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Metabolism and exercise

During power exercises such as sprinting, when the rate of demand for energy is high, glucose is broken down and oxidized to pyruvate, and lactate is then produced from the pyruvate faster than the body can process it, causing lactate concentrations to rise. The production of lactate is beneficial for NAD regeneration (pyruvate is reduced to lactate while NADH is oxidized to NAD), which is used up in oxidation of glyceraldehyde 3-phosphate during production of pyruvate from glucose, and this ensures that energy production is maintained and exercise can continue. During intense exercise, the respiratory chain cannot keep up with the amount of hydrogen ions that join to form NADH, and cannot regenerate NAD quickly enough, so pyruvate is converted to lactate to allow energy production by glycolysis to continue.

The resulting lactate can be used in two ways:

Lactate is continually formed at rest and during all exercise intensities. Lactate serves as a metabolic fuel being produced and oxidatively disposed in resting and exercising muscle and other tissues. Some sources of excess lactate production are metabolism in red blood cells, which lack mitochondria that perform aerobic respiration, and limitations in the rates of enzyme activity in muscle fibers during intense exertion. Lactic acidosis is a physiological condition characterized by accumulation of lactate (especially L -lactate), with formation of an excessively high proton concentration and correspondingly low pH in the tissues, a form of metabolic acidosis.

The first stage in metabolizing glucose is glycolysis, the conversion of glucose to pyruvate and H:

When sufficient oxygen is present for aerobic respiration, the pyruvate is oxidized to CO and water by the Krebs cycle, in which oxidative phosphorylation generates ATP for use in powering the cell.When insufficient oxygen is present, or when there is insufficient capacity for pyruvate oxidation to keep up with rapid pyruvate production during intense exertion, the pyruvate is converted to lactate by lactate dehydrogenase), a process that absorbs these protons:

The combined effect is:

The production of lactate from glucose (glucose → 2 lactate + 2 H), when viewed in isolation, releases two H. The H are absorbed in the production of ATP, but H is subsequently released during hydrolysis of ATP:

Once the production and use of ATP is included, the overall reaction is

The resulting increase in acidity persists until the excess lactate and protons are converted back to pyruvate, and then to glucose for later use, or to CO and water for the production of ATP.