Is it easy to lose muscle mass on a diet?

Not necessarily. Athletes lose weight for different reasons; to improve their body's power ratio, to compete in a particular weight class, or for aesthetic reasons. Weight loss can be achieved either by restricting energy intake, increasing energy expenditure, or both at the same time, resulting in an energy deficit. During a weight loss period, when the energy deficit is too high, muscle mass and fat mass are often lost. The less fat mass there is at the beginning of the weight loss period, the more likely it is that muscle mass will be lost during the weight-loss period.

Muscle growth is thought to be both myofibres and sarcoplasmic muscle growth. In myofibres muscle growth, the surface area of the protein components of the muscle increases, while in sarcoplasmic muscle growth, the other volumes of the muscle cell, such as fluid volume and the size of energy stores, increase. The potency of creatine, among other things, is thought to be partly due to this phenomenon. Creatine binds water, and the muscle cell's pressure increases as it contains more water than before, thus providing a stimulus to the cell to strengthen its structures, i.e., increase its size. This is one of the consequences of the weight-increasing effect of creatine.

Muscle mass is not necessarily as fleeting on a diet as is often thought, especially when the diet is carried out sensibly. This is because the 'burning' of muscle mass and reduction in muscle size may not happen so easily. One explanatory factor for the decrease in muscle mass during the weight-loss period may be that the reduction in muscle size is a so-called sarcoplasmic reduction. In this case, the amount of muscle cell fluid is reduced.

It is known that muscle contains about 75% water and when there is an energy deficit, the body's fluids are reduced, so this can also happen in the muscle cell. This may therefore help to explain the reduction in muscle mass during the weight-loss period. This is very noticeable in fitness athletes when they start to increase their energy intake after a diet, and suddenly their weight "bounces" up to several kilos, and they immediately think, "now I've gained fat." So it may just be a case of the muscle cells filling up with water, i.e., sarcoplasmic muscle gain.