HMB (Beta-Hydroxy Beta-Methylbutyrate) vs L-Citrulline

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-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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