ATP (adenosine triphosphate)
A molecule that serves as the universal energy currency of cells, produced from nutrients and used by every reaction that needs energy.
ATP is the form in which cells use energy. Everything the body does (contracting a muscle, transmitting a nerve impulse, building a protein, transporting a molecule) uses up ATP. Breaking it down releases the energy stored in its phosphate bonds.
It comes from the breakdown of carbohydrate, fat and, to a lesser extent, protein. Three pathways coexist: the anaerobic alactic pathway (phosphocreatine) for very brief efforts, the anaerobic lactic pathway (glycolysis) for intense efforts lasting a few minutes, and the aerobic pathway, which takes place in the mitochondria and supplies most of the energy at rest and during prolonged effort.
The body stores almost none of it. This is the most striking fact: ATP reserves cover a few seconds of intense effort. The body constantly recycles a considerable amount, in the order of its own body weight over a day, rather than building up stores.
Several micronutrients are essential for this production: magnesium, without which ATP cannot be used by enzymes, B vitamins as cofactors in the Krebs cycle, and iron for the respiratory chain. This is one of the physiological links between deficiency and the feeling of fatigue.