GNC Pro Performance L-Glutamine 5000 mg GNC Pro Performance L-Glutamine is a key amino acid for muscles. Supplementation may help maintain glutamine levels that have been depleted as a result of intense exercise. Glutamine is involved in protein synthesis and plays a pivotal role in the body's nitrogen balance.ˆ It serves as a fuel for the immune system and maintenance of proper intestinal function. Tested and certified Banned Substance Free♦ Gluten and lactose free Mixes easily with your favorite beverage. ˆWhen used in conjunction with an exercise program.