Energy Expenditure: How the Body Burns Calories

Energy Expenditure: How the Body Burns Calories

• Introduction to energy expenditure
• What causes the body to burn calories?
  • Essential functions
  • Physical activity and exercise
  • Growth, pregnancy and lactation
    • Energy expenditure in infants
  • Gender
  • Thermal effect of food
  • Menstruation
  • Age
  • Genetics

 

• How is physical activity measured?
  • Metabolic equivalents (METs)
  • Physical activity level (PAL)
    • Sedentary (1.0 ≤ PAL  1.4)
    • Low activity (1.4 ≤ PAL  1.6)
    • Active (1.6 ≤ PAL  1.9)
    • Highly active (1.9 ≤ PAL  2.5)
  • Physical activity levels in infants

energy. Fats and alcohol contain relatively high amounts of energy (i.e. 9 calories per gram in fat and 7 calories per gram in alcohol) compared to carbohydrates (3.75 calories per gram) and proteins (4 calories per gram). Thus the amount of energy consumed depends not only on the amount of food consumed, but also on the types of food consumed.

Energy produced from food in the human body is used to maintain the body’s essential functions (e.g. cell growth and repair, respiration, blood transport) and perform physical tasks including work, exercise and recreational activities. The body needs a set amount of energy just to perform its essential functions, and in most individuals, the majority of energy consumed will be expended performing these functions.

The vast majority of individuals also perform at least some form of physical activity throughout the day. Physical activity includes all forms of movement, including walking, lifting objects, cleaning, exercising and dancing. As the total amount of physical activity an individual performs increases, that individual’s energy requirement also increases.

The energy expended in performing physical tasks accounts for most of the individual’s remaining energy expenditure (after energy expended for essential functions). A range of factors including the types of physical activities performed, as well as the length of time for which they are performed, influence the amount of energy consumed.

Basal Energy Expenditure (BEE) and is typically the largest single component of energy expenditure.

The Basal Metabolic Rate (BMR) is the rate at which energy is expended for BEE while an individual is awake, but in a completely resting state (e.g. lying down and having not eaten, as eating increases energy expenditure because the body must expend energy to digest consumed food).

Total Energy Expenditure (TEE) (i.e. basal energy expenditure plus all other energy expenditure) therefore depends largely on the amount of physical activity an individual performs. In individuals who perform a lot of physical activity (e.g. athletes, labourers), the component of energy consumed during physical activity is often more than the component expended performing essential bodily functions. Even in individuals who do not perform obviously physical tasks, physical activities which are performed unconsciously and spontaneously (e.g. fidgeting, smiling), consume energy.

Physical activity is the only factor influencing energy expenditure which can be controlled by an individual (as opposed, for example to gender and age). An individual who wished to increase their energy expenditure, therefore must increase either the quantity or vigour of physical activities they perform.

Physical activity is any activity which causes the body to move. Exercise is a distinct category of physical activity, characterised by activity which is sufficiently vigorous to induce laboured breathing and sweating. The amount of energy expended during a period of physical activity depends mainly upon the type of exercise and the vigour with which it is performed. More vigorous physical activities expend more energy, for example an individual who jogged for 30 minutes would expend more than an individual who walked for 30 minutes, as jogging is a more vigorous form of exercise.

The body weight of the individual performing the exercise also influences energy expenditure and, on average, heavier individuals will consume slightly more energy than lighter individuals performing the same tasks. For example, an individual who weighs 57kg would expend 135 calories by walking briskly for 30 minutes. The same physical activity would result in energy expenditure equivalent to 165 calories, in an individual who weighed 70kg.

gestation or lactation.

The growth of placental and foetal tissues also requires additional energy, and thus pregnant women have increased energy requirements. This is particularly true in the second and third trimesters of pregnancy, when foetal growth occurs most rapidly. Lactation also creates a higher energy requirement, due to the additional energy required to produce breast milk.

luteal phase (the post-ovulation phase of the menstrual cycle), compared to the follicular phase (the first half of the menstrual cycle or the phase immediately following the start of menstrual bleeding) of the menstrual cycle.

Article Dates:

Modified: 26/11/2010

Created: 18/8/2009

Energy Expenditure: How the Body Burns Calories