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

Introduction to energy expenditure

The human body converts the food consumed by the individual into energy. The amount of energy provided by food is typically measured either in calories or kilojoules.

Different types of food contain different amounts of 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.

What causes the body to burn calories?

There are many factors which affect the amount of energy an individual expends. Performing essential bodily functions and performing physical activities are the two major expenders of energy, although energy expenditure is also influenced by other factors such as age, gender and even menstruation.

Essential functions

A person who is resting in bed and does not do any form of physical activity, (e.g. walking), will still require about 1200 calories in a 24 hour period to maintain their essential functions. This energy expenditure is known as 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).

Physical activity and exercise

Apart from basal energy expenditure, an individual expends most of their energy performing a range physical activities, and 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.

After physical activity

The effects of physical activity on energy expenditure are felt even after physical activity has ceased, as the body’s oxygen requirement increases in the post activity period. The extent of the increase in energy expenditure post-exercise is related to the vigour of the exercise performed. An individual can expect to expend an additional 15% of the energy expended performing a physical activity, in the post activity period. As more vigorous activities expend more energy while they are being performed, they also cause an individual to expend more energy in the post exercise period. Depending on the vigour of the activity performed, an individual may expend additional energy for a period of up to 24 hours after performing the physical activity.

Growth, pregnancy and lactation

Periods of growth, that is, periods in which new tissues are being grown by the body, require additional energy compared to periods of non-growth and thus an individual who is growing requires additional energy to maintain this growth. Such tissue growth occurs during childhood, adolescence, 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.

Energy expenditure in infants

The most intense period of growth occurs during the first year of life. The amount of energy expended by babies and very young children differ significantly from the amount expended by older children and adults. This is mainly because, in the first year of life, a considerable proportion of the energy consumed by a baby is used to assist growth. This growth-assisting energy is converted to tissues which increase the size of bones, muscles and body organs like the liver, kidney and brain and the process of converting energy to body tissues is known as tissue deposition. Babies typically double their body weight within six months, and triple it within 12 months, before their rate of growth stabilises. Thus the proportion of energy which is used for growth and tissue deposition is highest in early infancy. Up to one month of age, an infant uses some 35% of its energy for growing. This decreases to around 3% by 12 months of age. From there, growth remains relatively stable until the pubertal growth spurt, where up to 4% of energy is used to aid growth. In the first year of and extensive growth, energy expenditure per kilogram of body weight in infants is approximately double that of adults and basal energy expenditure accounts for almost all the infants total energy expenditure.

Energy expenditure also varies between babies depending on the age and sex of the child and whether they are being fed with breast milk or formulas and supplementary foods. Infants typically require more energy as they age, and male infants usually expend more energy than female infants. Infants consuming only breast milk typically expend less energy than those being fed with infant formulas and other foods, in the first 12 months of life. At age three months, a formula-fed infant requires 7% more energy than a breastfed infant, while at six, nine and twelve months of age the additional requirement for formula-fed infants is 8%, 9% and 3% respectively.

Gender

Energy expenditure in women is estimated to be 16% lower than energy expenditure in men. The exact reason for the difference is unclear, however it is thought to be related to the different hormonal environment of male and female bodies.

Thermal effect of food

Eating increases the body’s energy expenditure slightly, as the body must work harder, and thus use more energy, to digest and process food. This is known as the thermal effect of food. The extent to which food consumption increases energy expenditure above resting levels (i.e. above BEE) depends on the type of food which is consumed. The thermal effect of carbohydrates raises the body’s energy expenditure by 5-10% for the period taken to digest the food. Consumption of fat raises energy expenditure by <5%, while protein consumption raises energy expenditure by up to 30%, reflecting the more energy intensive process required to process protein in the body.

Menstruation

A women’s BEE appears to increase during the 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.

Age

As an adult ages, their BEE decreases. In men, the reduction in energy requirement commences at around 40 years of age, while women typically experience reduced energy expenditure following menopause, beginning at around 50 years of age.

Genetics

 

Energy requirements differ considerably between individuals and this is thought to be genetically related. While there is currently insufficient evidence to predict an individual’s energy requirements based on genetic characteristics, this may be possible in the near future.

 

How is physical activity measured?

Metabolic equivalents (METs)

Metabolic equivalents (METs) is a measure of the vigour of a physical activity and is based on the rate of oxygen consumption while that activity is being undertaken. The amount of oxygen consumed is related to the number of calories expended.

By calculating the METs of a physical activity, it is possible to also calculate the number of calories which will be expended for each minute that the activity is performed. As an individual’s body weight also influences they amount of oxygen consumed during physical activity, body weight is also considered when calculating how many calories an individual consumed during activity.

The following table provides the METs values for a range of common household and leisure activities and the energy expenditure (in calories) equivalent to 30 minutes of that activity for men and women weighing 70kg and 57kg respectively.

