The relationship between the energy a person consumes and the energy they burn is formally known as Energy Balance. This concept is central to understanding metabolism and how the body regulates weight and overall health. Energy Balance compares “Energy Intake,” the total calories consumed through food and beverages, with “Energy Expenditure,” the total calories the body uses for daily functions. The resulting state—balanced, surplus, or deficit—governs changes in the body’s stored energy reserves over time.
Defining Energy Balance
Energy Balance is governed by the first law of thermodynamics: energy cannot be created or destroyed, only transformed. In human biology, consumed energy must either be used for work and heat or stored within the body. The foundational equation is: Energy Balance equals Energy Intake minus Energy Expenditure.
When intake precisely matches expenditure, the body is in neutral energy balance, resulting in a stable body weight. This balance is a dynamic process, with both intake and expenditure constantly changing in response to daily activities and biological signals. The body strives for homeostasis, but this internal regulation can be overcome by environmental factors.
Components of Energy Intake
Energy Intake is the “calories in” side of the equation, derived from the chemical energy stored in macronutrients: carbohydrates, proteins, fats, and alcohol. The energy provided by each is measured in kilocalories (kcal), commonly called calories.
Fat is the most energy-dense macronutrient, providing about nine kilocalories per gram. Carbohydrates and protein provide roughly four kilocalories per gram. Alcohol yields about seven kilocalories per gram.
The body absorbs over 95% of consumed food energy. Excess energy not immediately needed for metabolic processes or physical activity is stored, primarily as adipose tissue, or body fat. The energy density of food plays a significant role in determining how much energy is consumed before a feeling of fullness is reached.
Components of Energy Expenditure
Energy Expenditure, or the “calories out” side, is the Total Daily Energy Expenditure (TDEE). TDEE is composed of three primary components: Basal Metabolic Rate (BMR), the Thermic Effect of Food (TEF), and physical activity. BMR accounts for the largest portion of daily energy burn, typically 50% to 70% of TDEE.
Basal Metabolic Rate is the energy required to maintain fundamental life-sustaining functions while the body is at rest. This rate is influenced by several factors:
- Body size.
- Age.
- Biological sex.
- The amount of lean muscle tissue.
Individuals with a higher proportion of muscle often have a higher BMR, as muscle tissue burns more calories at rest than fat tissue.
The Thermic Effect of Food (TEF) is the energy expended to digest, absorb, transport, and store nutrients. This process generally accounts for about 10% of TDEE. The energy cost varies by macronutrient: protein requires the most energy to process (20-30% of its caloric content), followed by carbohydrates (5-10%), and fat requires the least (0-3%).
The third component is energy expended through physical activity, the most variable part of TDEE. This includes planned exercise and Non-Exercise Activity Thermogenesis (NEAT). NEAT encompasses all energy burned for daily movements that are not formal exercise, such as standing, walking, and fidgeting. NEAT can significantly increase total expenditure.
States of Energy Balance and Body Weight
The comparison between energy intake and energy expenditure determines whether body weight is maintained, gained, or lost over time. When intake and expenditure are equal, the body is in a neutral energy balance, and weight remains stable.
A positive energy balance occurs when energy intake exceeds energy expenditure. The surplus energy is stored in the body, primarily as adipose tissue, leading to weight gain. A consistent excess of roughly 3,500 calories is needed to gain one pound of body mass.
Conversely, a negative energy balance is created when energy expenditure is greater than energy intake. The body must then draw upon its stored energy reserves, such as body fat, to cover the deficit, which results in weight loss. Over the long term, the body has mechanisms that try to push it back toward its previous weight, such as increasing hunger hormones and decreasing the metabolic rate.