What is lactic acid a by-product of?

Lactic acid is a by-product of glucose metabolism during intense exercise, particularly when the body relies on anaerobic glycolysis to produce energy. During high-intensity physical activity, the demand for energy increases significantly, and the body may not be able to supply sufficient oxygen to metabolize glucose completely through aerobic pathways. As a result, glucose is broken down into pyruvate, which is then converted into lactic acid when oxygen levels are low. This process allows for the regeneration of NAD+, which is necessary for continued anaerobic energy production.

In contexts where the other options are considered, fat metabolism primarily relies on aerobic pathways and does not produce lactic acid to the same extent as glucose metabolism during intense exercise. Protein synthesis is concerned with the creation of proteins and does not directly produce lactic acid. Mineral absorption relates to the process by which the body takes in minerals from food rather than involving energy production, so it does not generate lactic acid either. This understanding reinforces the relationship between high-intensity exercise, energy demands, and lactic acid production.