	METS	Energy expenditure (calories) 30 minutes (man 70kg)	Energy expenditure (calories) 30 minutes (woman 57kg)
Mild
Playing accordion	1.8	66	54
Flute	2.0	74	60
Horseback riding (walking)	2.3	85	69
Cello	2.3	85	69
Piano	2.3	85	69
Billiards	2.4	88	72
Canoeing (leisurely)	2.5	92	75
Golf (with cart)	2.5	92	75
Violin	2.5	92	75
Walking (2 mph)	2.5	92	75
Dancing (ballroom)	2.9	107	87
Volleyball (noncompetitive)	2.9	107	87
Moderate
Walking (3 mph)	3.3	121	99
Cycling (leisurely)	3.5	129	105
Calisthenics (no weight)	4.0	147	120
Golf (without cart)	4.4	162	132
Swimming (slow)	4.5	165	135
Walking (4 mph)	4.5	165	135
Vigorous
Chopping wood	4.9	180	147
Tennis (doubles)	5.0	184	150
Dancing ballroom (fast) or square	5.5	202	165
Ice skating	5.5	202	165
Cycling (moderately)	5.7	209	171
Dancing (aerobic or ballet)	6.0	221	180
Surfing	6.0	221	180
Skating Roller	6.5	239	195
Skiing (water or downhill)	6.8	250	203
Climbing hills (no load)	6.9	254	206
Swimming	7.0	257	209
Climbing hills (5 kg load)	7.4	272	221
Walking (5 mph)	8.0	294	239
Jogging (10 min miles)	10.2	375	305
Rope skipping	12.0	441	359
Squash	12.1	445	362

Physical activity level (PAL)

Physical activity is measured using a rating of Physical Activity Level (PAL), which quantifies the proportion of total energy expenditure caused by physical activity. Individuals with a low PAL expend only a small proportion of their total energy performing physical activities; the majority of their energy is expended to maintain essential bodily functions. Individuals with a high PAL on the other hand expend more than half of the total energy performing physical activities. The following PALs correspond to the categories of physical activity.

Sedentary (1.0 ≤ PAL < 1.4)

Sedentary individuals consume energy through basal energy expenditure, the thermal effect of food, and conducting the tasks necessary for independent living (e.g. sitting, standing, cooking, cleaning). They do not perform any other physical activity, such as recreational walking.

Low activity (1.4 ≤ PAL < 1.6)

Individuals who are categorised as low activity consume energy through the same mechanisms as sedentary individuals. In addition, an average weight individual (i.e. a man weighing 70kg or a woman weighing 57kg) will undertake physical activity which is equivalent to walking 2.2 miles each day at a rate of 3-4 miles per hour, or roughly 45 minutes of brisk walking. Brisk walking can be considered walking which increases the heart rate to such an extent that the individual becomes slightly out of breath, but can still talk as they perform the activity. Individuals who are much lighter need to undertake greater amounts of activity, while those who are much heavier do not need to undertake as much physical activity. For example, an individual who weighed only 44kg, would need to walked for 2.9 miles at the same speed, while one who weighed 120kg would need walk for only 1.5km.

Active (1.6 ≤ PAL < 1.9)

As with sedentary individuals, active individuals expend energy through eating and performing the tasks of everyday living. In addition, men and women weighing 70kg and 57kg respectively, undertake physical activity which is equivalent to walking 7.3 miles per day, at a rate of 3-4 miles per hour, or roughly two hours of brisk walking. The physical activity requirement for active individuals weighing only 44kg increases to 9.9 miles per day, while for those weighing 120kg, the requirement increases to 5.3 miles per day.

Highly Active (1.9 ≤ PAL < 2.5)

Highly active individuals expend energy through the thermal effect of food and conducting the tasks of day to day living. In addition individuals weighing 70kg undertake physical activity which is equivalent to 16.7 miles per day of walking at a speed of 3-4 miles per hour, or roughly six hours of brisk walking. Individuals who weigh only 44kg need to walk 22.5 miles per day to exert the same effect, while those weighing 120kg need walk only 12.3 miles per day to achieve a PAL equivalent to being highly active.

Physical activity levels in infants

As an infant ages, energy consumed through physical activity accounts for an increasingly greater proportion of TEE. At three months of age, an infant’s PAL is 1.2, that is, only around 17% of its total energy expenditure occurs from physical activity, while some 83% of energy is expended maintaining essential functions (as BEE). The infant’s PAL typically rises to 1.4 by 24 months of age. So by 2 years, 29% of a child’s energy expenditure occurs through physical activity.

Exclusively breastfed infants obtain an estimated 500 calories per day from breast milk, a value which is somewhat lower than the energy expenditure requirements of infants aged two months and older. However, exclusive feeding with breast milk is recommended as the most nutritious dietary option for infants under six months of age, and, because of the lower energy requirements of breastfed infants compared to formula fed infants, babies will receive all the macro and micro nutrients they need in the first six months of life from exclusive breast feeding.

More information

For more information on fitness and exercise, including stretches, types of exercise, exercise recovery and exercise with health conditions, as well as some useful videos, see Fitness and Exercise.

 

References

1. Panel on Macronutrients & Standing Committee for the Scientific Evaluation of Dietary Reference Intakes. Energy. Chapter 5. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids (Macronutrients), National Academy of Sciences. 2005. [cited 2009, June 6], Available from http://www.nap.edu/catalog/10490.html
2. Panel on Macronutrients & Standing Committee for the Scientific Evaluation of Dietary Reference Intakes. Physical Activity. Chapter 12. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids (Macronutrients), National Academy of Sciences. 2005. [cited 2009, June 6], Available from http://www.nap.edu/catalog/10490.html