3
Energy Metabolism
Arne Astrup and Angelo Tremblay
Key messages- Energy balance in the body is the balance between how much
energy is consumed and how much is expended. Positive balance
is when intake exceeds expenditure and is associated with
increases in body energy stores (weight gain). During negative
balance, as in periods of starvation, body energy stores are
depleted. - Energy intake corresponds to the energy content of macro-
nutrients in foods. Carbohydrate provides 16.8 kJ/g, protein also
16.8 kJ/g, and fat 37.8 kJ/g. In addition, alcohol provides
29.4 kJ/g. - Total energy expenditure constitutes approximately two-thirds of
the energy expended by the body to maintain basic physiological
functions plus the thermic effect of a meal and energy expended
during physical movement. The basic physiological functions
include heart beat, muscle function, and respiration (resting or
basal metabolic rate). The thermic effect of a meal is about 10%
of the caloric value of the meal needed to digest, metabolize,
and store ingested macronutrients. The energy expended during
physical activity is energy expended when skeletal muscles are
used for any type of physical movement. In infants and children,
the cost of growth is added. - Energy requirement is the amount of food energy needed to
balance energy expenditure in order to maintain body size, body
© 2009 A Astrup and A Tremblay.
composition, and level of physical activity, consistent with long-
term good health. This includes the energy needs for optimal
growth and development in children, and the needs of pregnancy
and lactation (deposition of tissue and secretion of milk).- Body mass index (BMI) classifi es weight relative to height squared
and is the most accepted and widely used crude index of obesity.
A BMI of 18–24.9 kg/m^2 is regarded as normal for adults,
between 25 and 29.9 kg/m^2 as overweight, and >30 kg/m^2 as
obese. - Energy expenditure can be measured by direct methods (calor-
imetry) or indirect methods, in which oxygen consumption and
carbon dioxide production are used to calculate energy expendi-
ture. However, the modern gold standard is measurement by
doubly labeled water, which is a noninvasive method used to
measure total energy expenditure over periods of 7–14 days
while subjects are living in their usual environments. - Hunger is the physiological need to eat and results in actions to
attempt to obtain food for consumption. Appetite is a psychologi-
cal desire to eat and is related to the pleasant sensations that
are often associated with food. Thus, hunger is more of an
intrinsic instinct, whereas appetite is often a learned response.
3.1 Introduction
Defi nition and conceptualization of
energy balance
The average adult human consumes close to 1 000 000
calories (4000 MJ) per year. Despite this huge energy
intake, most healthy individuals are able to strike a
remarkable balance between how much energy is
consumed and how much energy is expended, thus
resulting in a state of energy balance in the body. This
accurate balance between energy intake and energy
expenditure is an example of homeostatic control and
results in maintenance of body weight and body
energy stores. This regulation of energy balance is
achieved over the long term despite large fl uctuations
in both energy intake and energy expenditure within
and between days. The accuracy and precision by
which the body maintains energy balance is high-
lighted by the fact that even a small error in the system
can have detrimental consequences over time. If
energy intake chronically exceeds energy expenditure
by as little as 105 kJ/day, then, over time, a person
will become substantially obese. The achievement of
energy balance is driven by the fi rst law of thermo-