Energy Balance and Nutritional Strategies of Soccer Players During a Microcycle

Football is among the most popular team sports in the world. From a physiological and biomechanically complex perspective—correction: From a physiological and biomechanical standpoint, it is a highly complex intermittent sport in which players combine short, maximal-intensity sprints with dynamic changes of direction, accelerations, decelerations, and longer bouts of low- or moderate-intensity running. During a competitive match, an adult or adolescent player covers an average of 7 to 10 kilometers, with 10% to 15% of this distance consisting of high-intensity runs and sprints. The total volume and structure of physical load vary significantly depending on the player's position: central and wide midfielders cover the highest total volumes, while wingers and forwards exhibit the highest frequency of explosive accelerations.

In addition to somatic and physiological prerequisites, cognitive abilities, stimulus processing speed, tactical thinking, and the ability to make rapid decisions under fatigue play a key role. The weekly training load is structured with the upcoming match in mind. Training sessions with the highest physiological and mechanical load are scheduled for days further away from the match, whereas as Match Day (MD) approaches, training volume gradually decreases with an emphasis on tactical preparation and optimal recovery.

Physiological Demands on the Footballer's Organism

Energy supply for football performance is ensured by both aerobic and anaerobic metabolic pathways. Aerobic load accounts for 70% to 80%, occurring during longer periods of low and moderate intensity. Anaerobic metabolism (the phosphocreatine system and anaerobic glycolysis) dominates during short-term maximal sprints and duels, producing lactate as a byproduct. The primary source for muscle contraction is adenosine triphosphate (ATP); however, its immediate intramuscular reserves are very limited and only last for the first few seconds of exercise. Subsequent ATP resynthesis occurs immediately from phosphocreatine (PCr) and muscle glycogen.

Cardiovascular, Respiratory, and Muscular Adaptation

Long-term training loads induce a series of profound adaptive changes. The cardiac output of an untrained individual during exercise is around 20–25 L/min, whereas in trained athletes it can rise up to 35 L/min. Resting heart rate decreases due to parasympathetic adaptation (dropping as low as 35 beats/min in elite athletes). In muscle tissue, the activation of hypoxia-inducible factors (HIF-1$\alpha$) leads to the formation of new capillaries (angiogenesis), improved oxygen delivery, and an increase in mitochondrial capacity and the activity of key oxidative enzymes such as citrate synthase (CS) and cytochrome c oxidase (COX).

Clinical Note: Intensive mechanical loading in football leads to microtraumas within muscle tissue. Immediately post-match, markers of muscle damage (creatine kinase, myoglobin) and systemic inflammatory markers (e.g., CRP) demonstrably increase, placing enormous demands on post-exercise nutritional recovery and the replenishment of glycogen stores.

Biological Development and Specifics of Adolescent Players

The youth categories (age range 15–18 years) fall into the period of adolescence, which is defined by rapid somatic, hormonal, and psychosocial development. A key phenomenon of this period is the attainment of Peak Height Velocity (PHV). In practice, it is essential to strictly distinguish between chronological and biological age. The difference in biological maturity among peers can be up to 4 years, which fundamentally affects the strength, speed, and coordination capacities of players. Biologically more mature individuals exhibit higher muscular strength and better tolerance to training loads, whereas late-maturing players are exposed to a higher risk of overload and injury due to the imbalance between bone growth and muscle strength.

Energy Expenditure and Methods for Its Determination

The quantification of Total Daily Energy Expenditure (TEE) is a fundamental prerequisite for establishing adequate nutritional intake. TEE consists of Resting Metabolic Rate (RMR), the Thermic Effect of Food (TEF), Non-Exercise Activity Thermogenesis (NEAT), and the Thermic Effect of Activity (TEA). In adolescent football players, TEE ranges from 3,000 to 3,600 kcal/day, depending on the age category, body weight, and training volume.

For clinical and research purposes, the doubly labeled water (DLW) method represents the gold standard. In everyday practice, resting energy expenditure is estimated using predictive equations, with the Mifflin–St Jeor equation exhibiting the highest accuracy: