Energy Metabolism

Living beings continuously expend energy to sustain vital life functions and, when food is available, they periodically consume energy from fat, carbohydrate, and protein.  The difference between energy intake and energy expenditure determines whether, over time, body weight increases, decreases, or remains constant.  This is why understanding energy processes is very important.

The relationship between energy in and energy out is referred to as the energy balance equation and is expresses as follows:

Energy Intake = Energy Expenditure + Energy Storage

A person whose weight is stable successfully balances energy intake and expenditure.  Weight loss occurs when expenditure exceeds intake or, put another way, when intake is less than expenditure.  Weight gain occurs when intake exceeds expenditure or when expenditure is less than intake.

Your daily total energy expenditure (TEE) has three principal components:  basal or resting metabolic rate (BMR or RMR); adaptive thermogenesis; and activity energy expenditure (AEE).  For a typical sedentary person, BMR accounts for an estimated 65-75% of total daily energy output, adaptive thermogenesis accounts for about 10-15%, and physical activity is responsible for the rest.  In fact, activity energy expenditure accounts for most of the variability in TEE and has the greatest potential for increasing energy expenditure.

Energy intake occurs when we ingest foods that contain the energy nutrients (fat, carbohydrates, and protein).  When intake is adequate, all of the energy nutrients can be stored.  Fat, or lipid, is by far the most abundant source of stored energy.  One pound of stored body fat is equal to approximately 3,500 Kcal of energy.  Even a 140-lb person who is only 15% fat stores 21 lb of fat-the equivalent of more than 73,000 kcal.

The energy balance equation suggests that a decrease in food intake and/or an increase in any or all components of energy expenditure to equal 3,500 kcal will result in a loss of 1 pound.  Conversely, an increase in food intake and/or a decrease in energy expenditure to equal  3,500 kcal will yield a gain of 1 pound.  The conclusion is logical but overly simplistic.  Energy metabolism is neither so predicable nor consistent from person to person.  The billion dollar diet book industry is based on the assumption that energy in is the main culprit in weight gain:  "If overweight people could only be convinced to reduce their intake or eat certain foods, then excess pounds would disappear."  In fact, the often-overlooked aspect of energy out bears much responsibility for weight gain and regain, too.

Carbohydrates, lipids, and protein comprise the energy-in component of energy balance.  They are known as the energy nutrients because they contain calories.  The caloric value of food is expressed in kilocalories (kcal).  One kilocalorie is defined as the amount of heat required to raise the temperature of 1 kg of water 33.8 degrees Fahreheit.  One kilocalorie (or Calorie with a capital C) equals 1,000 cal. The terms calorie (with a small c) and kilocalorie are often used interchangeably.

How is Food Energy Transformed in the Body?

What is the Role of Carbohydrates in Energy Metabolism?  

What is the Role of Protein in Energy Metabolism:

How do we know which nutrient is being used for fuel?


How do dietary fats act as Key Regulators of Energy