There are several components to the energy burned side of the energy balance equation: (1)
Resting energy expenditure (REE): The amount of energy the body uses, at rest, to fuel basic cellular-level metabolic activities and to keep the heart, lungs, kidneys, and other organs functioning. REE varies according to body size, body composition, age, gender, and genetic differences. In people who are not very active, REE represents about two-thirds of their total daily energy expenditure; in people who have very active jobs, REE may represent only half of their daily energy expenditure.
Thermic effect of food: It takes energy to digest, absorb, and store the nutrients from foodabout 10 percent of a persons total daily energy expenditure. Fat, carbohydrate, and protein each require different amounts of energy to be processed. Protein has the highest thermic effect, and fat has the lowest.
Thermoregulation: Keeping the body temperature steady requires energy. In general, clothing, shelter, and climate control systems help people maintain their body temperature within comfortable limits, so thermoregulation is only a very small part of daily energy expenditure.
Physical activity: Any intentional body movement that burns calories is considered physical activity. For most people, exercise makes up only a small part of the energy they spend on physical activity. The bulk of it is spent on other routine daily activitiesfrom fidgeting and walking to household chores and office workcollectively referred to as non-exercise activity thermogenesis, or NEAT. (2) Calories burned through NEAT vary quite a bit from person to person, depending on genes, occupation, and environment, and declining NEAT levels are thought to play a role in the obesity epidemic.
1. Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, D.C.: National Academies Press; 2005.
2. Levine JA. Nonexercise activity thermogenesisliberating the life-force. J Intern Med. 2007; 262:273-87.