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The leucine threshold

In order to maximize the synthesis of muscle proteins, we must remember about the proper supply of proteins (we recommend reading: proteins). Proteins are made of amino acids. The most important from the point of view of muscle mass is the exogenous amino acids, i.e. those that the body is unable to synthesize and we must provide them with food. We can safely say that one of them should focus a bit longer due to its unique properties and a kind of uniqueness.

Leucine, because it will be the main topic of today’s article, is the initiator of the muscle protein synthesis process. Put simply – the appropriate concentration of leucine “lights the lamp” in the muscles, so that these begin their growth.

A short reminder
Both during the building of muscle mass and weight reduction, the main stimuli determining the success of the planned nutritional strategy is the intake of adequate amount of energy and protein throughout the day. Nevertheless, there are other factors in optimal building of muscle mass, such as the number of meals per day, protein content, essential amino acids (EAA), leucine and digestive kinetic of consumed protein.

Synthesis more important than disintegration.
Our muscles are constantly damaged and rebuilt. During the day, 1-2% of proteins in our body are remade [1]. The balance of muscle tissue depends on two processes called muscle protein synthesis (MPS) and muscle protein breakdown (MPB). We are dealing with the mass building when MPS dominates MPB. Analogically, when MPB surpasses MPS, we are dealing with catabolism or the breakdown of muscle tissue. Since we are dealing with MPB and MPS chronically, logic tells us that in order to optimally build muscle mass, we should maximally inhibit the breakdown of muscle tissue and intensify its synthesis.

In practice, however, it is a bit different. Changes in MPS dictated by training and eating are much greater than differences in MPB in response to these factors. By using a meal, we are able to slow down MPB by 50% (this is the maximum value), which happens due to a small increase in insulin concentration (30 mU / l) [2]. Importantly, the insulin release that maximally inhibits MPB can be achieved by ingesting only 25 g of whey protein isolate. The addition of carbohydrates to this amount of protein in no way affects MPB [3], so in practice, every meal will inhibit MPB to the same extent. Therefore, dreaming about big muscles, we should focus on maximizing the muscle protein synthesis process … and here leucine enters the game.

The concept of leucine threshold.
Leucine intensifies the synthesis of muscle proteins without the addition of other exogenous amino acids [4]. The significance of the leucine threshold is shown by two very similar studies. The first of them assessed the impact of five different supplementation protocols on MPS, and these were:
• 6.25 g of whey,
• 6.25 g whey + 2.25 g leucine (total of 3 g leucine),
• 6.25 g whey + 4.25 g leucine (total 5 g leucine),
• 6.25 g whey + 6g branched-chain amino acids (BCAA) (total 4.25 g of leucine),
• 25 g whey (total of 3 g leucine).

Three protocols proved to be the most beneficial – 25 g whey 6.25 g whey + 2.25 g leucine and 6.25 g whey + 4.25 g leucine, which gives basis to believe that the addition of leucine to the meal can compensate for the suboptimal supply of other amino acids exogenous, thus maximizing the synthesis of muscle proteins [4].

In the second study, the same author tested the same protocols and, to my surprise, 6.25 g of whey + 2.25 g of leucine did not increase MPS to the same extent as 6.25 g of whey + 4.25 g of leucine [5]. The only difference between the works was the fact that in the primary study they only received conditioner + leucine, and in the second, carbohydrates and fats were added to the shake, which may delay the absorption of amino acids and thereby reduce the leucine peak in the blood (and muscles) after a meal.

This gives a basis to believe that the kinetics of absorption (ie how fast the protein will be digested and absorbed) can affect the MPS process. This was confirmed by Pennings et al. [6] Evaluating the effect of whey, hydrolyzed casein and casein on the MPS response. The optimal choice turned out to be whey protein characterized by high leucine content and rapid digestion (thanks to which the peak was immediate and significant). The second place was taken by hydrolyzed casein, which is digested quickly but amount of leucine is less than in whey, and third place has unhydrolyzed casein, which not only has little content of leucine, but it also digests slowly. This leucine “peak”, which will maximize MPS, is called the leucine threshold. The value of the leucine threshold is individual and depends mainly on the degree of physical activity and age. Younger physically active people need less leucine than older, inactive. According to the ISSN, the content of leucine in each athlete’s meal should be 0.7-3 g or about 20-40 g of a full-value protein [7].

Leucine supplementation.
As in the previous studies [4,5], most of the leucine-dose work that maximizes MPS is based on the addition of the amino acid in the form of a supplements rather than typical meals, which may affect practical guidelines. In addition, Murphy et al. [8] showed that the addition of 5 g leucine to the three main meals in people using the low diet (0.8 g / kg) and high (1.2 g / kg body mass) protein content contributed to MPS growth. Interestingly, the differences in MPS between low and high protein supply groups were unnoticeable. Based on the above information, it can be concluded that supplementation of 5 g leucine 15-30 minutes before a conventional meal can maximize MPS and thus, hypothetically increase the content of muscle mass. Consumption of protein in the context of MPS has two functions, the first is the initiation of the process due to exceeding the leucine threshold, and the second is the supply of substrates in the form of essential amino acids to build muscle mass.

In summary, the leucine threshold is the leucine concentration achieved in the post-meal tissue, which maximally stimulates the muscle protein synthesis process. In order to exceed the leucine threshold, it is recommended to consume 0.7-3 g of leucine in each meal. There are scientific premises that supplementation with leucine 15-30 minutes before a meal helps to intensify MPS, which hypothetically could translate into the increased content of muscle tissue.

References:
1. Welle S., Thornton C., Statt M., McHenry B., Postprandial myofibrillar and whole body protein synthesis in young and old human subjects. „Am J Physiol.” 267(4 Pt 1), 1994, 599-604.
2. Greenhaff P.L., Karagounis L.G., Peirce N. et al., Disassociation between the effects of amino acids and insulin on signaling, ubiquitin ligases, and protein turnover in human muscle. „Am J Physiol Endocrinol Metab.” 295(3), 2008, 595-604.
3. Staples A.W., Burd N.A., West D.W. et al., Carbohydrate does not augment exercise-induced protein accretion versus protein alone. „ Med Sci Sports Exerc.” 43(7), 2011, 1154-61.
4. Churchward-Venne T.A., Breen L., Di Donato D.M.et al., Leucine supplementation of a low-protein mixed macronutrient beverage enhances myofibrillar protein synthesis in young men: a double-blind, randomized trial. „Am J Clin Nutr.” 99(2), 2014, 276-86.
5. Churchward-Venne T.A., Burd N.A., Mitchell C.J. et al., Supplementation of a suboptimal protein dose with leucine or essential amino acids: effects on myofibrillar protein synthesis at rest and following resistance exercise in men. „J Physiol.” 590(11), 2012, 2751-65.
6. Pennings B., Boirie Y., Senden J.M., Gijsen A.P., Kuipers H., van Loon LJ., Whey protein stimulates postprandial muscle protein accretion more effectively than do casein and casein hydrolysate in older men. „Am J Clin Nutr.” 93(5), 2011, 997-1005.
7. Jäger R., Kerksick C.M., Campbell BI., International Society of Sports Nutrition Position Stand: protein and exercise. „J Int Soc Sports Nutr.” 20, 2017, 14-20.
8. Murphy C.H., Saddler N.I., Devries M.C., McGlory C., Baker S.K., Phillips S.M., Leucine supplementation enhances integrative myofibrillar protein synthesis in free-living older men consuming lower- and higher-protein diets: a parallel-group crossover study. „Am J Clin Nutr.” 104(6), 2016, 1594-1606.

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