The balance between protein synthesis and breakdown in muscle is influenced by energy balance and exercise. A negative energy balance favors the loss of both fat body mass and lean body mass. Resistance training favors a net increase of lean body mass provided that the individual is at energy balance, so that diet is supporting the exercise she/he is performing. In fact, it has been pointed out that nutritional interventions in athletes may have their biggest impact on performance by supporting consistent intensive training and thus promoting the physiological and biochemical adaptations that will, in turn, lead to muscle hypertrophy and improved performance.75
There is some evidence that a high protein intake may favor a preferential loss of fat versus lean body mass during weight loss, by favoring muscle protein deposition, among other mechanisms (see Section 4.5.2). Likewise, there is some evidence that protein intake shortly after resistance training may facilitate exercise-induced muscle hypertrophy,76 although, overall, there is little evidence to support the premise that extra protein intake is essential for maximal performance in the athletes.77
The amino acid leucine has specifically been implicated in the promotion of net muscle protein anabolism, through effects on protein synthesis and/or protein breakdown (see references 78 and 79). In rats, it is well established that leucine promotes protein synthesis in muscle, mainly via activation of mTOR (mammalian target of rapamycin) kinases, which leads to the activation and increased expression of key translational factors and other proteins involved in protein synthesis (reviewed in reference 80). In humans, it appears that the main effect of leucine is on muscle protein breakdown, reducing breakdown without increasing muscle protein synthesis (reviewed in reference 79). The mechanism behind leucine-induced reduction of protein breakdown is not known, but it is noteworthy that synthetic leucine aldehyde peptides - such as N-acetyl-leucyl-leucyl-norleucinal (LLN) and CBZ-leucyl-leucyl-leucinal (MG132) - are commonly used inhibitors of an important component of the cell machinery for protein breakdown, the proteosome.
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