HMB (Beta-Hydroxy Beta-Methylbutyrate) vs L-Carnitine (ALCAR / Acetyl-L-Carnitine)

Mechanism

HMB is a metabolite of leucine (produced endogenously at ~0.3g/day from typical leucine intake). It reduces proteolysis by inhibiting the ubiquitin-proteasome proteolytic pathway and attenuating caspase-mediated apoptosis in muscle cells. HMB also stimulates muscle protein synthesis via mTOR/p70S6K pathway activation (though less potently than leucine itself) and enhances sarcolemmal membrane integrity by serving as a precursor for de novo cholesterol synthesis in muscle cells (via HMG-CoA). The net effect is anti-catabolic rather than primarily anabolic — it prevents muscle breakdown more than it builds new muscle.

Standard Dosing

3g daily (1g 3x/day)

Timing

Split into 1g doses with meals (HMB-Ca) or 1-2g 30-60 min pre-exercise (HMB-FA for rapid absorption). Timing matters more for HMB-FA (free acid) than HMB-Ca (calcium salt).

Cycle Duration

ongoing during training periods

Side Effects

• Generally very well tolerated
• Mild GI discomfort
• Bloating

Contraindications

• Few known. Renal impairment (monitor at high doses).

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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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