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11.2: Balancing Energy Intake with Energy Output

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  • Learning Objectives

    • Describe energy balance.
    • Define basal metabolism and explain the factors that affect basal metabolic rate.
    • Summarize factors that affect energy intake and energy output.

    Energy Balance

    Recall that the macronutrients you consume are either converted to energy, stored, or used to synthesize macromolecules. A nutrient’s metabolic path is dependent upon energy balance. When you are in a positive energy balance the excess nutrient energy will be stored or used to grow (e.g., during childhood, pregnancy, and wound healing). When you are in negative energy balance you aren’t taking in enough energy to meet your needs, so your body will need to use its stores to provide energy. Energy balance is achieved when intake of energy is equal to energy expended.

    Weight can be thought of as a whole body estimate of energy balance. Body weight is maintained when the body is in energy balance. Weight is lost when the body is in negative energy balance. Weight is gained when the body is in positive energy balance. In general, weight is a good predictor of energy balance, but many other factors play a role in energy intake and energy expenditure. Some of these factors are under your control and others are not. Let us begin with the basics of energy intake and energy output. Then we will consider the other factors that play a role in maintaining energy balance and hence, body weight.

    Energy Intake

    The amount of energy taken in each day comes from the foods you eat and the beverages you drink. Energy intake is measured in kilocalories as described in Chapter 1, which are commonly referred to as calories.

    Energy Expenditure

    The amount of energy you expend every day includes not only the calories you burn during physical activity, but also the calories you burn while at rest (basal metabolism), and the calories you burn when you digest food (Figure \(\PageIndex{1}\)).

    Basal metabolism refers to those metabolic pathways necessary to support and maintain the body’s basic functions (e.g., breathing, heartbeat, liver and kidney function) while at rest. The basal metabolic rate (BMR) is the amount of energy required by the body to conduct its basic functions over a certain time period. The great majority of energy expended (between 60 and 75%) daily is from conducting life’s basic processes. BMR is dependent on body size, body composition, gender, age, nutritional status, and genetics (Table \(\PageIndex{1}\)). People with a larger frame size have a higher BMR simply because they have more mass. Muscle tissue burns more calories than fat tissue even while at rest and thus the more muscle mass a person has, the higher their BMR. Since females typically have less muscle mass and a smaller frame size than men, their BMRs are generally lower than men’s. As we get older, muscle mass declines and thus so does BMR. Nutritional status also affects basal metabolism. Caloric restriction, as occurs while dieting, for example, causes a decline in BMR. This is because the body attempts to maintain homeostasis and will adapt by slowing down its basic functions to offset the decrease in energy intake. Body temperature and thyroid hormone levels are additional determinants of BMR.

    The energy required for all the enzymatic reactions that take place during food digestion and absorption of nutrients is called the “thermic effect of food” and accounts for about 5-10% of total energy expended per day.

    The other energy required during the day is for physical activity. Depending on lifestyle, the energy required for this ranges between 15 and 35% of total energy expended. The main control a person has over total energy expenditure is to increase physical activity.

    Components of energy expenditure include basal metabolic rate (60-75%), thermic effect of food (5-10%), and physical activity (15-35%).
    Figure \(\PageIndex{1}\): Energy expenditure is the sum of energy expended at rest (BMR), during digestion (TEF), and during physical activity.
    Table \(\PageIndex{1}\): Factors Affecting Basal Metabolic Rate (BMR)
    Factors That Increase BMR Factors That Decrease BMR
    Higher lean body mass Lower lean body mass
    Greater height Shorter height
    Younger age Older age
    Male gender Female gender
    Elevated levels of thyroid hormone Depressed levels of thyroid hormone
    Stress, fever, illness Fasting, very low-calorie diets

    Factors Affecting Energy Intake

    Physiological Influences

    How much we eat and what we eat is controlled not only by our own desires, but is also regulated physiologically. The hypothalamus in the brain is the main control point of hunger. It receives hormonal and neural signals, which determine if you feel hungry or full. Hunger is an unpleasant sensation of feeling empty that is communicated to the brain by both mechanical and chemical signals from other parts of the body. Conversely, satiety is the sensation of feeling full and it also is determined by mechanical and chemical signals relayed from other parts of the body.

    Hunger pangs are real and so is a “growling” stomach. When the stomach is empty it contracts, producing the characteristic pang and “growl.” The stomach’s mechanical movements relay neural signals to the hypothalamus, which relays other neural signals to parts of the brain. This results in the conscious feeling of the need to eat. Alternatively, after you eat a meal the stomach stretches and sends a neural signal to the brain stimulating the sensation of satiety and relaying the message to stop eating. The stomach also sends out certain hormones when it is full (peptide YY) and others when it is empty (ghrelin). These hormones communicate to the hypothalamus and other areas of the brain either to stop eating or to find some food.

    Adipose (fat) tissue also plays a role in regulating food intake. Adipose tissue produces the hormone leptin, which communicates to the satiety center in the hypothalamus that the body is in positive energy balance.

