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

Mechanism

Glycine is the simplest amino acid with profound neurological and metabolic roles. It is an inhibitory neurotransmitter acting at glycine receptors (strychnine-sensitive) in the brainstem and spinal cord, inducing a drop in core body temperature that facilitates sleep onset. It is also an obligatory co-agonist at the NMDA receptor glycine binding site, modulating excitatory neurotransmission. Metabolically, glycine is the rate-limiting amino acid for glutathione synthesis (glutathione = glycine + cysteine + glutamate), a key substrate for collagen synthesis (every 3rd amino acid), essential for creatine synthesis, bile acid conjugation, heme synthesis, and one-carbon metabolism.

Standard Dosing

3-5g daily

Timing

3g before bed for sleep (core body temperature reduction). Divided doses during day for glutathione support. Powder in water has a mildly sweet taste.

Cycle Duration

ongoing

Side Effects

• Mild drowsiness
• Soft stools at high doses
• Nausea (rare)
• Generally extremely well-tolerated

Contraindications

• Very few absolute contraindications. Caution in severe renal disease.

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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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