Final answer:
The basal metabolic rate (BMR) is the primary contributor to energy expenditure, accounting for about 70 percent of daily energy use at rest, as opposed to physical activity and adaptive thermogenesis which contribute less.
Step-by-step explanation:
The major contributor to energy expenditure in humans is the basal metabolic rate (BMR). This measurement accounts for about 70 percent of all daily energy expenditure necessary for the basic functions of the body's organs while at rest. On the other hand, physical activity accounts for approximately 20 percent, and adaptive thermogenesis, which is the energy used for body temperature regulation, accounts for the remaining 10 percent. As individuals become more active or have more lean body mass, their BMR increases. It is also important to note that as one ages, the BMR generally declines as lean muscle mass decreases.