In neonatal pigs, long-term leucine infusion does not increase protein synthesis in the heart, kidney, or jejunum, despite measurable mTOR activation in some tissues, indicating tissue-specific responsiveness to leucine and suggesting that not all organs are equally sensitive to leucine’s anabolic signaling.

When leucine levels rise, it turns on a molecular switch called mTORC1 that tells cells to start making proteins. But if other amino acids are not present in sufficient amounts, the cells cannot build proteins even though the switch is on. This happens in some organs like the heart, kidney, and jejunum, where leucine activates the switch but the lack of other building blocks stops protein production.