6 Energy Balance

The energy balance relationship

Energy balance describes how energy taken in through foods and beverages relates to energy used by the body over time. The change in body energy stores is expressed as:

When intake and expenditure are similar over time, body energy stores tend to remain relatively stable. A sustained surplus tends to increase stored energy, while a sustained deficit tends to reduce it. This relationship follows the conservation of energy, but it does not imply that body weight changes at a fixed, predictable rate: intake, expenditure, and body composition can all change as weight changes.

Components of daily energy expenditure

Total daily energy expenditure has several main components that interact:

• Resting energy expenditure (REE): Energy used for essential functions such as breathing, circulation, and maintaining tissues. It is generally the largest component and varies with body size and composition.

• Thermic effect of food: Energy used to digest, absorb, and process food.

• Physical activity expenditure: Energy used in planned exercise and everyday movement. Everyday movement, such as walking around the home, standing, and doing chores, is often called non-exercise activity thermogenesis (NEAT).

• Adaptive thermogenesis: Changes in energy use in response to conditions such as altered food intake, body weight, or temperature.

A larger body generally uses more energy at rest and requires more energy to move. As body weight decreases, both resting needs and the energy cost of movement usually decrease.

Energy balance and changes in body weight

When average energy intake is close to average energy expenditure, energy stores are relatively stable over time. If intake repeatedly exceeds expenditure, excess energy is stored mostly as body fat, and body weight tends to rise. If expenditure repeatedly exceeds intake, the body draws on stored energy and weight tends to fall.

The pattern over time matters more than any single day: eating more than usual on one day does not necessarily cause a lasting change in body weight. Short-term scale changes can also reflect water, glycogen, food in the digestive tract, and other factors—not only changes in body fat.

A calorie estimate is not a fixed prescription. The intake that maintains one person's weight may not maintain another person's, and needs can change as body weight and activity change. The National Institute of Diabetes and Digestive and Kidney Diseases' Body Weight Planner accounts for changes over time rather than assuming that the same calorie gap produces a constant rate of weight change.

Physiological regulation of energy

Energy balance is governed by linked physiological processes, not by intake or expenditure in isolation. The brain integrates signals from the digestive tract, endocrine system, and energy stores to help regulate hunger, fullness, and energy use.

Leptin is a hormone produced by fat cells and is one signal related to stored energy. When weight is lost, leptin levels commonly fall; appetite may increase, and energy expenditure may decline. These responses can make continued weight loss and maintenance of a lower weight more difficult.

Factors that influence body weight

Several influences contribute to differences in energy needs, appetite, body composition, and body weight:

• Body size and composition: Maintaining more tissue generally requires more energy. Muscle and organs use energy even at rest.

• Age, growth, and life stage: Growth and pregnancy increase energy needs. Needs may also change with age and with changes in body composition or activity.

• Activity and movement: Exercise and everyday movement contribute to expenditure, but activity levels and spontaneous movement vary among people and across days.

• Genetics and hormones: Inherited traits and hormonal signals contribute to differences in appetite, body composition, and energy expenditure.

• Adaptation to weight change: During weight loss, a smaller body requires less energy. Additional metabolic and behavioral adjustments may further reduce expenditure or increase hunger. The size and persistence of these adaptations vary, and their precise measurement is challenging.

Together, these influences help explain why energy balance is a useful scientific framework but body weight is not controlled by willpower alone. Calorie-need estimates are starting points, and observed patterns over time provide important context. The NIDDK Body Weight Planner estimates are intended for adults and not for people who are pregnant or breastfeeding.