what is L-leucine powder?

Leucine is one of the three branched chain amino acids and sometimes referred to as the 'main' amino acid due to the most popular benefit of BCAAs (muscle building) being mostly due to leucine.

Similarly to many other amino acids, leucine is found in many foods that are high in protein. Examples include meats (such as fish, chicken, and turkey), dairy products (such as yogurt and cheese), and soybeans. Other foods like eggs, nuts, seeds, and fruit also contain leucine, but to a lesser extent.