Which of the following is produced during energy metabolism and determines calories expended during activity?

During energy metabolism, carbon dioxide is produced as a byproduct when the body breaks down nutrients for energy. This process primarily occurs within the mitochondria of cells, where nutrients like glucose and fatty acids undergo oxidation during aerobic metabolism. The production of carbon dioxide reflects the level of metabolic activity taking place; thus, measuring the amount of carbon dioxide exhaled can be used to estimate the number of calories expended during physical activity.

As the body converts food into energy, some of this energy is stored in the form of ATP (adenosine triphosphate), which is vital for muscle contractions and energy transfer within cells but does not directly indicate the caloric expenditure itself. Water is also produced as a byproduct of metabolism but does not have the same direct relationship to caloric expenditure. Oxygen is consumed during the metabolic processes but is not a byproduct and is primarily used to help produce ATP. Therefore, carbon dioxide serves as a critical indicator of the metabolic rate and the energy used during activity.