Strenuous workouts are beneficial but should not be expected to sustain you on their own. The harder you train, the more you need to replenish the energy you exert, causing you to exhaust yourself toward the end of your workout. If you do it all alone, with no way of putting back into your body all of the nutrients you lose, you will burn yourself out and not be able to operate at your greatest potential.
One of the biggest causes of energy depletion stems from the affects of the immune system. Each time you train, and take on strenuous workouts during your routine, your immune system takes a hit. When your immune system is at a lowered capacity, you are at a much higher risk for injury, infection or even disease. This is where Glutamine can greatly influence and enhance your immune system's ability to deal with the stresses you place upon it when you take your body through an intense workout.
Glutamine can help the muscles replenish their glycogen reserves, which are depleted after a strenuous workout. This is also why it is highly recommended to take in a post-workout meal or meal replacement as soon as possible after training. The faster you can replenish those stores, the faster you will recover from your workout, which prepares you for going back into the gym for your next session.
Glutamine can be purchased in pill form or powder. However, with powder, it can be added to your post workout drinks, which will in turn speed up the recovery process, repairing muscle tears and tissue, relieving soreness, and helping with fatigue after a grueling workout. Also, speeding up this process will increase your ability to build more muscle tissue over time, as research has shown that Glutamine can help to produce growth hormone levels. In short, Glutamine is a primary energy source for your immune system. In fact, studies have found that 1.6 grams of Glutamine per day has a 92% cure rate of ulcers. (See more studies: Harvard Gazette Report).
There are a ton of supplements on the market that will claim to offer benefits to your workouts. Some are necessary, others are a waste of money. But if you are a bodybuilder or someone who trains with great weight and intensity each workout, be sure to incorporate Glutamine in your regimen so that you can always be ready and able to take on every "next" workout.