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Aging muscle needs a bigger protein signal
Turnover With Age
The muscle a person has today is not the muscle they will have in 10 years, because the body keeps breaking down and rebuilding tissue. That turnover grows less efficient with age. Young children are highly anabolic and build on small amounts of protein.
The Signals
Three influences build muscle: mechanical tension, hormones, and amino acids, especially leucine. They all feed into mTOR, a protein kinase at the center of muscle protein synthesis. When hormones are robust, as in youth, the body depends less on leucine, and enough protein through leucine lowers the dependence on hormonal status.
The Leucine Threshold
Muscle ages and responds only when a meal clears a leucine threshold. A breakfast of two eggs and fruit delivers protein, yet it may never stimulate muscle. Activity shifts the balance as well, on a U-shaped curve: marathoners need more protein, and sedentary people need more because they have no mechanical tension to lean on. The worst combination is a sedentary aging woman on a low-protein diet.
Why does aging muscle respond less to protein?
Older adults show anabolic resistance: the same amount of amino acids raises muscle protein synthesis less than it does in the young. In a 2005 study, Daniel Cuthbertson and colleagues at the University of Nottingham found that older men had a smaller synthesis response to essential amino acids than younger men. Proposed causes include weaker mTOR signaling and reduced blood flow carrying amino acids to muscle.
Why is leucine special for muscle?
Leucine is one of the branched-chain amino acids, and cells sense it through a protein called Sestrin2, identified by David Sabatini's lab in 2016. When leucine binds, Sestrin2 releases its hold on a pathway that switches on mTORC1, the complex that starts muscle protein synthesis. Whey and dairy proteins are especially rich in leucine, which is why they are widely used in muscle research.
How did scientists discover mTOR?
A soil sample from Easter Island in the 1960s yielded rapamycin, a compound made by a bacterium. In the 1990s, researchers identified the protein it blocks and named it TOR, for target of rapamycin. The mammalian version became mTOR. Its name still carries the island's Polynesian name, Rapa Nui.
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