After examining this issue, I went on to explore what happens during a period of low protein feeding. What I discovered is there are several mechanisms that preserve muscle proteins in favor of, for example, liver proteins [which include more labile proteins], during this condition. First of all, muscle proteins have a longer life span than liver proteins, so initially [during the first few days of protein deprivation], liver proteins, rather than muscle proteins, are lost, and muscle mass is remarkably well preserved.7 Furthermore, the urea- cycle enzymes are down-regulated [interestingly, the same thing happens during overfeeding3]; thus, less urea is formed, and this urea can, under these conditions, even be recycled by something called the urea-salvage pathway to create amino acids again.10 This also takes place during training and recovery.2 Even more interesting are the events within the muscle cell. During the first day of a low protein intake, protein synthesis is decreased while the degradation is constant. After three days, however, the degradation is significantly lowered.23 Thus, in essence, a three-day, low-protein diet actually stimulates anti-proteolytic mechanisms, or "anti-catabolism" as it is often referred to in the muscle magazines.
Now comes the very interesting part—when you switch back to a high-protein diet, you create the perfect environment for super-compensation of muscle proteins [GROWTH!] to take place. Here's why:
The amino acids [nitrogen] will stay in the body since urea-cycle enzymes are still down-regulated, and the urea-salvage pathway is still operating.