    Nutrients themselves also play a role in influencing food intake. After eating, blood glucose levels are high which then stimulates the release of insulin. Insulin binds to receptors in the hypothalamus to help indicate that you're full. When there are low levels of glucose in the blood, insulin is not released and therefore doesn't block the receptors in the hypothalamus.

    Genetic Influences

    Genetics certainly play a role in body fatness and weight and also affects food intake. Children who have been adopted typically are similar in weight and body fatness to their biological parents. Moreover, identical twins are twice as likely to be of similar weights as compared to fraternal twins. The scientific search for obesity genes is ongoing and a few have been identified, such as the gene that encodes for leptin. However, overweight and obesity that manifests in millions of people is not likely to be attributed to one or even a few genes, but rather to the interactions of hundreds of genes with the environment.

    Psychological/Behavioral Influences

    When your mouth waters in response to the smell of food you are experiencing a psychological influence on food intake. A person’s perception of good-smelling and good-tasting food influences what they eat and how much they eat. Mood and emotions are associated with food intake.

    Certain behaviors can be predictive of how much a person eats. Some of these are how much food a person puts on their plate, how often they snack on calorie-dense, salty foods, how often they watch television or sit at a computer, and how often they eat out.

    Societal Influences

    Food preferences, cultural customs, and religious practices are just a few of the societal influences that can impact energy intake. Special occasions with family or friends can lead to overeating. In addition, there tends to be easy access to high-calorie, convenient, fast food which can influence eating behavior.

    Factors Affecting Energy Expenditure

    Physiological and Genetic Influences

    Why is it so difficult for some people to lose weight and for others to gain weight? There are several theories that attempt to explain the impact of genetics including:

    • Set Point Theory. One theory is that every person has a “set point” of energy balance. This "set point" works to maintain our weight within a narrow range by triggering changes in energy intake or expenditure to maintain body weight within the target range. Some believe that this theory provides an explanation as to why after dieting, most people return to their original weight not long after stopping the diet.
    • Thrifty Gene Theory. The thrifty gene theory proposes that a gene (or genes) causes people to be energetically thrifty. In theory, people with this gene expend less energy than other people and therefore may gain weight more easily and be more resistant to weight loss.
    • FTO Gene. Although it is likely that there is no one 'obesity gene', there are some genes that have been identified - such as the fat mass and obesity (FTO) gene. The FTO gene is associated with higher levels of hunger, increased intake of high fat foods & refined starches. It's estimated that 44-65% of people have at least one copy of the FTO gene.
    • Protein Leverage Hypothesis. This hypothesis proposes that the body has a fixed daily dietary protein target that must be reached. The hypothesis suggests that overeating may occur because our diets are proportionally higher in carbohydrate and fat (and lower in protein).
    • Drifty Gene Hypothesis. The drifty gene hypothesis proposes that random mutation and drift in genes control the upper limit of body fatness. Individuals with "drifty genes" are prone to obesity because the genes do not control the upper limit of body fatness.

    Psychological/Behavioral Influences

    Sedentary behavior includes sitting, reclining, or lying down while awake. The sedentary lifestyle of many Americans contributes to their lower average energy expenditure in daily life. Simply put, the more you sit, the less energy you expend. Americans also partake in an excessive amount of screen time, which is a sedentary behavior that not only reduces energy expenditure, but also may contribute to weight gain because of the exposure to aggressive advertising campaigns for unhealthy foods.

    Societal Influences

    In the United States, many societal factors influence the number of calories burned in a day. Escalators, moving walkways, elevators, and cars are common modes of transportation that reduce average daily energy expenditure. Office work, high-stress jobs, and occupations requiring extended working hours are all societal pressures that reduce the time allotted for exercise of many Americans. Even remote controls contribute to being less active.

    Socioeconomic status has been found to be related to weight gain; people of lower economic status have higher rates of obesity and related chronic diseases than people with higher incomes.1 One reason for this relationship is that inhabitants of low-income neighborhoods have reduced access to safe streets and parks for walking. Another is that fitness clubs are expensive and few are found in lower-income neighborhoods. In addition, some are economically forced to purchase cheap, unhealthy foods, such as that at a nearby gas station.

    Key Takeaways

    • Energy balance is achieved when energy intake is equal to energy expended.
    • The amount of energy you expend every day includes not only the calories you burn during physical activity, but also the calories you burn at rest (basal metabolism), and the calories you burn when you digest food.
    • Basal metabolic rate (BMR) is dependent on body size, body composition, gender, age, nutritional status, genetics, body temperature, and thyroid hormone levels. The great majority of energy expended (between 60 and 75%) daily comes from conducting life’s basic processes.
    • The main control a person has over total energy expenditure is to increase physical activity.
    • Energy intake and energy expenditure are regulated by complex physiological responses and are influenced by genetics, behavior, and society.


    1. Pan L, Sherry B, Njai R, Blanck HM. Food insecurity is associated with obesity among US adults in 12 states. J Acad Nutr Diet. 2012;112(9):1403-1409. doi:10.1016/j.jand.2012.06.011