain English

Your body makes the "go-go-go" neurotransmitters — dopamine (DA), norepinephrine (NE), epinephrine (EPI) — from the amino acid tyrosine. The pathway is:

L-Tyrosine → (TH) → L-DOPA → (AADC) → Dopamine → (DBH) → Norepinephrine → (PNMT) → Epinephrine

The enzyme tyrosine hydroxylase (TH) does the first step and is the rate-limiting enzyme of the entire pathway. Anything that changes TH activity changes how much DA/NE/EPI you can make.

Why it (usually) doesn't matter at baseline

Under normal conditions, TH is more-or-less saturated with tyrosine — there is plenty of substrate, and neurons aren't firing fast enough to deplete it. Adding more tyrosine in this state is like adding more flour to a bakery that already has plenty: the bottleneck is somewhere else (oven space, baker's hands — i.e., TH activity, BH4 cofactor, vesicle loading), not the ingredient. This is why most baseline cognitive-enhancement trials of tyrosine in well-rested unstressed adults show null or near-null effects.

Why it matters under stress / sleep dep / cold

When neurons fire at high frequency (acute stress, sustained cognitive demand, hypothermia, sleep loss), several things happen that flip the bottleneck:

1. TH gets multiply-phosphorylated during high-frequency firing, which dramatically increases its affinity for its cofactor BH4 and increases the enzyme's V_max. The enzyme can now convert tyrosine much faster than baseline.
2. Local intracellular tyrosine pools deplete because the high-firing neurons are consuming the substrate faster than transport can replenish it.
3. Net effect: tyrosine availability becomes the new rate-limiting variable. Adding more substrate now produces more catecholamines.

This is the entire mechanistic basis for why tyrosine works in stress conditions but not at baseline. It's substrate replenishment for a bottleneck that only exists when neurons are firing hard.

LAT-1 / LNAA competition at the blood-brain barrier

Tyrosine is a Large Neutral Amino Acid (LNAA). It crosses the BBB via the LAT-1 transporter, which is a competitive transporter — at physiological concentrations it is more-or-less saturated, so all the LNAAs (phenylalanine, tryptophan, leucine, isoleucine, valine, methionine, histidine, tyrosine) compete for transport. Practical implications:

• High-protein meals raise plasma branched-chain amino acids (leucine/iso/val) and crowd tyrosine off the transporter. A whey-protein shake right before tyrosine dosing will measurably reduce brain tyrosine uptake.
• Pure tyrosine on an empty stomach is the high-uptake condition — minimal LNAA competition.
• Phenylalanine competes hardest because it is a larger LNAA with higher LAT-1 affinity. People taking phenylalanine separately are partially blocking tyrosine transport.

NALT: marketing vs reality

N-Acetyl-L-Tyrosine has an acetyl group bolted onto the amino nitrogen, which makes it more water-soluble. Supplement marketing leans on "better solubility = better bioavailability" to charge a premium. The pharmacokinetic data does not support this claim.

• Oral L-tyrosine: plasma tyrosine increases 130-276% over baseline (well-replicated)
• IV NALT: plasma free tyrosine increases only 0-25%; most of the NALT is excreted in urine unchanged because the human kidney clears it faster than tissue deacetylases can hydrolyze it
• Oral NALT: roughly comparable issue — only a small fraction gets deacetylated to free tyrosine that can cross the BBB

The deacetylation step is enzymatically inefficient in humans (acetylated tyrosine is not a high-Km substrate for the relevant N-acyl-amino-acid hydrolases). Most of the NALT dose is wasted. For raising plasma tyrosine, plain L-tyrosine is the correct form. NALT may have a niche only in niche IV nutrition contexts where its solubility matters; for oral supplementation it is inferior.

Pharmacokinetics (oral L-tyrosine)

• Onset: Plasma tyrosine begins rising within 30-60 min on empty stomach
• Peak: 90-120 min after dosing
• Duration: Plasma elevation persists 4-8 hours; does not return to baseline within 4 hours
• Dose-response: Roughly linear 50-150 mg/kg; some non-linearity at very high doses due to saturation of intestinal absorption

Downstream catecholamine handling matters

Tyrosine alone is just substrate. What actually happens to the DA/NE/EPI made from it depends on the rest of the pathway — BH4 availability, vesicle loading (VMAT2), reuptake (DAT, NET), and clearance (COMT, MAO-A, MAO-B, DBH). This is why pharmacogenomics matters (see Pharmacogenomics section) and why pairing tyrosine with compounds that act downstream (e.g., MAO-B inhibition by selegiline, DAT inhibition by modafinil, NE reuptake inhibition by bupropion) is mechanistically synergistic.