β-Hydroxy-β-methylbutyrate (HMB)HMB · β-羟基-β-甲基丁酸

Place · Level 3 · Supplement

亮氨酸代谢中间产物 · 主作用是抗分解不是促合成 · 老年保肌 + 卧床抗萎缩 B-A 证据 · 健康年轻运动员边际小

Leucine 5% derivative

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身份 · 亮氨酸下游 5%

HMB (β-hydroxy-β-methylbutyrate) is an intermediate in the leucine metabolic pathway, not an essential nutrient.

Metabolic pathway:

Leucine (Leu) → transamination → α-ketoisocaproate (KIC)Normal route (95%): KIC → KICD → isovaleryl-CoA (IVA-CoA) → enters breakdown cycle → energyHMB route (5%): KIC → KIC dioxygenase (KICD) → HMB → HMG-CoA → cholesterol / ketone synthesis
Key facts:

Your body already produces a small amount of HMB every day (~0.2–0.4 g/day, depending on leucine intake)Food HMB content is trivial: avocado, grapefruit, cauliflower in trace amounts (mg level)Reaching the research dose of 3 g/day is essentially only possible via supplementation
Two commercial forms:

HMB-Ca (calcium salt): most common, cheap, but fast-peaking — 90 min to peak then dropsHMB-FA (free acid): more expensive, faster absorption (30–60 min peak) with more stable bioavailability; some research prefers this form
HMB-leucine equivalence math:

1 g HMB ≈ 60 g leucine (reverse-calculated from 5% conversion rate)But the body uses 'leucine' and 'HMB' for different functions (next scene), so this is not a direct conversion — it is theoretical raw-material coverage3 g HMB/day ≈ 60 × 3 = 180 g leucine (far beyond any diet), which is HMB's commercial pitch as a concentrated leucine-derivative
Why supplement HMB instead of leucine directly:

Leu is an anabolic activator ( trigger, see protein/muscle L4)HMB is an anti-catabolic signal (next scene); it is not a simple leu substitute but a specialized downstream fate of leu

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