Glutamine is the most common and abundant amino acid that is found in the skeletal muscle that is used for the biosynthesis of proteins. During intense training, Glutamine is depleted due to the need for muscle growth. This subsequently decreases strength, stamina and recovery times. Typically, Glutamine is non-essential and conditionally essential (meaning that your body can make sufficient levels), however, during some instances of high strain such as high intensity workouts, the demand increases and your body struggles to provide enough.
This is why Glutamine is a great addition to any workout stack as it promotes muscle growth and retention.