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6.5: Balancing Energy Input with Energy Output

Skills to Develop

  • Estimate your daily energy requirement.
  • Define basal metabolism and explain the factors that affect the basal metabolic rate.
  • Summarize why the amount of food we eat (appetite) is not completely under our conscious control.

To Maintain Weight, Energy Intake Must Balance Energy Output

Before you begin your journey to learn about energy balance, we need to define a few terms. These are shown in Table \(\PageIndex{1}\). 

Table 6.5.1: Terms for understanding energy balance.
Term Definition
Hunger physiological drive to eat
Appetite psychological drive to eat
Satiety State in which both drives are satisfied and there is no longer a desire to eat. Being satisfied.
Hypothalamus A group of cells at the base of the brain which participates in many regulatory functions including hunger
Leptin Hormone produced by body fat that plays a role in regulating food intake and physical activity
Ghrelin Hormone produced by the stomach that regulates appetite
Peptide YY (PYY) Protein produced by gastrointestinal tract that regulates appetite and food intake
Calorie or kilocalories amount of heat energy needed to raise temperature of 1 kg of water 1 C (unit of energy measurement)

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 the 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, lost when it is in negative energy balance (intake or input < expenditure or output), and gained when it is in positive energy balance (intake or input > expenditure or output). When your energy intake matches your energy output, you will maintain your weight, whatever that is. People who are overweight or obese can be in energy balance. In other words, their intake matches their energy output but at some point in their life, their energy intake exceeded their energy intake. 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 on how to estimate energy intake, energy requirement, and energy output. Then we will consider the other factors that play a role in maintaining energy balance and hence, body weight. In Figure 6.5.1, energy output exceeds energy input, so this person would be losing weight.

Figure 6.5.1: Energy is a balance between energy input and energy output. When one exceeds the other you will lose or gain weight.

One pound of body fat is approximately 3,500 kcal and 1 kg is 7,700 kcal. If you consume 10 more kcal per day than what you need, you will gain 1 lb per year. If you eat 100 kcal more than you expend per day, that is one slice of white bread, you will gain 10 lbs in a year. Portion sized have increased. In 1950, a burger, fry, and drink were 590 kcal. Today, a quarter-pound burger with cheese, large fries and drink is 1,550 kcal. Wow! What a huge increase in half a century - almost 300%.

Total Energy Expenditure (Output)

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), the calories you burn when you digest food and heat production (minor). The sum of caloric expenditure is referred to as total energy expenditure (TEE). 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 50 and 80 percent) daily is from conducting life’s basic processes. Of all the organs, the liver requires the most energy (see Table 6.5.5). Unfortunately, you cannot tell your liver to ramp up its activity level to expend more energy so you can lose weight. BMR is dependent on body size, body composition, sex, age, nutritional status, and genetics. 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, thyroid hormone levels, and pregnancy and lactation are additional determinants of BMR. 

Usually, we measure RMR (resting metabolic rate) or REE (resting energy expenditure). Both are measured shortly after awakening in the morning after at least a twelve hour fast. RMR or REE are slightly higher than BMR because the individual may have ambulated, for example, walked to urinated before measuring RMR. BMR is measured with no ambulatory movement. 

Table 6.5.2: Energy Breakdown of Organs
Organ Percent of Energy Expended
Liver 27
Brain 19
Heart 7
Kidneys 10
Skeletal muscle (at rest) 18
Other organs 19

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 percent of the total energy expended per day. Fat is metabolized more efficiently than protein or carbohydrate so the TEF for high-fat foods is less. Also, dietary fat is usually stored, not burned, after eaten.

The other energy required during the day is for physical activity. Depending on lifestyle, the energy required for this ranges between 15 and 50 percent of the total energy expended. The percent varies among individuals. The energy cost of physical activity depends on the activity, for example walking vs running, the length the activity is sustained, body weight, and training. The benefits of physical activity continue after the activity has stopped. The main control a person has over TEE is to increase physical activity.

Figure 6.5.2: Total energy expenditure is the sum of energy expended at rest, during digestion, and during physical activity.

