What component of Total Daily Energy Expenditure (TDEE) is most susceptible to reduction during prolonged calorie restriction and significantly impacts overall energy expenditure without direct exercise?

The component of Total Daily Energy Expenditure (TDEE) most susceptible to reduction during prolonged calorie restriction and significantly impacting overall energy expenditure without direct exercise is Non-Exercise Activity Thermogenesis (NEAT). Total Daily Energy Expenditure (TDEE) represents the total number of calories an individual burns in a day, comprising four main components: Basal Metabolic Rate (BMR), Thermic Effect of Food (TEF), Exercise Activity Thermogenesis (EAT), and Non-Exercise Activity Thermogenesis (NEAT). Basal Metabolic Rate (BMR) is the energy expended at rest for vital bodily functions like breathing, circulation, and cell production. The Thermic Effect of Food (TEF) is the energy required to digest, absorb, and metabolize food. Exercise Activity Thermogenesis (EAT) refers to the energy burned during planned, structured physical exercise. Non-Exercise Activity Thermogenesis (NEAT) encompasses all the energy expended for physical activities that are not formal exercise, sleeping, or eating. This includes a wide range of everyday movements such as walking, standing, fidgeting, typing, talking, and even unconscious muscle contractions. During prolonged calorie restriction, the body initiates adaptive thermogenesis, a physiological response to conserve energy. This involves a decrease in overall energy expenditure beyond what can be explained by changes in body weight or lean mass alone. While Basal Metabolic Rate also reduces as part of this adaptation, NEAT is particularly susceptible because the body unconsciously and spontaneously reduces the amount of energy expended on these seemingly minor daily movements. For instance, an individual might fidget less, walk more slowly, choose to sit rather than stand, or take fewer spontaneous strolls. These small, often unnoticed reductions accumulate throughout the day, leading to a significant decrease in total energy expenditure without requiring any reduction in structured exercise. This reduction in NEAT is a powerful, autonomous mechanism the body employs to mitigate energy deficit and maintain energy balance, directly impacting how many calories are burned outside of planned physical activity.