292 OLIVE OILS AND HEALTH METABOLISM The term metabolism refers to the set of physical and chemical processes that occur both in the cells and throughout the body, by which energy is converted or used. On the one hand, nutrients from food are first digested and then metabolically converted into energy substrates that the body can use to perform its vital functions: breathing, brain and nervous system activity, muscle contraction, digestion, blood circulation, temperature regulation, and the elimination of waste through urine and faeces. On the other hand, the body also uses this energy not only when in motion during everyday activities, but also to sustain those same vital functions when at rest and, of course, during physical exercise. Metabolism includes two opposing but closely related types of activity: anabolic (constructive metabolism) and catabolic (destructive metabolism). In anabolic processes, the body synthesises and stores its own carbohydrates, proteins, and lipids (fats) from the basic units released during digestion: simple sugars (such as glucose), amino acids, and fatty acids. Anabolism contributes to the growth and maintenance of body tissues, and to the storage of energy for later use. Catabolic processes, in contrast, generate the energy necessary for all cellular activities by breaking down carbohydrates, proteins, and lipids (fats). This release of energy provides the fuel needed for constructive metabolism (anabolism) and helps maintain body temperature. - aerobic capacity The ability of the body to perform work (e.g., muscle movement) by means of aerobic metabolism. - oxygen debt The body stores approximately two litres of oxygen in the lungs, the haemoglobin of red blood cells, body fluids, and the myoglobin of muscle fibres (a molecule that binds oxygen in a similar way to haemoglobin). These reserves can be used for aerobic metabolism and are rapidly and fully consumed during moderate to high-intensity exercise. However, the body continues to obtain the required oxygen by increasing the respiratory rate (hyperventilation). Once the physical activity ends, these oxygen reserves must be restored. When the oxygen supply to the muscles is insufficient to meet their demands, anaerobic metabolic reactions (in the absence of oxygen) take place, leading to the accumulation of lactic acid as a by-product. This lactic acid is later removed by converting it back into glucose, a process that requires oxygen. Additional oxygen is also needed after exercise to restore the phosphagen energy system. This system is the fastest route for generating energy and is used during explosive movements (such as throwing, jumping, sprinting, etc.) when there is no time to rely on other energy substrates. It provides a maximum energy output but only for the first few seconds of physical activity. The oxygen required to replenish these reserves, eliminate lactic acid, and restore the phosphagen system is referred to as oxygen debt. The magnitude of this oxygen debt depends on the intensity, duration, and type of exercise performed. - anabolic effect Stimulation of anabolic metabolism. This refers to synthesis processes in which larger molecules (carbohydrates, proteins, and lipids -fats-) are produced from smaller ones, such as simple sugars (like glucose), amino acids, and fatty acids.
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