TA: My interest in protein cycling originated about a year ago when I realized that if you change from a diet with normal protein intake to one with a high intake, after about a week, you will have less amino acids in your blood than before upping your protein intake.13 The reason for this is not only enzymatic adaptation but also hormonal changes. A high protein intake stimulates the release of a hormone called glucagon, which is a hormone that opposes the effects of the anabolic hormone insulin.
With increased protein intake, the urea cycle runs at a faster pace to excrete the nitrogen from the excess amino acids. Glucagon is also "consuming" amino acids, but in this case, to create glucose [gluconeogenesis] by up-regulating gluconeogenic enzymes. Unfortunately, these two systems overcompensate and thus decrease the amount of available amino acids in the bloodstream. However, it seems that the transport of amino acids into muscle is initially improved.
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.
The nitrogen balance in muscle is dramatically elevated because the synthesis is rapidly increased due to improved availability of amino acids20 and because it takes two days for protein degradation to increase23 back to your baseline value, which is still lower than average due to the high protein intake.8,16
These very important observations are the basis for protein cycling within the framework of my Anabolic Burst Cycling System. By doing these micro-cycles, especially during the low-calorie phase, you can experience muscle growth, even though you are on a restricted-calorie diet—you can build muscle and burn fat at the same time! This is what "nutrient repartitioning" is all about—you drive energy stores from fat to fuel muscle tissue. It is a "rob Peter to pay Paul" phenomenon. Unfortunately, this "primed condition" exists for only about two or three days.