L-Citrulline vs L-Tyrosine

Mechanism

L-Citrulline is converted to L-arginine in the kidneys via argininosuccinate synthase and argininosuccinate lyase (bypassing hepatic first-pass metabolism that degrades oral L-arginine). L-arginine is then the substrate for endothelial nitric oxide synthase (eNOS), producing nitric oxide (NO). NO activates soluble guanylyl cyclase (sGC), increasing cGMP, causing vascular smooth muscle relaxation and vasodilation. Citrulline also participates in the urea cycle (ammonia detoxification), and supplementation increases plasma arginine more effectively than arginine supplementation itself due to bypass of intestinal and hepatic arginase.

Standard Dosing

3-6g L-citrulline daily or 6-8g citrulline malate

Timing

30-60 minutes pre-workout for exercise performance. For blood pressure: split AM/PM doses. Powder form in water.

Cycle Duration

ongoing

Side Effects

• Mild GI discomfort
• Bloating
• Headache (vasodilation)
• Generally very well-tolerated even at high doses

Contraindications

• Concurrent nitroglycerin/nitrate therapy
• Severe hypotension
• Urea cycle disorders (citrullinemia)

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Mechanism

L-Tyrosine is the precursor amino acid for catecholamine neurotransmitter synthesis: tyrosine hydroxylase converts tyrosine to L-DOPA (rate-limiting step), which is then converted to dopamine, norepinephrine, and epinephrine. It is also the precursor for thyroid hormones (iodination of tyrosine residues on thyroglobulin produces T3/T4) and melanin (via tyrosinase). Under conditions of acute stress, catecholamine turnover increases dramatically, depleting brain tyrosine pools. Supplementation provides substrate to maintain catecholamine synthesis during stress, sleep deprivation, cold exposure, and cognitive demand.

Standard Dosing

500-2000mg daily

Timing

Morning on empty stomach (competes with other large neutral amino acids for BBB transport). 30-60 min before stressful tasks or exercise.

Cycle Duration

As needed or cycle 4-8 weeks on, 2 weeks off

Side Effects

• Headache
• GI upset
• Irritability
• Anxiety (excess catecholamines)
• Insomnia if taken late
• Heart palpitations (high doses)

Contraindications

• MAO inhibitor therapy
• Hyperthyroidism
• Melanoma (tyrosine is a melanin precursor)
• Phenylketonuria (tyrosine from phenylalanine metabolism)

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