Hormonal
Elevating forearm insulin to high physiologic levels (approx. 157 uU/ml) blocks approximately 74% of skeletal muscle amino acid nitrogen release in postabsorptive humans, primarily by inhibiting the output of branched-chain and aromatic amino acids.
In a fasted state, raising insulin levels significantly stops muscle from breaking down and releasing most amino acids (like leucine and isoleucine) into the blood. However, this effect is not universal; alanine release continues largely unchanged. This suggests insulin's primary job in fasting is to preserve muscle protein by stopping its breakdown, rather than actively building new muscle tissue from scratch.
Elevation of forearm insulin in eight subjects from postabsorptive (12 uU/ml) to high physiologic levels (157 /IU/ml) in addition to stimulating muscle glucose uptake blocked muscle alpha amino nitrogen release by 74%. Consistent declines in output were seen for leucine, isoleucine, tyrosine, phenylalanine, threonine, glycine, and a-aminobutyric acid.
Why this rating
Controlled human study with direct intra-arterial infusion allowing precise local dosing and measurement, though sample size is small (n=8 for high dose).
Source
Amino acid balance across tissues of the forearm in postabsorptive man. Effects of insulin at two dose levels
Thomas Pozefsky et al. · Journal of Clinical Investigation · 1969
DOI 10.1172/jci106193
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