Estimating Energy Requirement

To maintain body weight you have to balance the calories obtained from food and beverages with the calories expended every day. Here, we will discuss how to calculate your energy needs in kilocalories per day so that you can determine whether your caloric intake falls short, meets, or exceeds your energy needs. The Institute of Medicine has devised a formula for calculating your Estimated Energy Requirement (EER). It takes into account your age, sex, weight, height, and physical activity level (PA). The EER is a standardized mathematical prediction of a person’s daily energy needs in kilocalories per day required to maintain weight. It is calculated via the following formulas:

  • Adult male:

\[EER = 662 − [9.53 \times age (y)] + PA \times [15.91 \times wt (kg) + 5.39.6 \times ht (m)]\]

  • Adult female:

\[EER = 354 − [6.91 \times age (y)] + PA \times [9.36 \times wt (kg) + 726 \times ht (m)]\]

Note: to convert pounds to kilograms, divide weight in pounds by 2.2. To convert feet to meters, divide height in feet by 3.3.

The Harris Benedict Equation is a way to calculate RMR. You must add additional kcal for activity.

  • Men: RMR = 66.47 + 13.75(kg) + 5 (cm) - 6.76(age)
  • Women: RMR = 6.55.1 + 9.56(kg) + 1.85(cm)-4.68(age)

Examples of physical activity adjustments are as follows. Multiple the RMR by this number you calculate above by this number to get the total energy requirement: males and females bed rest - 1.1-1.3; males and females very sedentary - 1.2-1.4; males and females sedentary/maintenance - 1.3-1.5; males and females light activity - 1.4-1.5.

Estimating Caloric Intake

In Chapter 2, you learned how to calculate the number of calories in food. To determine your caloric intake per day requires that you conduct a dietary assessment and record the number of calories you eat.

Interactive 6.5.1

To begin your dietary assessment, go to MyPlate, which is available on the US Department of Agriculture (USDA) website:

Table 6.5.3: Physical Activity (PA) Categories and Values
Activity Level Men PA Value Women PA Value Description
Sedentary 1.00 1.00 No physical activity beyond that required for independent living
Low 1.11 1.12 Equivalent to walking 1.5 to 3 miles per day
Moderate 1.25 1.27 Equivalent to walking 3 to 10 miles per day
High 1.48 1.45 Equivalent to walking 10 or more miles per day
These values only apply to normal-weight adults and not to children or pregnant or lactating women.

The numbers within the equations for the EER were derived from measurements taken from a group of people of the same sex and age with similar body size and physical activity level. These standardized formulas are then applied to individuals whose measurements have not been taken, but who have similar characteristics in order to estimate their energy requirements. Thus, a person’s EER is, as the name suggests, an estimate for an average person of similar characteristics. EER values are different for children, pregnant or lactating women, and for overweight and obese people. Also, remember the EER is calculated based on weight maintenance, not for weight loss or weight gain.

Table 6.5.4: Estimated Daily Calorie Needs
Sex Age (years) Sedentary Moderately Active Active
Child (female and male) 2–3 1,000–1,200 1,000–1,400 1,000–1,400
Female 4–8 1,200–1,400 1,400–1,600 1,400–1,800
9–13 1,400–1,600 1,600–2,000 1,800–2,200
14–18 1,800 2,000 2,400
19–30 1,800–2,000 2,000–2,200 2,400
31–50 1,800 2,000 2,200
51+ 1,600 1,800 2,000–2,200
Male 4–8 1,200–1,400 1400–1,600 1,600–2,000
9–13 1,600–2,000 1,800–2,200 2,000–2,600
14–18 2,000–2,400 2,400–2,800 2,800–3,200
19–30 2,400–2,600 2,600–2,800 3,000
31–50 2,200–2,400 2,400–2,600 2,800–3,000
51+ 2,000–2,200 2,200–2,400 2,400–2,800

Source: US Department of Agriculture. 2010 Dietary Guidelines for Americans. 2010.

The 2010 Dietary Guidelines provides a table (Table 6.5.4) that gives the estimated daily calorie needs for different age groups of males and females with various activity levels. The 2010 Dietary Guidelines also states that while knowing the number of calories you need each day is useful, it is also pertinent to obtain your calories from nutrient-dense foods and consume the various macronutrients in their Acceptable Macronutrient Distribution Ranges (AMDRs) (Table 6.5.5).

Table 6.5.5: Acceptable Macronutrient Distribution Ranges
Age Carbohydrates (% of Calories) Protein (% of Calories) Fat (% of Calories)
Young Children (1–3) 45–65 5–20 30–40
Older children/adolescents (4–18) 45–65 10–30 25–35
Adults (19 and older) 45–65 10–35 20–35