What component of energy expenditure accounts for the largest proportion of total energy output?

Basal metabolism accounts for the largest proportion of total energy expenditure in the body. This component represents the energy expended while at rest in a neutrally temperate environment and is necessary to maintain vital physiological functions such as breathing, circulation, and cell production. Basal metabolic rate (BMR) is influenced by factors including age, sex, body composition, and hormonal levels, and it typically constitutes about 60-75% of a person's total energy expenditure.

In contrast, physical activity varies widely among individuals and usually contributes about 15-30% of daily energy output, depending on lifestyle and activity levels. The thermic effect of food, which is the energy required for digestion, absorption, and processing of nutrients, typically contributes a smaller percentage, around 5-10%. Adaptive thermogenesis refers to the energy used to regulate body temperature and respond to environmental changes, but it is generally less significant in overall energy expenditure compared to basal metabolism. Therefore, basal metabolism is the primary factor in determining total daily energy expenditure.