All cutting phases and bulking phases may not be at the extreme ends of the spectrum. They may be a modified version just to continue to lose body fat or build muscle, while giving the body a break. A true cutting diet will take the body into a slightly catabolic state, so it can not be sustained for long periods without losing muscle. On the other side of the coin, a true bulking diet will result in the addition of some body fat. The two diets must offset each other to achieve absolute bodybuilding success.
A good cutting diet should help the bodybuilder slowly lose body fat, while minimizing muscle loss. This requires a close focus on nutrient, as well as calorie, intake. It also requires close inspection of the body each day in the mirror. Calorie reduction should be done slowly to keep the metabolism working and enhance fat loss.
Severe calorie restriction will lead to the metabolism slowing down which means fewer calories burned each day. Dropping below 1800 calories for an extended period of time is never a good idea for the hard working bodybuilder. Many people increase cardiovascular exercise to offset this, the problem, however, is that the body doesn’t like to use the fat stores for energy in this state. The body will try to preserve its energy stores for future needs so it will use glycogen stores from the muscles and liver first. The body will also rob the muscles of much needed protein to repair and build messengers and hormones if enough protein is not taken in through the diet. It is important to make sure enough protein is ingested to support the needs of the body and muscle maintenance. This will reduce the catabolism so often associated with cutting diets.
A bulking diet is used when it is time to build more mass. A good bulking diet is one that provides enough calories to support muscular repair and growth. Some bodybuilders require as many as 8000 to 10,000 calories to increase muscular size. When the muscles are worked they break down and require protein to repair, and then a little extra to build new muscles fibers to make the muscle stronger. The muscle stores of carbohydrates (glycogen) are also depleted and require replenishing through the diet. These stores can be built up and increased just like muscle fibers. Just like a cutting diet, this diet requires watching your progress closely and using the mirror to make sure you are not gaining too much fat.
To recap, the major difference between the cutting and bulking diets is the amount of calories taken in. A good cutting diet requires slow, even calorie reduction. A bulking diet requires increasing the calories slowly to reach a point of muscle gain. Both diets require focus on the gains (or losses) to make sure the calorie content is at the optimum level to meet your goals. The cutting diet will be required to lose some of the body fat gained during the bulking phase. Conversely, the bulk diet will be needed to offset the catabolism of the cutting phase and push the body into an anabolic state again.