Energy balance is the relationship between the energy you consume and the energy your body uses. This balance, often compared to a seesaw, dictates whether body weight is gained, lost, or stays the same. The energy stored in food is converted into forms the body can use for work, heat, and storage. Maintaining this equilibrium over time is an aspect of sustaining a healthy weight.
Understanding Energy Intake
The “energy in” side of the equation comes from the food and drinks we consume. This energy is measured in units called calories, or more accurately, kilocalories (kcal). Nutrition labels use “calories” to refer to these kilocalories. The body breaks down what we consume to release this stored energy, which fuels everything from basic cell functions to intense physical activity.
The primary sources of this energy are the three macronutrients: carbohydrates, fats, and proteins. Each macronutrient provides a different amount of energy. Both carbohydrates and proteins offer approximately 4 calories per gram. Fats are the most energy-dense, providing about 9 calories for every gram consumed.
The body digests these macronutrients into smaller units—carbohydrates into sugars, proteins into amino acids, and fats into fatty acids. These basic units are then absorbed and used to build tissues, support growth, and provide the immediate energy needed for daily activities. While carbohydrates are the quickest energy source, fats provide energy more slowly.
Components of Energy Expenditure
The “energy out” side of the balance equation is your total daily energy expenditure (TDEE). This is the total number of calories your body burns in a 24-hour period and is the sum of several components that determine how much energy your body uses.
The largest portion of this expenditure is the Basal Metabolic Rate (BMR), which accounts for about 60-75% of the total calories burned daily. BMR represents the energy required to sustain life-sustaining functions while the body is at complete rest, including breathing, circulating blood, and maintaining body temperature. Building more muscle mass is the primary way to increase BMR, as muscle tissue burns more calories at rest than fat tissue.
Another component is the Thermic Effect of Food (TEF), which is the energy your body uses to digest, absorb, and process the nutrients from what you eat. TEF accounts for roughly 10% of your total daily energy expenditure. The energy cost varies by macronutrient; protein has the highest thermic effect, followed by carbohydrates and then fats.
The final component is Activity Energy Expenditure (AEE), which covers all energy burned through movement. This is often divided into two subcategories. Exercise Activity Thermogenesis (EAT) is the energy burned during planned, structured physical activities like running, weightlifting, or playing sports. The other part, Non-Exercise Activity Thermogenesis (NEAT), includes all other movements, such as walking, fidgeting, doing chores, or even typing. NEAT can vary greatly among individuals and plays a significant role in overall daily calorie burn.
The Three States of Energy Balance
The interplay between energy intake and expenditure determines your body’s energy status, which exists in one of three states. Each state has a direct consequence on body weight over time.
A state of positive energy balance occurs when your calorie intake is greater than your calorie expenditure. When you consume more energy than your body needs, the surplus energy is stored, primarily as body fat, leading to weight gain over time.
Conversely, a negative energy balance happens when your energy expenditure exceeds your energy intake. In this state, the body needs more fuel than it is receiving from food and drink. To make up for this deficit, it turns to its own stored reserves, breaking down body fat to release the necessary energy. This mobilization of stored energy results in weight loss.
When energy intake is equal to energy expenditure, the body is in a state of equilibrium. In this state, you are providing your body with the precise amount of energy it needs, with no surplus or deficit. This balance results in the maintenance of a stable body weight. Long-term balance, rather than perfect day-to-day equality, is the goal for weight maintenance.
Physiological Regulation of Energy Balance
Energy balance is more complex than a simple calorie-counting equation because the body has physiological systems to regulate it. These systems involve a constant conversation between the brain and the body through hormones that signal hunger and fullness. This regulation helps the body maintain a stable state, or homeostasis.
Two of the most well-known hormones in this process are ghrelin and leptin. Ghrelin, the “hunger hormone,” is produced in the stomach and its levels rise when the stomach is empty, signaling the brain to stimulate appetite. Leptin, the “satiety hormone,” is produced by fat cells. As fat stores increase, more leptin is released, signaling to the brain that the body has enough energy stored, which reduces food intake.
The body can also adjust its “energy out” side of the equation in response to changes in intake, a process known as metabolic adaptation. When you are in a negative energy balance (like during a diet), your body may perceive this as a threat of starvation. In response, it can become more efficient, reducing its overall energy expenditure to conserve resources. This can make continued weight loss more challenging and is a key reason why maintaining weight loss can be difficult. This adaptive response highlights the body’s drive to maintain its energy stores.