Protecting muscle mass with leucine

As people age retaining muscle mass is harder. The loss of muscle mass is termed sarcopenia. This is the Greek term for “loss of flesh”. Human muscles are typically classified as either Type I or Type II. Sarcopenia affects Type II muscles. These are the muscles that contact quickly and are associated with power and strength. Type I muscles are associated with physical endurance and are slower to contract.

Exercising can help to lessen the loss of muscle mass as the body ages. However, the rate of Type II disintegration persists. The body slows the production of new muscle fibres and instead increases the bulk of existing fibres. Strength and coordination also decreases due to less control of muscle activation.

By the time people reach their forty’s muscle mass starts to decline and this accelerates by the time people reach their fifty’s.  However, there are ways to slow this decline and help to maintain muscle mass, strength and coordination.

How do amino acids affect muscle mass?

Researchers from Zhejiang University in China have been investigating the effects of amino acids on sarcopenia. Specifically, Zhe-rong Xu and colleagues focused on leucine. One of the nine essential amino acids, leucine is particularly beneficial for muscle building.

Many body builders subscribe to a diet high in leucine or take supplements enriched with this amino acid. It is one of the three branched-chain amino acids (BCAAs) together with valine and isoleucine.  Compared to any other amino acid, leucine is the most powerful synthesiser of protein.

One of the ways in which muscle growth is stimulated is through a pathway called the “mechanistic target of rapamycin”, more commonly referred to as the mTOR pathway. This pathway is very sensitive to leucine concentration and responses to the presence of this amino acid by building muscle1, 2.

Decreasing levels of leucine will signal to the mTOR pathway that new skeletal muscle production must cease. This reduces the production of new muscle. Even with regular exercise muscle mass is unable to build from new fibres and must rely on the thickening of existing fibres. Consequently, Zhe-rong Xu and colleagues investigated how altering the dietary intake of leucine may help to prevent sarcopenia as people age.

Increasing dietary intake of leucine may stop muscle wasting

Zhe-rong Xu and colleagues conducted a meta-analysis review of nine published clinical studies that investigated the effects of leucine on muscle mass3.

These randomised control trials assessed protein synthesis and the secondary outcome of lean body and leg mass. In their review, researchers found that protein synthesis did increase in response to leucine supplementation. No changes were observed for leg or body lean mass.

Consequently, Zhe-rong Xu and colleagues concluded that “leucine supplementation is useful to address the age-related decline in muscle mass in elderly individuals, as it increases the muscle protein fractional synthetic rate.”

Leucine supplementation

There are many foods rich in leucine. These include nuts, seeds, fish, seafood, cheese, soybeans, beef, chicken, and pork. Increasing the dietary intake of these foods can help to bolster leucine levels, especially as people get older. There are also supplements available which include this amino acid.  As the body gets older, physical demands increase and the body’s response is not as efficient. Consequently, good quality supplements can be very beneficial.


  1. “Anthony J. (2000). Leucine stimulates translation initiation in skeletal muscle of postabsorptive rats via a rapamycin-sensitive pathway. Journal of Nutrition. Volume 130, Issue 10, (pp. 2413-9).”
  2. “Bolster, D. (2002). AMP-activated protein kinase suppresses protein synthesis in rat skeletal muscle through down-regulated mammalian target of rapamycin (mTOR) signaling. Journal of Biological Chemistry. Volume 277, Issue 27, (pp. 23977-80).”
  3. “Xu, Z. (2014). The effectiveness of leucine on muscle protein synthesis, lean body mass and leg lean mass accretion in older people: a systematic review and meta-analysis. British Journal of Nutrition, Volume 19, (pp. 1-10).”