What is Doctor's Best Pure L-Glutamine Powder? Doctor's Best L-Glutamine Powder contains Glutamine, an amino acid that is a building block of protein. Glutamine is most commonly used to support muscle growth, skeletal muscles and athletic endurance. Supplementing with L-Glutamine allows the body the much-needed amino acids to increase muscle hydration. This is an essential process in the body that supports muscle recovery from high intensity workouts.