32 Introduction to Human Nutrition
dynamics, which states that energy can be neither
destroyed nor created. This principle necessitates that
when energy intake equals energy expenditure, body
energy stores must remain constant. This chapter
explains how the body is able to achieve this state of
energy balance through control of energy intake and
energy expenditure. In addition, the various ways that
body energy stores can be measured and some exam-
ples of conditions in which energy balance may be
disrupted are summarized. Particular emphasis is
placed on obesity, which is the end-result of a positive
energy balance and is now considered one of the
major nutritional disorders.
Components of energy balance
Energy intake
Energy intake is defi ned as the caloric or energy
content of food as provided by the major sources of
dietary energy: carbohydrate (16.8 kJ/g), protein
(16.8 kJ/g), fat (37.8 kJ/g), and alcohol (29.4 kJ/g).
Energy storage
The energy that is consumed in the form of food or
drinks can either be stored in the body in the form of
fat (the major energy store), glycogen (short-term
energy/carbohydrate reserves), or protein (rarely
used by the body for energy except in severe cases of
starvation and other wasting conditions, as discussed
later in the chapter), or be used by the body to fuel
energy-requiring events.
Energy expenditure
The energy that is consumed in the form of food is
required by the body for metabolic, cellular, and
mechanical work such as breathing, heart beat, and
muscular work, all of which require energy and result
in heat production. The body requires energy for a
variety of functions. The largest use of energy is
needed to fuel the basal metabolic rate (BMR), which
is the energy expended by the body to maintain basic
physiological functions (e.g., heart beat, muscle con-
traction and function, respiration). BMR is the
minimum level of energy expended by the body to
sustain life in the awake state. It can be measured after
a 12 hour fast while the subject is resting physically
and mentally, and maintained in a thermoneutral,
quiet environment. The BMR is slightly elevated
above the metabolic rate during sleep, because energy
expenditure increases above basal levels owing to the
energy cost of arousal. Because of the diffi culty in
achieving BMR under most measurement situations,
resting metabolic rate (RMR) is frequently measured
using the same measurement conditions stated for
BMR. Thus, the major difference between BMR and
RMR is the slightly higher energy expended during
RMR (~ 3%) owing to less subject arousal and non-
fasting conditions. Because of this small difference,
the terms basal and resting metabolic rate are often
used interchangeably. RMR occurs in a continual
process throughout the 24 hours of a day and remains
relatively constant within individuals over time. In
the average adult human, RMR is approximately
4.2 kJ/min. Thus, basal or resting metabolic rate is the
largest component of energy expenditure and makes
up about two-thirds of total energy expenditure.
In addition to RMR, there is an increase in energy
expenditure in response to food intake. This increase
in metabolic rate after food consumption is often
referred to as the thermic effect of a meal (or meal-
induced thermogenesis) and is mainly the energy that
is expended to digest, metabolize, convert, and store
ingested macronutrients, named obligatory thermo-
genesis. The measured thermic effect of a meal is
usually higher than the theoretical cost owing to a
facultative component caused by an activation of
the sympathoadrenal system, which increases energy
expenditure through peripheral β-adrenoceptors.
The energy cost associated with meal ingestion is pri-
marily infl uenced by the composition of the food that
is consumed, and also is relatively stable within indi-
viduals over time. The thermic effect of a meal usually
constitutes approximately 10% of the caloric content
of the meal that is consumed. The third source of
energy expenditure in the body is the increase in
metabolic rate that occurs during physical activity,
which includes exercise as well as all forms of physical
activity. Thus, physical activity energy expenditure
(or the thermic effect of exercise) is the term fre-
quently used to describe the increase in metabolic rate
that is caused by use of skeletal muscles for any type
of physical movement. Physical activity energy expen-
diture is the most variable component of daily energy
expenditure and can vary greatly within and between
individuals owing to the volitional and variable nature
of physical activity patterns.
In addition to the three major components of
energy expenditure, there may be a requirement for
energy for three other minor needs.