The energy a person uses each day is measured in calories and represents the total energy required to power every bodily function. This overall daily expenditure is known as Total Daily Energy Expenditure (TDEE). Understanding TDEE is fundamental for managing body weight, as it determines the caloric intake required to maintain, lose, or gain mass. TDEE is a composite of three distinct components: the baseline need to sustain life, the energy used to process food, and the energy spent on movement.
The Baseline Energy Requirement
The largest share of daily energy expenditure is used simply to keep the body functioning at rest. This baseline requirement is measured by the Basal Metabolic Rate (BMR) or the Resting Energy Expenditure (REE). BMR is the minimum number of calories needed to sustain life when an individual is completely rested, fasted, and in a neutral temperature environment. REE is a more practical measurement, taken under less strict conditions, and is generally about 10–20% higher than BMR.
These metabolic rates account for approximately 60–75% of the total calories burned each day. Energy is allocated to involuntary physiological processes like maintaining body temperature, circulating blood, and continuous organ function. The liver, brain, heart, and kidneys are the most metabolically active organs at rest and consume a large amount of this energy.
Several factors influence BMR. Lean body mass is a significant determinant because muscle tissue is more metabolically demanding than fat tissue. Age is also a factor; BMR typically declines by 1–2% per decade after age 20, often due to gradual loss of fat-free mass. Biological sex and genetics also play a role in determining foundational energy needs.
Energy Used for Digestion
The process of eating, digesting, absorbing, and storing nutrients requires a measurable amount of energy known as the Thermic Effect of Food (TEF). TEF, also referred to as Diet-Induced Thermogenesis, makes up the smallest component of TDEE, typically accounting for about 10% of the total calories consumed.
The energy cost of digestion varies significantly based on the type of macronutrient being processed. Protein requires the highest energy expenditure, using 20–30% of its caloric value for processing. Carbohydrates are less costly, requiring 5–15% of their energy value. Fats are the most easily processed, demanding only 0–5% of their caloric content for digestion and storage. Consequently, a high-protein meal results in a higher temporary increase in the metabolic rate compared to a meal of the same caloric value composed primarily of fat.
Energy Used for Movement
The most variable element of TDEE is the energy expended for movement, which is broadly categorized into two types of activity. Exercise Activity Thermogenesis (EAT) refers to the calories burned during structured, intentional exercise. However, the majority of movement-related energy comes from Non-Exercise Activity Thermogenesis (NEAT).
NEAT is the energy used for all physical activities that are not sleeping, eating, or dedicated exercise. This includes small actions like fidgeting, maintaining posture, walking around the office, standing, and performing household chores. NEAT is the primary reason TDEE can vary so widely between two people of the same size.
The difference in NEAT between individuals can be substantial, varying by up to 2,000 calories per day, depending on occupation and lifestyle. A person with a physically demanding job, such as construction, may burn over 1,500 more calories in NEAT than an office worker who sits for most of the day. Simple changes, like opting for a standing desk, can significantly increase daily NEAT expenditure over time.
Calculating Total Daily Energy Use
To estimate TDEE, the three components—BMR, TEF, and physical activity—must be combined. Since measuring these factors directly is complex and requires specialized equipment, estimates rely on formulas and multipliers.
The process begins by calculating BMR using predictive equations such as the Mifflin-St Jeor or the Harris-Benedict formula, which take into account weight, height, age, and sex. The Mifflin-St Jeor equation is widely used for its accuracy in a non-laboratory setting.
Once BMR is estimated, it is multiplied by an activity factor to approximate TDEE. This factor is a single number that mathematically accounts for both the energy used for movement and the Thermic Effect of Food. The multiplier is selected based on the person’s typical activity level throughout the week.
Activity multipliers range from 1.2 for a sedentary lifestyle (little to no exercise) to 1.9 for an extremely active lifestyle (hard daily exercise or a physically demanding job). A moderately active person, who exercises three to five days a week, typically uses a multiplier of 1.55. This final calculated figure provides an estimate of the total calories the body burns in a 24-hour period.