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What is it Leucine
Leucine is an essential branched-chain amino acid (BCAA). Amino acids are the building blocks of protein.
An essential amino acid is considered an amino acid that our body cannot produce, and we must obtain it through the consumption of foods that contain it. Leucine is required for the growth and repair of muscles, skin and bones.
Leucine is thought to be the only amino acid that can stimulate muscle growth and help prevent muscle deterioration with age.
What is Leucine’s role in the body?
Leucine, along with isoleucine and valine, is one of the three branched-chain amino acids (BCAAs).
Compared to the other two amino acids, leucine is the most potent activator of protein synthesis. Cells are able to detect leucine levels and in response activate protein synthesis through an enzyme (mTOR), a master regulator of protein synthesis.
It also helps to better regulate blood sugar levels. It stimulates the secretion of insulin by the pancreas and thus facilitates glucose uptake by cells. In a state of starvation, it acts as a source for the synthesis of glucose from non-carbohydrate sources. It acts as a muscle healer and increases insulin sensitivity in skeletal muscles.
mTOR is a complex of two components, one of which is activated by leucine. This complex includes mTOR which works in parallel with other proteins such as Raptor (TOR-associated regulatory protein), GβL (G-protein β-subunit like protein) and PRAS40 (proline-rich PKB/Akt substrate of 40kDa) .
This combination of proteins works to facilitate a myriad of cellular signals which then enhance protein synthesis.
In simpler terms, increased leucine stimulates mTOR , which in turn stimulates a specific enzyme (i.e. p70S6 kinase) via a genetically encoded protein pathway. P70S6 kinase then positively enhances muscle protein synthesis .
Given the well-established protein synthesis activating properties of leucine, several human trials have been conducted to determine whether adding supplemental leucine to various protein sources can increase muscle protein synthesis; results in older individuals have been consistently positive.
Anabolic resistance is an established phenomenon during aging, with cells becoming more resistant to activation of protein synthesis. This may be improved in part by additional leucine, as research shows that older people need twice as much leucine compared to younger adults for similar activation of muscle protein synthesis.
This increased leucine requirement for muscle protein synthesis in the elderly may be partially explained by the increased retention of leucine taken orally in the gut, which is retained twice as much as in young adults, limiting the amount of leucine entering the bloodstream.
Resistance to leucine-stimulated protein synthesis also occurs at the cellular level in aged cells. Studies of leucine supplementation in younger adults have been positive, with some caveats.
In a study comparing a low (6 g) dose of whey protein plus 3 or 5 g of extra leucine with 25 g of whey protein alone, the 6 g whey meal plus 5 g of leucine stimulated muscle protein synthesis to a similar, but lesser degree than 25 g of whey protein. 
Another study comparing the same dose of 25 g whey protein to 6 g whey protein with an amount of leucine equivalent to 25 g whey protein found that only the 25 g whey protein group had increased muscle protein synthesis 3 to 5 hours after exercise in young adults. 
Overall, this work suggests that supplemental leucine can increase muscle protein synthesis in the context of limited total protein, but a 25 g dose of whey whey alone is probably better, at least in non-elderly adults.
Leucine and athletic performance
Leucine is by far the most “anabolic” (protein synthesis activating) amino acid, but the results of studies differ.
Scientists first published in 1986 that amino acids are required to activate protein synthesis in rat skeletal muscle. Subsequent work looked at which amino acids or mixtures of amino acids were able to stimulate protein synthesis (mTOR activity) in cultured cells.
One particularly informative study examined the ability of amino acid mixtures lacking certain amino acids to activate mTOR signaling; both leucine and arginine were found to be essential for mTOR activation in cultured cells, but protein synthesis was suppressed overall when cells lacked the remaining 18 amino acids. 
Leucine deficiency had the strongest effect, reducing protein synthesis by 90%, compared to an 81% reduction when the amino acid mixture had no arginine.
The study also revealed that neither leucine nor arginine alone could stimulate maximal protein synthesis, with arginine or leucine alone stimulating protein synthesis by only 7.1% or 10.8%, respectively, compared to the full complement of amino acids. Leucine and arginine in combination did a little better, but not much.
Leucine and arginine combined only stimulated protein synthesis at 28% of the maximum levels reached when all 20 amino acids were consumed.
