Physical activity and the muscular contextual work involve an increased demand for energy substrates, depending on the intensity and duration. Muscular work is made possible only by consuming adenosine triphosphate (ATP), the main molecule of metabolism. Each muscular contraction consumes ATP, which must be resynthesized from scratch since the size of the molecule does not allow it to be accumulated in large quantities in muscles and cells. Therefore, the neo- synthesis of ATP becomes possible starting from the so- called metabolic substrates, depending on the energy demand’s rapidity and oxygen availability, as synthetically described in Table 1.

Table1. Characteristics of metabolic substrates

In general, and in line with the premises, it is therefore evident how the consumption of different substrates can vary substantially depending on the type of effort required. In this case, an exercise of high intensity and short duration (e.g., less than 10 s) will be based on purely anaerobic metabolism and will require almost exclusively the consumption of ATP and/or phosphocreatine, without affecting the stocks of other substrates. Nevertheless, as the duration of the effort increases and the intensity inevitably decreases, the stocks of ATP phosphocreatine will tend to be rapidly depleted and the metabolism will first become lactic anaerobic, with prevalent consumption of carbohydrates, and will then become progressively aerobic, with prevalent oxidation of carbohydrates (as long as available) and lipids. Should the availability of these stocks run out progressively, the organism would then be forced to use proteins as the last resort of metabolic substrates (“muscular cannibalism”), broken down into amino acids and then reconverted to glucose through gluconeogenesis. The dynamics of the evolution of muscle metabolism as a function of intensity and duration of physical effort are well-summarized in Fig. 1.

Fig1. The dynamics of the evolution of muscle metabolism as a function of intensity and duration of effort. (Copyright EDISES 2021. Reproduced with permission)

It is essential to plan for the correct availability of energy substrates based on the programmed physical exercise. In this case, this means providing the body with the right food at the right time. If it, therefore, seems completely unreason able to provide foods rich in lipids or complex carbohydrates (pasta, bread, rice, legumes) during intense but short efforts (e.g., weightlifting), then it is just as unreasonable to feed the athlete with large quantities of foods rich in simple and rap idly absorbed carbohydrates (sucrose, honey, fruit) in preparation for a marathon. Additionally, considering that the maximum degree of lipid oxidation occurs in correspondence with moderate-intensity exercise, the maximum effect on fat consumption in order to obtain a significant weight reduction will be obtained by dedicating oneself to moderate- intensity physical activity (60–80% of maximum oxygen consumption (VO2max)) for a sufficiently long period (e.g., at least 30–60 min). A highly intense physical activity (90 95% of VO2max) will produce a large consumption of carbo hydrates (above all muscular glycogen), but it cannot be sustained for a long time and will therefore have little effect on lipid reserves.

As discussed above, physical exercise tends to progressively consume energy, even the proteins that make up the muscle itself, if other substrates (essentially carbohydrates or lipids) are unavailable. This is because physical exercise simultaneously stimulates the synthesis and degradation of proteins, the balance of which is defined by the intensity of the exercise (the more intense the effort, the greater the muscular catabolism) and by the availability of substrates for the neo-synthesis of amino acids and proteins (the lower the availability of substrates, the greater the muscular catabolism). In all subjects, especially those who are more physically active, it is therefore also necessary to supply the body with a constant quantity of proteins to maintain constant (or increase) muscle tissue. Protein intake can vary depending on body mass and needs. In particular, if a sedentary person generally needs 1 g of protein per kilogram of body mass, then this limit should be raised to 1.5–1.7 g of protein per kilogram of body mass for muscle maintenance and even to 2.0–3.0 g of protein per kilogram of body mass for muscle development (“strengthening”). It should also be remembered that the essential amino acids (phenylalanine, isoleucine, leucine, lysine, methionine, threonine, tryptophan, valine, histidine, and arginine) are pivotal for building muscle proteins and are mainly contained in foods of animal origin, such as meat, eggs, milk, and cheese.

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