Research
Hormonal
Oral supplementation with beta-hydroxy-beta-methylbutyrate (HMB) increases plasma concentrations to levels sufficient to activate mTORC1 and stimulate protein synthesis, whereas endogenous conversion from oral leucine fails to reach these effective thresholds.
If your goal is maximizing muscle protein synthesis via the mTORC1 pathway, direct HMB supplementation is more effective than relying on your body to convert Leucine into HMB. Direct HMB intake raises plasma levels to ~400 µM, while Leucine only raises it to ~10 µM, which is insufficient for significant anabolic effects.
GoodQualifiesHIGH confidence
HMB ingestion (2.42 g) by participants was found to increase the plasma HMB concentration to 400 µM; however, ingestion of leucine (3.42 g) increased plasma HMB levels to 10 µM [49]. Therefore, endogenous HMB derived from the ingestion of leucine does not appear to be a major contributing factor in skeletal muscle anabolism.
Why this rating
The paper cites specific human plasma concentration data comparing HMB and leucine ingestion, though it is a review summarizing other studies.
Source
Regulation of Skeletal Muscle Function by Amino Acids
Yasutomi Kamei et al. · Nutrients · 2020
DOI 10.3390/nu12010261
narrative_reviewCited 246×
Read the paper DOI resolved against Crossref · corpus check 2026-06-10
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