Both leucine and arginine alone are capable of activating protein synthesis in cultured cells, but leucine is more potent than arginine.
Neither leucine alone, nor arginine alone, nor leucine and arginine combined were sufficient to fully activate protein synthesis in the absence of other amino acids.
Subsequent research examining the effects of individual essential amino acids found that although each of the essential amino acids (EAAs, valine, leucine, isoleucine, phenylalanine, tryptophan, lysine, histidine, methionine and threonine) is capable of stimulating protein synthesis in cultured cells to some extent, leucine was the most potent.
Although studies in cultured cells have consistently identified leucine as the amino acid that most stimulates protein synthesis, the degree to which this occurs depends on the cell type, as leucine is the only amino acid that can activate the protein synthesis mechanism in adipocytes that are depleted of amino acids.
It should be emphasized that this or leucine’s anabolic effect is further enhanced by muscle contraction during physical activity, with some researchers suggesting that there is an additional benefit of using it before exercise.
After exercise, our muscles are in a state of negative protein balance, where degradation rates outweigh synthesis. Combining muscle contraction with adequate leucine allows for both muscle recovery and enhanced synthesis after exercise.
For example, in a study with college-aged men, researchers found that supplementation with leucine did not improve strength or skeletal muscle mass during a 3-month trial period. Researchers did, however, see cellular changes in muscle that may yield benefits if supplementation and training continued longer. 
Another study published in 2017 found that taking leucine supplements (3 grams per day after training) did not increase strength or muscle mass in healthy young men who consumed enough protein overall. 
However, other studies have shown that leucine supplements may help boost muscle mass during intense strength training. Further research on leucine supplements for athletes is ongoing.
Leucine and weight loss
For several years, researchers have been studying the effect of leucine on weight loss. Some scientists believe that leucine can help the body maintain muscle mass when one follows a weight loss program.
Maintaining muscle mass is important for both initial weight loss and weight maintenance because these muscles help the body burn more calories each day. Other scientists believe that leucine can help boost glucose and insulin homeostasis  – a great benefit for dieters who have a constant appetite for food.
There is research suggesting that leucine helps to reduce appetite and reduce weight, but no general conclusion can be drawn because they were either done over a short period of time, in a small sample of individuals or in a sample of mice.
Research on l-leucine supplements has failed to show statistically significant results that leucine can cause weight loss. In fact, since many leucine supplements are aimed at weightlifting or bodybuilding athletes who want to gain weight, the products may contain significant calories.
Which foods are rich in leucine?
Foods high in leucine include chicken, beef, pork, fish (especially tuna), tofu, canned beans, milk, cheese, pumpkin seeds and eggs.
The recommended daily intake of leucine is 39 mg per kilogram of body weight. A person weighing 70 kg should consume about 2730 mg of leucine per day.
|Food||Leucine (mg)||% RDI|
|Chicken drumstick (1 medium)||5160||189|
|Beef (170 gr)||5007||183|
|Pork steak (1 medium)||4501||163|
|Tuna (170 gr)||4133||151|
|Tofu (1 bowl)||3508||128|
|Beans (1 bowl)||1674||61|
|Cheese ricotta (1/2 bowl)||1531||56|
|Pumpkin seeds (30 gr)||678||25|
|Eggs (1 large)||538||20|
Are leucine supplements necessary?
Leucine deficiency is not often seen, and when it does occur, symptoms such as muscle weakness and poor blood glucose regulation manifest themselves. Instead, it can become toxic if accumulated in large amounts and affect brain functions.
L-Leucine supplements are popular in the bodybuilding and athletic community. Since BCAAs are known to help boost muscle growth, powders and pills are widely sold online and in health food stores.
Most leucine supplements provide about 3 to 5 grams of leucine per serving; a small study in 5 healthy men who were given a graded intake of up to 1,250 mg/kg of leucine (25 times the estimated average requirement) noted that oral doses of 500-1. 250mg caused increases in serum ammonia and therefore the upper limit of consumption was agreed to be established at 500mg/kg.
If a generic protein formulation is chosen, the percentage of leucine concentration in it should be checked, because wheat, soy, egg and whey proteins have different leucine concentrations.
Of the above and if one compares exactly the same doses, only egg and whey protein have sufficient amount of leucine. The others should be fortified with additional leucine to perform accordingly.
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