How is metabolism best defined?

Metabolism can be defined as the combination of all chemical reactions that occur within living organisms. It encompasses both the processes that build up substances (anabolism) and those that break them down (catabolism). This definition captures the holistic nature of metabolism, as it is not limited to just fat burning or any single activity. Rather, metabolism is essential for various bodily functions, including energy production, growth, and repair of cells, as well as regulation of important physiological processes.

In contrast, other options focus too narrowly—limiting the definition to fat burning, physical exercise, or aerobic activities—thus failing to encompass the full range of metabolic processes that occur in the body. Metabolism occurs continuously, regardless of whether the organism is active or at rest, making it an essential aspect of overall biological function.