Total Daily Energy Expenditure (TDEE) measures a person’s daily calorie burn. For individuals with minimal physical activity, this total is primarily driven by the constant work the body performs to sustain life, not by movement. The vast majority of calories burned maintain fundamental biological processes. Understanding the minimum daily calorie burn requires examining the three main components of TDEE: the energy needed for basic function, the calories used for digestion, and the small amount expended through non-exercise movement.
The Core Calorie Burn: Basal Metabolic Rate
The single largest component of TDEE is the Basal Metabolic Rate (BMR), accounting for approximately 60 to 75 percent of total daily calories burned. BMR represents the minimum energy required to keep the body functioning at complete rest. These calories fuel involuntary, life-sustaining functions, such as blood circulation, breathing, and the complex operations of the brain and nervous system.
BMR is measured under specific conditions, typically after a prolonged fast and a night of complete rest, ensuring no energy is used for digestion or physical activity. Even when completely still, internal organs like the heart and liver continuously demand a significant energy supply. This means a person with a sedentary lifestyle still burns a substantial number of calories just by being alive. This baseline expenditure is the foundational number for calculating total daily caloric needs.
Resting Metabolic Rate (RMR) is a closely related term often used interchangeably with BMR, though RMR measurements are taken under less strict conditions. For practical purposes in estimating daily calorie needs, BMR provides the reliable minimum energy cost. Since this rate is responsible for the largest fraction of daily energy use, it forms the non-negotiable floor for a person’s calorie burn before any movement is considered.
Calculating the Daily Minimum Expenditure
To determine TDEE for a person with minimal activity, scientists use predictive formulas to estimate BMR and then incorporate an activity factor. The Mifflin-St Jeor equation is a widely accepted method for estimating BMR based on an individual’s weight, height, age, and biological sex. This calculation provides the number of calories required simply to exist.
Since BMR accounts only for absolute rest, the result must be multiplied by an Activity Factor (AF) to account for the few calories burned during daily life. For an individual who engages in little to no exercise and primarily has a desk job, the lowest standard Activity Factor is typically set at 1.2. Applying this factor to the calculated BMR yields the estimated TDEE for a sedentary individual.
For example, a 45-year-old woman weighing 165 pounds (75 kg) and 65 inches (165.1 cm) tall might have an estimated BMR of approximately 1,396 calories. Multiplying this BMR by the sedentary activity factor of 1.2 results in a TDEE of about 1,675 calories. This final number represents the minimum daily caloric burn for someone who spends most of their day sitting or lying down.
How Physiological Factors Influence the Burn Rate
While the Activity Factor is low for a sedentary lifestyle, BMR can vary significantly between individuals due to unique physiological characteristics. A primary determinant of BMR is body composition, specifically the proportion of muscle mass to fat mass. Muscle tissue is metabolically more active than fat tissue, meaning it requires more calories to maintain, even while resting.
Individuals with greater overall body weight or larger body size generally have a higher BMR because a larger mass requires more energy to support its functions. Biological sex also plays a role, as men typically have a naturally higher BMR than women, largely due to carrying a greater percentage of muscle mass. Furthermore, BMR gradually decreases with age, primarily due to the natural loss of muscle tissue over time.
These biological inputs explain why two people with the same minimal activity level can have different total calorie burns. Factors like genetics and hormonal balance, particularly thyroid hormones, also contribute to individual differences in metabolic rate. The physiological makeup of the body establishes the unique baseline for energy expenditure before daily movement is considered.
The Smallest Contributors: Digestion and Minimal Movement
The remaining, smaller components of TDEE are the Thermic Effect of Food (TEF) and Non-Exercise Activity Thermogenesis (NEAT). TEF is the energy required to digest, absorb, and metabolize consumed nutrients, typically accounting for about 10 percent of total caloric intake. This energy is expended regardless of a person’s activity level, as long as they are eating.
NEAT is the energy expended for all physical activities other than structured exercise, sleeping, or eating. For a person with minimal activity, NEAT includes extremely low-level movements like fidgeting, shifting posture while sitting, or the small energy cost of typing and speaking. While a low contributor to the total daily burn, NEAT still adds to the TDEE above the BMR baseline.
These spontaneous, minor movements expend energy, making NEAT the most variable component of non-exercise energy expenditure among sedentary individuals. The sum of BMR, TEF, and minimal NEAT provides the final total of the calories burned by a highly sedentary person.