Every time people eat or drink, they're giving the body what it needs to function: energy. When consumed, carbohydrates, fats, proteins and alcohol are metabolised to produce energy; this energy is needed to maintain body function, for physical activity and to grow and repair body tissues. Energy is measured in calories; energy expenditure is the term for the total amount of energy, or calories, the body uses. Energy expenditure is divided into three components: basal metabolic rate, thermogenesis and physical activity.
Basal Metabolic Rate
Basal metabolic rate (BMR), also referred to as resting metabolic rate (RMR), is the minimal amount of energy the body requires when it's at rest. BMR represents 60 to 75 per cent of the body's total energy expenditure. Total body weight and muscle to fat ratio are the biggest determinants of BMR. Men tend to have a higher BMR than women because they have more muscle; muscle requires more energy to maintain than fat. Infants and children have a higher BMR for their size due to their rapid development, while older adults have a lower BMR since they carry less muscle.
The process of food digestion, food absorption and nutrient transportation expends energy; this is known as thermogenesis, or the thermogenic effect of food. The energy used during thermogenesis accounts for 5 to 10 per cent of the body's total energy expenditure. Since the rate of energy expenditure naturally decreases later in the day, food has an increased thermogenic effect when eaten earlier.
The most variable factor in energy expenditure is physical activity. This is comprised of all muscle movement, ranging from gross muscular work and exercise to small movements like fidgeting and shivering. On average, physical activity accounts for 15 to 30 per cent of a person's total energy expenditure. This figure varies, however, depending on activity level. Energy expenditure also varies according to body weight --- a heavier person expends more energy. The rate of energy expenditure during exercise depends on the duration, intensity and frequency of the exercise.
When the calories coming into the body do not equal the calories, or energy, being expended, an energy imbalance occurs and weight is gained or lost. Weight gain results when a person eats more calories than he can physically burn; weight loss happens when a person consumes too few calories for his physical activity. This imbalance is fixed by adjusting calorie consumption, physical activity or both.