How is metabolism best defined?

Metabolism is best defined as the rate at which the body uses energy. This definition encompasses the overall biochemical processes that occur in the body, including both catabolic (breaking down molecules for energy) and anabolic (building up molecules) reactions. Metabolism is crucial because it determines how efficiently the body utilizes food for energy, supporting various functions such as physical activity, maintaining body temperature, and repairing tissues.

Other choices focus on specific aspects of metabolism rather than capturing its comprehensive nature. For instance, breaking down food into nutrients is just one part of the metabolic process, specifically related to digestion and absorption. Similarly, the body's capacity to gain weight relates more to the balance of energy intake and expenditure rather than the definition of metabolism itself. Converting sugars to fats is a metabolic process, but it does not define metabolism in a broader sense; it is merely a specific example of how energy can be stored in the body. Therefore, defining metabolism as the rate at which the body uses energy covers a broader and more accurate scope of what metabolism entails.