Amino acids are much more than passive building blocks for protein. Many have specific and important functions of their own. The amino acid glutamine is a great example.
Your body's most important fuel source is glucose (blood sugar), which you get from the sugars and starches (carbohydrate) in your diet. Your body does everything in its power to preserve its glucose supply. But when glucose is in short supply your body can also make it from certain amino acids like glutamine.
So as you eat less carbohydrate you convert more glutamine into glucose. This process requires energy, which in this case comes from burning fat. This is at least part of the reason why glutamine supplementation can stimulate the metabolic (calorie-burning) rate in healthy humans.
Being an important source of glucose, glutamine may be especially valuable if you are following a carb-restricted diet. Intense workouts lower muscle and liver glycogen contents (glycogen is the storage form of glucose). After working out, your body relies heavily on glutamine to replenish glycogen.
Five to eight grams of L-glutamine consumed post-workout seems to be effective in accelerating glycogen recovery. Make sure your protein powder or meal replacement has a few grams of glutamine in each serving (or add L-glutamine to them) if you want to 'swell' your muscles up with glycogen faster than normal after training.