Section 1: Sources of Energy for the Body

Section 2: Uses of Energy by the Body

Basal metabolic rate
Physical activities
Growth and stored energy

Section 3: Energy Balance

Energy balance equation
Adults
Children, adolescents, and teenagers

Section 4: Weight Gain in Adolescents

Sources of Energy - Food

The human body uses food as a source of energy and for raw materials to maintain the body and produce new body tissues. The major nutrients in foods are carbohydrates, fats, and proteins. The energy from these nutrients is represented as E_in.

E_in = energy from food consumed

Carbohydrates are the major source of energy for the body.

sources of carbohydrates, including bread, greens, tomato and orange
1 g carbohydrate = 4 calories

Fats are important for energy storage.

sources of fats, including red meat, cheese, olives and oil
1 g fat = 9 calories

Proteins are used to build new body tissues, but they can also be energy sources.

sources of protein, including milk, eggs, beans, poultry and red meat
1 g protein = 4 calories

Uses of Energy - Basal Metabolic Rate

Basal metabolic rate, or BMR, represents the energy used to carry out necessary body activities such as breathing, regulating body temperature, and maintaining a heart beat. The majority of daily energy output, 60 to 70 percent, is used for these activities.

BMR varies across individuals:

Age: younger people have higher BMR
Growth: children and pregnant women have higher BMR
Height: tall, thin people have higher BMR
Body composition: people with more lean tissue have higher BMR

BMR varies across time for individuals:

Fever: fever increases BMR
Stress: stress increases BMR
Temperature: hot and cold outside temperatures raise BMR
Fasting: drastic dieting lowers BMR

Uses of Energy - Physical Activities

For most people, physical activities account for about 20 to 30 percent of the body’s total energy output. The number of calories used for an activity varies with a person’s age, weight, and gender. The total number of calories used for an activity depends on the intensity level and duration of the activity.

Activity Level	Activity	Calories per hour
Resting	sleeping, lying quietly	60
Very Light	watching television, eating	85
Light	walking, household chores	140
Moderate	bicycling, dance practice	285
Heavy	swim practice, basketball	400

Uses of Energy - Growth and Storage

In addition to BMR and physical activities, the bodies of growing children, adolescents, and teenagers use energy to produce new body tissues such as bone, muscle, and blood. This energy is represented by E_growth:

E_growth = energy used to produce new body tissues

Young people's bodies also store energy in the form of fat as a normal part of development. This stored energy, represented by E_stored, may be used later for growth. It takes 3500 calories to make 1 pound of stored fat.

E_stored = energy stored as fat

Energy Output

The energy used for basal metabolic rate and physical activities is represented by E_out.

E_out = energy used for BMR + energy used for activities

The Energy Balance Equation

E_in = E_out

The energy balance equation shows the relationship between energy input (E_in) and energy output (E_out). Energy balance is determined over long periods of time, like months and years, not over short periods of time, like days.

Energy input and output are expressed in calories.

Energy Balance in Adults

Adults who, over time, consume the same number of calories in food that they expend in BMR and physical activities are in energy balance: E_in = E_out. They maintain a constant body weight.

Adults who consistently consume more calories than they expend are in positive energy balance: E_in > E_out. They gain weight.

Adults who consistently expend more calories than they consume are in negative energy balance: E_in < E_out. They lose weight.

Energy Balance in Children, Adolescents, and Teenagers

In order to grow properly and maintain a healthy state, children, adolescents, and teenagers must be in positive energy balance: E_in > E_out. They need to consume more calories than needed for BMR and physical activities (E_out). The extra calories they consume are used for increasing the amount of important body tissues such as bone, muscle, and blood (E_growth). Extra calories also may be stored as fat (E_stored).

For healthy children, adolescents, and teenagers, the energy balance equation is E_in = E_out + E_growth + E_stored.

Weight Gain in Adolescents

Adolescence is a time of rapid growth and development, processes that require extra energy and result in weight gain. The amount of normal weight gain expected varies with age, gender, weight, and height.

A 12-year-old girl is expected to gain 8 to 13 pounds over the next year. The average weight gain is 10 pounds.

A 12-year-old boy is expected to gain 8 to 15 pounds over the next year. The average weight gain is 11 pounds.

A 13-year-old girl is expected to gain 7 to 11 pounds over the next year. The average weight gain is 7 pounds.

A 13-year-old boy is expected to gain 9 to 14 pounds over the next year. The average weight gain is 12 pounds.

Because growth occurs in spurts, young teens and pre-teens may gain several pounds in one month and none at all the next month. This is normal. Nutritionists consider weight gain over longer periods of time to evaluate whether the amount of weight gain is too little or too much.