L-Carnitine (ALCAR / Acetyl-L-Carnitine) vs L-Citrulline

Mechanism

L-Carnitine's primary function is transporting long-chain fatty acids across the inner mitochondrial membrane via the carnitine shuttle (CPT-I/CPT-II system) for beta-oxidation. Acetyl-L-Carnitine (ALCAR) additionally donates its acetyl group to form acetyl-CoA (bypassing pyruvate dehydrogenase) and serves as a precursor for acetylcholine synthesis. ALCAR crosses the blood-brain barrier, providing neuroprotective effects through mitochondrial energetics, reduction of lipofuscin accumulation, enhancement of NGF receptor sensitivity, and modulation of synaptic plasticity. It also reduces oxidative stress via upregulation of heme oxygenase-1.

Standard Dosing

500-2000mg ALCAR daily or 1000-3000mg L-Carnitine L-Tartrate (for exercise performance)

Timing

Morning on empty stomach for cognitive effects. Pre-workout for fat oxidation/performance. ALCAR for brain; L-Carnitine L-Tartrate for muscle/exercise.

Cycle Duration

ongoing

Side Effects

• Fishy body odor (TMA production by gut bacteria)
• GI upset/nausea
• Restlessness/insomnia (ALCAR)
• Increased appetite
• TMAO elevation (cardiovascular concern with L-Carnitine, less with ALCAR)

Contraindications

• Hypothyroidism (may worsen)
• Seizure disorders (conflicting data)
• TMAO concerns (see notes)

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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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