There has been a lot of talk lately regarding leucine as a supplement. It seems like every journal I read has an article on the benefit of leucine in the diet.
Leucine is one of the three Branch Chain Amino Acids (BCAA's) - isoleucine and valine are the other two. There are 9 essential amino acids (can not be made in the body) and 11 non essential amino acids (made in the body). The beauty about the three BCAA's (essential AA's) is that they can be metabolized in the muscle, rather than in the liver (where most dietary protein is metabolized such as deamination and transamination of amino acids). Once protein is broken down into individual amino acids, the aminos are either used to build new proteins or to be burned as fuel for energy. After exercise (ex. recovery), it's most important that you properly refuel with protein and carbs, so that carbs and fats (primarily fats) can be used for resting energy expenditure, and BCAA's can be used for tissue repair. BCAA's are the most abundant amino acids in muscle tissue and make up 1/3 of the muscle. Therefore, because BCAA's are essential amino acids, it's important that we receive adequate amounts of these wonderful nutrients. However, more than anything, it's important that you don't deprive yourself of any macronutrient (carbs, protein and fat) so that you can create a balanced diet which encourages the use of food for fuel. Keep in mind that you will only sabotage your weight goals and exercise routine if you keep your body in a calorie deficit and neglect vital nutrients which aid in tissue repair, glycogen synthesis, immune system functioning and brain health. Additionally, overloading the body with too many calories at one time will also lessen the chance of performance gains and/or meeting weight and body composition goals.

Here is a great segment of the Staying Strong article from Nutrition Action April 2011
It's not just the amount, but the kind of protein that may matter for muscle building. Our bodies can make 11 of the 20 amino acids that are the building blocks of protein and therefore of muscle tissue. Since we have to rely on our food for the other nine, they're called "essential".
"Only the essential amino acids in food stimulate protein synthesis," says Katsanos. One of the nine-leucine - appears to be the most powerful for making protein.
Leucine is more than a building block of new protein.
"It is the key amino acid that drives the majority of the protein synthesis response," explains Drummon.
In a one-day study, older men who were given extra leucine synthesized more protein than those who didn't get extra leucine. In younger men, leucine didn't matter.
While researchers don't know exactly how much leucine is optimal, early research suggests that it may be around three grams per meal, says Drummond.
"The proteins riches in essential amino acids and leucine are the animal proteins, such as eggs, dairy, meat, poultry and fish," explains Paddon-Jones.
Whey protein, which makes up about 20% of the protein in milk (the rest is casein) , has the highest concentration of leucine compared to other proteins says Katsanos. That's one reason why whey, which is the byproduct of cheese-making, is the source of protein in many bodybuilding powders.
"plan proteins are okay, too," notes Elena Volpi. "But they have lesser amounts of leucine, so individually they may not be as efficient as animal proteins." The best of the vegetable proteins seems to be soy, she adds.
Just don't expect extra leucine to make you look like a bodybuilder.
When Dutch researchers gave 30 healthy older men either 7.5 grams of leucine or a placebo every day for three months, they saw no difference in muscle strength or mass. But the extra leucine may not have mattered because the men were getting enough protein -they averaged 83 grams per day from their food.