Have you ever wondered why some people seem to eat whatever they want without gaining weight, while others struggle to lose weight despite their best efforts?  The answer begins with a concept called energy balance. At its simplest, energy balance means matching the calories you eat with the calories your body uses. When energy intake and energy expenditure are equal, body weight tends to remain stable over time.

Although the idea sounds straightforward, maintaining a healthy body weight is often more complicated than simply “eating less and moving more.” Physical activity, age, body composition, genetics, hormones, sleep, stress, and even the environmental temeperature you can influence how your body uses energy. Some of these factors are under your control, while others are not.

In this section, you will learn what energy balance is, explore the components of energy expenditure, and examine the many factors that influence body weight. Understanding energy balance provides a foundation for your current and future health.

Energy Balance

Your body weight is influenced by the energy (calories) you consume and the energy you expend. This relationship is defined by the energy balance equation:

Energy Balance = Energy Intake minus Energy Expenditure

When you are in energy balance, energy intake equals energy expenditure.  Over time, body weight tends to remain relatively stable because calories consumed are balanced by calories expended by the body, as illustrated in Figure 7.3. ^[1]

Energy intake refers to the calories you consume from foods and beverages. These calories provide the energy your body needs to support basic functions, physical activity, growth, and overall health.

You may have heard the common rule of thumb that approximately 3,500 calories are stored in one pound of body fat. Based on this estimate, consuming 3,500 fewer calories than your body needs would result in about a 1-pound loss, while consuming an extra 3,500 calories could lead to about a 1-pound gain in body fat.

In reality, weight change is rarely this predictable. As body weight changes, the body often responds by adjusting metabolism and appetite. Because of these adjustments, actual weight gain or loss may be greater or less than what simple calorie calculations predict. For this reason, the 3,500-calorie rule is best viewed as a general guideline rather than an exact formula.

Components of Energy Expenditure

Where do all the calories you burn each day go? Some are used to keep your heart beating and lungs breathing, some help digest food, and some fuel your physical activity. Together, these make up your total energy expenditure (TEE), which has three main components:

1. Basal metabolic rate (BMR)
2. Thermic effect of food (TEF)
3. Physical activity

1. Basal Metabolic Rate (BMR)

Basal metabolic rate (BMR) is the energy your body uses to support essential functions such as breathing, blood circulation, and maintaining body temperature while at rest. In everyday discussions, BMR and resting metabolic rate (RMR) are often used interchangeably.

For most sedentary or moderately active adults, BMR is the largest component of total energy expenditure, accounting for about 60–75% of the calories burned each day. For example, a person who needs 1,800 calories per day may use about 1,200 calories simply to keep the body functioning—even while resting.

One of the biggest factors affecting BMR is lean body mass, which includes muscles, organs, bones, and other nonfat tissues. Because these tissues require more energy to maintain than body fat, people with more lean body mass generally burn more calories at rest. Increasing muscle mass through regular exercise can therefore help increase BMR.

BMR is also influenced by body size, age, sex, hormones, genetics, and overall health (Table 7.1). In general, larger individuals have a higher BMR because they have more body tissue to maintain. As people age, muscle mass often declines, contributing to a gradual decrease in BMR.

2. Thermic Effect of Food (TEF)

The energy used to digest, absorb, and store the nutrients in foods is the thermic effect of food.  TEF accounts for 5 to 10 percent of total energy expenditure and does not vary greatly amongst individuals.  High-protein foods have a higher thermic effect than high-fat ones, making a protein diet popular for weight loss, as noted in Table 7.2.

Protein requires more energy to digest and process than carbohydrates or fat. This higher thermic effect is one reason high-protein foods tend to be more filling.

3. Physical activity

Physical activity accounts for approximately 15–30% of total energy expenditure and includes both exercise and everyday movement.

• EAT (exercise activity thermogenesis): planned activities such as running, sports, or strength training.
• NEAT (non-exercise activity thermogenesis): everyday activities such as walking, shopping, cleaning, maintaining posture, and fidgeting.

Although exercise is important for health, NEAT can vary considerably from person to person and may have a large impact on daily calorie expenditure.^[2]

Hunger versus Appetite

Hunger is the body’s physical need for food. When your stomach is empty, it sends signals to the brain that encourage you to eat. One of these signals comes from ghrelin, often called the “hunger hormone.” Ghrelin levels rise before meals and fall after eating.

Appetite is the psychological desire to eat. Unlike hunger, appetite can be influenced by factors such as the sight and smell of food, emotions, habits, and social situations.

Satiety is the feeling of fullness that develops after eating. As the stomach fills and stretches, signals are sent to the brain that help reduce hunger and promote satiety. Another hormone involved in this process is leptin, which is produced by fat tissue and helps signal the brain that the body has sufficient energy stored.

Researchers once hoped leptin could be used as a weight-loss treatment. However, many individuals with obesity already have high leptin levels and appear resistant to its effects, limiting its usefulness as a therapy.

The Complexity of Energy Balance

As you’ve learned, how much we eat is not simply a matter of willpower. Hunger and appetite are influenced by powerful biological and psychological signals that help regulate energy balance. When energy intake falls below energy needs, the body responds by increasing hunger and encouraging us to seek food.

Energy expenditure also changes over time. During periods of calorie restriction or weight loss, the body adapts by conserving energy. Basal metabolic rate (BMR) decreases, muscles become more efficient, and people often unconsciously reduce their level of NEAT. These adaptations can make continued weight loss and long-term weight maintenance challenging.

People also respond differently to changes in calorie intake. In one study, identical twins consumed an extra 1,000 calories per day for 100 days, yet weight gain varied from about 10 to 30 pounds among participants. These findings suggest that genetics and other biological factors influence how individuals gain and lose weight.^[3]

Although maintaining a healthy body weight can be challenging, healthy eating patterns, regular physical activity, adequate sleep, stress management, and social support can improve long-term success.