If a person maintains a healthy weight over time, the person is in energy balance. Food energy intake equals energy expenditure: deposits of fat made at one time have been compensated for by withdrawals made at another. In other words, the body uses fat as a savings account for energy. But, unlike money, having more fat is not better; there is an optimum.
A day’s energy balance can be stated like this: change in energy stores equals the food energy taken in (calories) minus the energy spent on metabolism and physical activities (calories). More simply:
Change in energy stores = energy in (calories) = energy out (calories).
The energy in food and beverages is the only contributor to the “energy in” side of the energy balance equation. Before you can decide how much food will supply the energy you need in a day, you must first become familiar with the amounts of energy in foods and beverages.
One way to do so is to look up the calories provided by various foods and beverages in the Table of Food Composition. Alternatively, computer programs can readily provide this information. Food composition data would reveal that an apple provides about 70 calories from carbohydrates and a candy bar supplies about 250 kcalories, mostly from fat and carbohydrate.
You may have heard that for every 3500 calories you eat in excess of expenditures, you store 1 pound of body fat—a general rule that has previously been used for mathematical estimations. Keep in mind, however, that this number can vary widely with individual metabolic tendencies and efficiencies of nutrient digestion and absorption. Currently, the dynamics of energy storage are a topic of intense scientific investigation. The fat stores of even a healthy-weight adult represent an ample reserve of energy—50,000 to 200,000 calories.
The body expends energy in two major ways: to fuel its basal metabolism and to fuel its voluntary activities. People can change their voluntary activities to expend more or less energy in a day, and over time they can also change their basal metabolism by building up the body’s metabolically active lean tissue.
Basal metabolism supports the body’s work that goes on all the time without the person’s conscious awareness. The beating of the heart, the inhaling and exhaling of air, the maintenance of body temperature, and the transmission of nerve and hormonal messages to direct these activities are the basal processes that maintain life.
Basal metabolism represents about two-thirds of the total energy a sedentary person expends in a day. In practical terms, a person whose total energy needs are 2000 calories per day may expend 1000 to 1300 of them to support basal metabolism.
The basal metabolic rate (BMR) is the rate at which the body expends energy for these life-sustaining activities. This rate varies from person to person and may vary for an individual with a change in circumstance or physical condition. The rate is slowest when a person is sleeping undisturbed, but it is usually measured when the person is awake, but lying still, in a room with a comfortable temperature after a restful sleep and an overnight (12- to 14-hour) fast. A similar measure of energy output—called the resting metabolic rate (RMR)—is slightly higher than the BMR because its criteria for recent food intake and physical activity are not as strict.
BMR. For the most part, the BMR is highest in people who are growing (children, adolescents, and pregnant women) and in those with considerable lean body mass (physically fit people and males). One way to increase the BMR, then, is to maximize lean body tissue by participating regularly in endurance and strength-building activities. The BMR is also fast in people who are tall and so have a large surface area for their weight, in people with fever or under stress, in people taking certain medications, and in people with highly active thyroid glands. The BMR slows down with a loss of lean body mass and during fasting and malnutrition.
Energy for Physical Activities
The number of calories spent on voluntary activities depends on three factors: muscle mass, body weight, and activity. The larger the muscle mass required for the activity and the heavier the weight of the body part being moved, the more calories are spent. The activity’s duration, frequency, and intensity also influence energy costs: the longer, the more frequent, and the more intense the activity, the more calories are expended.
Energy to Manage Food
When food is taken into the body, many cells that have been dormant become active. The muscles that move the food through the intestinal tract speed up their rhythmic contractions, and the cells that manufacture and secrete digestive juices begin their tasks.
All these and other cells need extra energy as they participate in the digestion, absorption, and metabolism of food. This cellular activity produces heat and is known as the thermic effect of food. The thermic effect of food is generally thought to represent about 10 percent of the total food energy taken in. For purposes of rough estimates, though, the thermic effect of food is not always included.
Estimating Energy Requirements
In estimating energy requirements, the DRI committee developed equations that consider how the following factors influence energy expenditure:
- Gender. Women generally have a lower BMR than men, in large part because men typically have more lean body mass. In addition, menstrual hormones influence the BMR in women, raising it just prior to menstruation. Two sets of energy equations—one for men and one for women—were developed to accommodate the influence of gender on energy expenditure.
- Growth. The BMR is high in people who are growing. For this reason, pregnant and lactating women, infants, children, and adolescents have their own sets of energy equations.
- Age. The BMR declines during adulthood as lean body mass diminishes. Physical activities tend to decline as well, bringing the average reduction in energy expenditure to about 5 percent per decade. The decline in the BMR that occurs when a person becomes less active reflects the loss of lean body mass and may be prevented with ongoing physical activity. Because age influences energy expenditure, it is also factored into the energy equations.
- Physical activity. Using individual values for various physical activities is time-consuming and impractical for estimating the energy needs of a population. Instead, various activities are clustered according to the typical intensity of a day’s efforts.
- Body composition and body size. The BMR is high in people who are tall and so have a large surface area. Similarly, the more a person weighs, the more energy is expended on basal metabolism. For these reasons, the energy equations include a factor for both height and weight.
Energy needs vary among individuals depending on such factors as gender, growth, age, physical activity, and body composition. Even when two people are similarly matched, however, their energy needs will still differ because of genetic differences. Perhaps one-day genetic research will reveal how to estimate requirements for each individual.
A person takes in energy from food and, on average, expends most of it on basal metabolic activities, some of it on physical activities, and about 10 percent on the thermic effect of food.
Because energy requirements vary from person to person, such factors as age, gender, and weight must be considered when calculating energy expended on basal metabolism, and the intensity and duration of the activity must be taken into account when calculating expenditures on physical activities.