The concept of energy balance seems simple on paper. Balance the calories that you eat with the calories you burn.   But many factors play a role in this concept. Some factors are under our control and others are not. In this section, we will define energy balance then look at the different components of expenditure and the factors that influence it. We’ll also consider why energy balance is more complex than it seems.

Energy Balance

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

Energy Balance = energy intake minus energy expenditure

When you’re in energy balance, energy intake equals energy expenditure, and body weight should remain stable, as illustrated in Figure 7.3. ^[1]

Negative energy balance is when energy intake is less than expenditure, usually resulting in weight loss. (This is desirable for those on weight loss diets.)

Energy intake refers to the calories you consume from foods and beverages. These calories come from the macronutrients—carbohydrates, protein, and fat—as well as alcohol.

A common rule of thumb is that consuming about 3,500 extra calories can lead to a gain of approximately 1 pound of body fat, while creating a 3,500-calorie deficit may result in losing about 1 pound. This is because one pound of stored body fat contains roughly 3,500 calories.

However, weight change is not always this predictable. People differ in how their bodies respond to overeating or calorie restriction, and over time, the body adapts by adjusting metabolism and appetite. These adaptations help explain why losing weight and keeping it off can be challenging.

Although energy balance is more complex than “calories in versus calories out,” it remains a useful concept for understanding changes in body weight.

Components of Energy Expenditure

The sum of caloric expenditure is called total energy expenditure (TEE). The three main components of TEE are:

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

1. Basal Metabolic Rate (BMR)

BMR is the energy used by your body at rest. These are the “behind-the-scenes” activities that are required to sustain life. Examples include:

• Breathing
• Blood circulation
• Protein synthesis
• Nervous system activity
• Body temperature regulation

If a person is sedentary or moderately active, BMR is the largest component of energy expenditure, making up about 60 to 75 percent of total energy used. For example, a sedentary person might need about 1800 calories in a day, with about 1200 of them used for basal metabolism.

BMR can vary widely amongst individuals. An individual’s lean body mass—made up of organs, bones, and muscles—is the biggest determinant of BMR, because lean body tissue uses more calories than body fat. This means that a muscular person burns more calories than a person of similar weight with more body fat. Likewise, increasing your muscle mass can raise your BMR. However, at rest, skeletal muscles account for only about 18 percent of the total energy expended by lean body mass. Most energy is used to meet the calorie needs of vital organs. The liver and brain, for example, together account for nearly half of the energy expenditure by lean mass, as noted in the figure below.

BMR depends not only on body composition but also on body size, sex, age, nutritional status, genetics, body temperature, and hormones (Table 7.1). People with a larger frame size have a higher BMR simply because they have more mass. On average, women have a lower BMR than men because they typically have a smaller frame size and less muscle mass. As we get older, muscle mass declines, and therefore BMR declines as well.

If someone is fasting, starving, or even just cutting calories to lose weight, his or her BMR will decrease. This is because our bodies attempt to maintain homeostasis and adapt by slowing down basic bodily functions to conserve energy. This is a protective mechanism during times of food shortages, but it also makes intentional weight loss more difficult.

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.

3. Physical activity

Physical activity is another important way our bodies use energy. Physical activity usually accounts for 15 to 30 percent of energy expenditure and can be divided into two categories: EAT and NEAT.

• EAT (exercise-related activity thermogenesis)  is planned, structured, and repetitive physical activity designed to improve health (participating in a sport like or strength training at the gym, for example).
• NEAT (non-exercise activity thermogenesis) is the energy expenditure for unstructured and unplanned activities. This includes daily living activities like cleaning the house, yard work, walking to class, and shopping, as well as the energy required to maintain posture and to engage in spontaneous movements such as fidgeting and pacing. NEAT can vary by up to 2,000 calories a day for two people of similar size.^[2]

Hunger versus Appetite

Hunger is the physiological need to eat. When your stomach is empty, it contracts and starts grumbling and growling. The stomach’s mechanical movements signal the hypothalamus in your brain. (Of course, your stomach also contracts when it’s full and hard at work digesting food, but we can’t hear these movements as well because the stomach’s contents muffle the noise.) Your stomach also produces and secretes the “hunger hormone,” ghrelin.   Levels of ghrelin are high before a meal and fall quickly after eating.²

Appetite is the psychological desire to eat. Satiety is the sensation of feeling full. After you eat a meal, your stomach stretches and sends a signal to the brain, stimulating the sensation of satiety and relaying the message to stop eating. Leptin, known as the “satiety hormone,”  is produced by adipose cells as body fat increases.  Leptin signals the brain that you are full and to stop eating. ^[4]

The discovery of leptin’s functions sparked a craze in the research world and the diet pill industry, as it was hypothesized that giving leptin to individuals trying to lose weight would make them eat less. In several clinical trials, it was found that people who are overweight or obese are actually resistant to the hormone, meaning their brains do not respond as well to it. Therefore, when you administer leptin for weight loss, there is generally no sustained effect on food intake. ^[5]

The Complexity of Energy Balance

As we’ve already discussed, how much food we eat each day is not just a matter of willpower or self-control. It’s the result of powerful physiological and psychological forces that tell us if we need to eat, or if we’ve had enough. Our brains are hard-wired to seek food if we’re in a negative energy balance, an instinct required for survival. This means that if you start to exercise more—increasing your energy expenditure—you will also feel hungrier, because your body needs more fuel to support the increase in physical activity.

Even measuring how much energy is consumed is not as simple as you might think. We can measure the caloric content of food from a chemical standpoint, but we can only estimate how much energy a person will absorb from that food. This will depend on how well the food is digested and how well the nutrients are absorbed—factors that vary depending on the food itself, the person’s digestive efficiency, and even the microbes living in their gut. Two people may eat the exact same meal, yet not absorb the same number of calories.

Energy expenditure is also dynamic and changes under different conditions, including changes in caloric intake. Decreased caloric intake and negative energy balance lead to a drop in BMR to conserve energy. Muscles also become more efficient, requiring less energy to work, and without realizing it, people in negative energy balance often decrease their NEAT activity level. These adaptations help to conserve energy and make it more difficult to shed pounds. People may still be able to lose weight despite their bodies working to prevent it, but maintaining a new, lower weight requires constant vigilance, and weight regain is common.

Research has also shown that people respond differently to changes in caloric intake. When a group of people is overfed, the amount of weight gained among study participants varies widely. In a study of identical twins who were given an extra 1,000 calories per day for 100 days, participants gained 10-30 pounds. Weight gain between twins was more similar (though not identical), which may be attributed to genetic factors.^[6] People gain and lose weight differently; we don’t necessarily follow formulas.

When people say that the answer to the obesity epidemic is to eat less and move more, they’re not wrong. But this is also an oversimplified answer, given the complexities underlying energy intake and expenditure.