The number of calories your body burns each day is known as Total Daily Energy Expenditure (TDEE). In a nutritional context, a calorie is a unit used to quantify the energy your body receives from food and the energy it expends to function. TDEE represents the sum of all energy used over a 24-hour period, powering continuous biological processes essential for life.
The Baseline: Basal Metabolic Rate (BMR)
The largest portion of your total daily calorie burn is dedicated to the Basal Metabolic Rate, or BMR, which accounts for approximately 60% to 75% of TDEE for most people. This rate represents the absolute minimum amount of energy required to sustain life when the body is at complete rest. It is the energy cost of maintaining fundamental physiological functions.
This baseline energy powers essential functions like breathing, blood circulation, and temperature regulation. Organ function is a major component of BMR, with the liver, brain, and heart demanding a steady energy supply. Cellular processes, such as repair and growth, also contribute to this resting expenditure.
BMR is measured under strict conditions, typically after a prolonged fast and a full night’s sleep, ensuring the body is at complete rest. This rate establishes the foundational number of calories the body must burn regardless of physical activity.
The Energy Cost of Physical Activity
The energy expended through physical activity is the most variable component of your total daily expenditure. This category is typically divided into two distinct parts: calories burned during intentional exercise and energy used for general movement. Intentional activity includes structured, planned exercise like running, lifting weights, or participating in a fitness class. This type of activity can significantly increase your calorie burn for a short period.
The second, often overlooked, part of this expenditure is Non-Exercise Activity Thermogenesis, or NEAT. NEAT encompasses all the calories burned through non-structured movements, ranging from standing and walking around the office to fidgeting and cleaning the house. For sedentary individuals, NEAT can account for a substantial portion of their total activity burn, sometimes contributing more to TDEE than formal exercise.
The difference in NEAT between two people of similar size can be significant, varying by up to 2,000 calories a day based on occupation and lifestyle. A person with a physically demanding job will have a much higher NEAT than someone with a desk job, making this a highly individual factor in daily energy use. Small, consistent movements throughout the day can accumulate a considerable total energy cost.
The Thermic Effect of Food (TEF)
The Thermic Effect of Food is the energy required to process the food you eat, including its digestion, absorption, transport, and storage of nutrients. This component is the smallest contributor to TDEE, typically accounting for about 10% of the total calories burned each day. It represents a metabolic “processing fee” for turning food into usable energy or stored reserves.
The energy needed for this process varies significantly depending on the macronutrient composition of the meal. Protein requires the most energy to process, using approximately 20% to 30% of its caloric content for digestion. Carbohydrates have a lower energy cost (5% to 10%), while dietary fat is the least expensive to process (0% to 3%).
Personal Factors That Determine Your Calorie Burn
While the three components of TDEE are universal, a number of personal biological factors create significant differences in calorie burn between individuals. Body composition is a major determinant, as muscle tissue is metabolically more active than fat tissue. A person with a higher percentage of lean muscle mass will have a higher BMR because muscle requires more energy to maintain at rest. Body size and weight also play a role, as larger individuals require more energy to support their greater mass and to power basic functions like circulation and respiration.
As people age, the metabolic rate naturally slows down, primarily because of a gradual decrease in lean muscle mass. Genetic predisposition can influence an individual’s baseline metabolic speed, contributing to natural variations in BMR. Hormonal conditions, such as the function of the thyroid gland, also directly regulate metabolic rate.