Protein synthesis is building new skeletal muscles. When that occurs on a large scale, it’s known as skeletal muscle hypertrophy (increase), or otherwise known as a process thanks to which our muscles become bigger. We will therefore introduce what it is and how the amino acid, also known as leucine, works. 
Leucine is an exogenous amino acid, one of the three branched amino acids, ketogenic, and its isomer is isoleucine. It influences the anabolic hormone production (insulin combined with carbohydrates), acts on enzymes (mTOR kinase) by beginning the process of muscle protein synthesis and controlling its progress. Protein synthesis through leucine influences the development of bones, muscles and skin. It works reductively on fat tissue. Without access to leucine, protein synthesis is impossible. It’s naturally found in animal products (such as eggs, meat, dairy and fish), as well as plant products and in form of post-workout supplements.
In a series of complicated studies, scientists wanted to directly test the muscle protein synthesis after giving animals different products containing amino acids and comparing them after consuming glucose. When complete protein (which contains all amino acids) is used up, protein synthesis increased a little. When only the BCAA amino acids were used up, the protein synthesis didn’t increase as much. And finally, when only leucine was used up, the protein synthesis increased to the same amount. These findings provided strong evidence that leucine was the driving force for proteins ability to stimulate synthesis.
This mechanism works by decreasing the concentration of leucine, which signals mTOR that there isn’t enough protein in the diet, which triggers the synthesis of new skeletal muscle proteins. 
In another study, scientists from Columbia University studied rats that were provided food with high fat content. Then, they were given supplements containing leucine, and their fat mass decreased by 25%. Leucine also showed to have impact on better control of sugar levels in blood, as well as completely decreasing the “bad” cholesterol. Leucine increases the resting metabolism by increasing the levels of UCP3 (uncoupling protein 3), which in result makes the body lose the supplied energy in form of heat, instead of storing it as fat tissue.
A lot more evidence is coming up, showing that increasing the intake of leucine can have a lot of benefits. It’s an important substance for muscle proteins, activates the most important actions in a complicated process of protein synthesis, increases weight loss, improves the body composition and corrects the metabolic disorders, such as increased glucose levels or cholesterol.
An optimal amount of leucine is still being discussed, when the single doses were being tested, consuming even 2,5g of leucine stimulates protein synthesis. In long term studies, daily doses of leucine that contained 8 grams or more are recommended to be in divided doses, so around 2,5g of leucine in every meal.
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