Nutritional science is increasingly seen in sports practice as an instrument capable of influencing the performance, recovery and gene expression of the athlete. Vice versa, the performance itself is able to modify human metabolism and the use of substrates for energy purposes. Since even small changes are important for the athlete, nutritional science must be considered a precision medicine in which standardized protocols apply to the needs of the individual athlete (based on the type of sport, the frequency of training and its intensity, the goals in terms of weight and muscle mass) in order to minimize measurement errors. The study of the baseline composition of the athlete’s body is essential to build up a nutritional plan that follows the athlete before, during and after competition, with the aim of optimizing performance and preventing the onset of fatigue. The purpose of this review is to sum up the most recent guidelines, underlining the key points of the current state of art on the best strategies to achieve specific goals in terms of changes or maintenance of body weight, preparing adequately the athlete for competition and encouraging recovery, with a brief mention to the psycho-behavioural dimension that nutrition acquires in sports practice.