BCAAs are especially helpful for maintaining muscle mass while on a calorie-deficit diet. They're particularly useful for bodybuilding competitors who take their physiques to the lean extreme.
Dieting is catabolic, which means it can lead to muscle breakdown, for several reasons. The leaner a body gets, the more likely it is to lose muscle mass as the body tries harder and harder to hold onto body fat stores. In doing so, the body will turn to muscle to satisfy its energy needs.
On the molecular level, muscle loss occurs because the body increases protein breakdown (catabolism) in order to liberate muscle amino acids for fuel. If this isn't bad enough, muscle loss is compounded by the fact that levels of protein synthesis will also decrease due to reduced energy intake.
The basic equation for muscle mass is:
Muscle mass = rate of protein synthesis - rate of protein breakdown
When the rate of synthesis equals the rate of breakdown, you don't gain or lose muscle. If the rate of synthesis exceeds the rate of breakdown, you gain muscle. When the rate of breakdown exceeds the rate of synthesis, you lose muscle. If you're dieting, you may be burning the candle at both ends: elevating muscle breakdown and reducing protein synthesis.
Working out compounds the metabolic effects of dieting. The leaner one becomes, the more lethargic one can become. Decreased energy intake and decreased glycogen storage make for some rough training sessions. If you're too tired or weak to lift as heavy as your body was getting used to, your muscles will adapt, and they won't use as much energy to get the work done.
That means your body won't increase lean muscle mass; it might also mean that your body will use lean muscle for energy because you aren't using it to lift a heavy load.
How do you defend against this three-headed monster of muscle loss?
It's well established that BCAA's (particularly leucine) stimulate protein synthesis, and might do so to a greater extent than a normal protein on its own. BCAAs also increase synthesis of the cellular machinery responsible for carrying out the process of protein synthesis. Therefore, BCAAs not only increase the rate of protein synthesis, but they also increase the cell's capacity for protein synthesis!
BCAAs also work in your favor by reducing the rate of protein breakdown They do this (primarily) by decreasing the activity of the components of the protein breakdown pathway, and also by decreasing the expression of several complexes involved in protein breakdown. (In this case, they decrease the amount of mRNA produced from the gene that codes for these components.)
If we revisit our original equation for muscle mass, it's plain to see that increasing synthesis and decreasing breakdown will equate to muscle gain/maintenance. This is how we fight the Cerberus of muscle loss!