Protein supplementation enhances the synthesis of skeletal muscle protein. Such myofibrillar protein synthesis is thought to occur through activation of a signal cascade involving rapamycin-1 complex (a protein kinase that regulates cell growth, protein synthesis and cell proliferation amongst other cellular functions). Studies have shown that the stimulation of myofibrillar protein synthesis is dependent on the essential amino acids within protein, as use of non-essential amino acids do not have this effect. In particular it is the branched chain amino acid isoleucine, leucine and valine that have been identified as playing an important and dominant role in this stimulation of muscle growth through the activation of myofibrillar protein synthesis. Studies on animals and humans have shown that the stimulation of myofibrillar protein synthesis is dose dependent, with higher intake stimulating more protein synthesis. In addition the process is saturable, showing an upper limit to the benefits of ingesting protein in the diet.
Although administration of the branched chain amino acids has been shown to be required for the stimulation of myofibrillar protein synthesis, evidence indicates that it is leucine that may be the most important of the branched chain amino acids. In this regard researchers have assessed the effects of supplemental whey protein containing various concentrations of leucine after exercise1. Subjects performed knee extensions and then consumed either 25 grams of whey containing 3 grams of leucine, 6.25 grams of whey containing 0.75 grams leucine, 6.25 grams of whey containing 3 grams of leucine, 6.25 grams of whey containing 5 grams of leucine, or 6.25 grams of whey containing 5 grams of leucine, isoleucine and valine. As a control the same supplemental regimen was followed but no exercise was performed. Muscle biopsies were used to assess the effects of the treatments on muscle signalling pathways. The results showed that for 1.5 hours, all treatments stimulated myofibrillar protein synthesis.
However, between 1.5 and 4.5 hours post exercise, myofibrillar protein synthesis rates remained elevated above baseline levels, but this effect was more significant for the 25 gram whey protein containing 3 grams leucine and the 6.26 gram whey protein containing 5 grams of leucine, which increased myofibrillar muscle synthesis rates by 276 and 220 %, respectively. In addition the plasma leucine concentrations were higher following supplementation with the 6.25 grams of whey protein containing 5 grams of leucine, compared to the 6.25 grams of whey protein containing the 3 or 0.75 grams of leucine. These results therefore suggest that leucine fortified whey protein increases plasma leucine concentrations to a greater degree than unfortified whey. In addition, leucine fortified whey protein improves the myofibrillar protein synthesis rates to a greater degree than to a larger dose of whey with lower amounts of leucine. Leucine may therefore be a key component of the muscle building effects of whey protein.
Dr Robert Barrington’s Nutritional Recommendation: Fortification of whey protein with additional leucine may therefore be an effective consideration for those who undertake resistance training, as leucine appears to be pivotal in stimulating muscle protein synthesis in response to resistance training. However, interestingly there was no difference statistically between the muscle stimulating effects of leucine when taken at rest or after exercise. Therefore exercise is not a prerequisite for the stimulation of protein synthesis pathways by leucine. Practical recommendations based on this research suggest that while leucine may have protein stimulating effects, plain whey protein with a regular amount of leucine appears to be effective, especially when the cost of additional leucine is considered. Hydroxymethylbutyrate (HMB) is a metabolite of leucine, and studies have indicated that it too has myofibrillar protein stimulating effects. Leucine may therefore share common effects because it is an upstream metabolite of HMB.
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