For some athletes, there may be a twofold need: slimming on the one hand, and increasing muscle mass on the other. Now, the direction the metabolism can take, whether anabolic or catabolic, is mostly absolute. By this I mean that if the organism is oriented towards anabolism, an increase in both fat mass and lean mass usually takes place, whereas if it is directed towards catabolism, a decrease in both occurs. What one normally does when intervening on diet and exercise is to make sure that the change in body composition mainly affects one component in particular (muscle mass in anabolism and fat mass in catabolism). However, no matter how much more muscle mass can be put on in the anabolic regime than fat mass and how much more fat mass can be lost than lean tissue in the slimming phase, the change will affect both components, which will take the same direction. This is the reason why it is generally advisable to follow a cyclic diet, alternating periods dedicated to increasing muscle mass, minimising fattening, and periods dedicated to eliminating fat mass, seeking maximum retention of lean tissue. To this rule, however, there are sometimes exceptions. Under certain circumstances, in fact, it is possible to lose fat and gain muscle tissue simultaneously, with a single diet (and training regime). This process is known as body recomposition. It mostly affects individuals whose body shape is far from ideal perfection. Generally speaking, the further one is from this ideal, the easier it is to get closer to it. The reason for this is that a very unfavourable body composition, far removed from what one would hope the average athlete to have, is often caused by incorrect eating habits. To improve the body composition of individuals in this circumstance, it is sufficient to correct these dietary errors. Intervening on all those habits at the root of which excess fat tissue can be placed and at the same time on all those that are the cause of low muscle mass can trigger the process of body recomposition. For example, it is sufficient to prohibit overweight people (with a not too long history of excess weight) from consuming sugary drinks, fried food or food with more than 15% fat to observe their slimming down. Similarly, suggesting a higher protein intake, perhaps by increasing consumption of lean meat, fish or eggs, may be sufficient to generate an increase in muscle mass. Individuals who are not in shape and who aspire to body recomposition will benefit from a diet relatively low in carbohydrates and rich in protein. As for calories, it is suggested, only at first, to neglect this aspect altogether, leaving the burden of determining how much to eat to the appetite. Individuals already on a diet can leave the calorie intake unchanged, possibly introducing small corrections following feedback. Of course, if diet plays its part, training must also play a part. Once the process of body recomposition has been completed, or once it has been ascertained that the organism is unable to lose weight and gain muscle mass at the same time, then the cyclic diet already illustrated above will be followed, alternating between anabolic and catabolic phases.