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Protein Synthesis

L-Leucine Insulin and mTOR


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mTOR Activation

Stimulating protein synthesis


  • “Insulin is the most potent anabolic hormone known, and promotes the synthesis and storage of carbohydrates, lipids and proteins, while inhibiting their degradation and release into the circulation43.”
  • “Although the mechanism of activation of mTOR remains unclear, it seems to require the presence of amino acids in the media for full activation by growth factors, and thus may also represent a nutrient sensor.43
  • Signalling through mTOR [mammalian target of rapamycin], is through amino acids and in particular the mTORC1 type.
  • Starving cells of amino acids will within minutes impair mTORC1 signalling and will make mTORC1 refractory to stimulation by insulin44.


  • Having carbohydrate without a full complement of amino acids means that mTORC1 signalling will be impaired and the insulin response of a carbohydrate load will not lead to protein synthesis.
  • Protein and in particular Whey Protein Isolate and L-Leucine [a Branched Chain Amino Acid - BCAA],  are powerful stimulators of insulin and protein synthesis with or without carbohydrate. 
  • The study by Esmarck et al8 showed that protein synthesis was not dependent on insulin but by hyperaminoacidaemia [high blood amino acid levels]
  • So why use an iso-osmotic glucose polymer with protein post-exercise - the reason is to re-establish glycogen stores rapidly. Only high intensity exercisers who take this level of exercise beyond 90 minutes will deplete muscle and liver glycogen and 'hit-the-wall' as a result.
  • The vast majority of individuals going to the gym to exercise for weight loss, toning, sculpting and even hypertrophy won't be exercising to this level of intensity and so glucose polymers should be avoided unless necessary as this will only increase caloric intake unnecessarily.

  • L-Leucine appears to be the important essential amino acid that stimulates mTORC1 signalling44.

  • Insulin binds to its receptor and causes the GLUT4 receptors bound to the membranous vesicle to migrate to the adipose cell membrane [a process called translocation]22.
  • Glucose can be used for energy, for glycogen synthesis after exercise or for fatty acid production to store energy as fat if there is a calorie excess.

The myth that large amounts of protein are needed for protein synthesis

  • There is a distinct feeling among individuals who perform resistance training for strength or hypertrophy that a large amount of protein is needed to stimulate protein synthesis.
  • Studies by Rasmussen45 and Esmarck8 used only 6g and 10g respectively in their studies to show the benefit of post-exercise protein ingestion in relation to protein synthesis.
  • In her book 'The Complete guide to Food for Sports Performance: Peak Nutrition for Your Sport', Dr Louise Burke stated:
    "10-20 of a good-quality protein seems the ideal size of a protein recovery snack. There is substantial increase in protein synthesis with a 10g protein serve, but the response seems to level out at 20-25 grams. In fact, more that 25 g of protein will lead to increased use of protein as a fuel source which negates the purpose of consuming protein46."
  • It is important to remember that protein excess beyond your caloric needs can be converted to fat easily and this is a biochemical fact, plain and simple.
    See Protein and Fat Accumulation

Link to Exercise Topic References

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