Energy Metabolism 33● The energy cost of growth occurs in growing indi-
viduals, but is negligible except within the fi rst few
months of life.
● Adaptive thermogenesis is heat production during
exposure to reduced temperatures, and occurs in
humans, e.g., during the initial months of life and
during fever and other pathological conditions, but
also as a contributor to daily energy expenditure.
● Thermogenesis is increased by a number of agents
in the environment, including in foods and bever-
ages. Nicotine in tobacco is the most important
one, and heavy smokers may have a 10% higher
energy expenditure than nonsmokers of similar
body size and composition and physical activity.
Caffeine and derivatives in coffee, tea, and choco-
late, capsaicin in hot chilies, and other substances
in foods and drinks may possess minor thermo-
genic effects that affect energy expenditure.
Energy balance
Energy balance occurs when the energy content of
food is matched by the total amount of energy that is
expended by the body. An example of energy balance
would be the scenario cited at the outset of this
chapter in which, over a year, the average adult con-
sumes and expends 1 000 000 calories, resulting in no
net change in the energy content of the body. When
energy intake exceeds energy expenditure, a state of
positive energy balance occurs. Thus, positive energy
balance occurs when excessive overfeeding relative to
energy needs occurs, and the body increases its overall
energy stores. Examples of positive energy balance
include periods around major festivals when overeat-
ing and inactivity generally prevail, and during preg-
nancy and lactation when the body purposefully
increases its stores of energy. When energy intake is
lower than energy expenditure, a state of negative
energy balance occurs, for example during periods of
starvation. In this regard, evidence suggests that,
under conditions of substantial energy imbalance, be
it positive or negative, energy expenditure may reach
a level that is beyond what could be predicted by body
weight changes. This so-called “adaptive thermogen-
esis” might contribute to the occurrence of resistance
to lose fat in the context of obesity treatment or the
achievement of a new body weight plateau following
overfeeding. It is important to note that energy
balance can occur regardless of the levels of energy
intake and expenditure; thus, energy balance can
occur in very inactive individuals as well as in highly
active individuals provided that adequate energy
sources are available. It is also important to think
of energy balance in terms of the major sources of
energy, i.e., carbohydrate, protein, and fat. For
example, carbohydrate balance occurs when the body
balances the amount of carbohydrate ingested with
that expended for energy.3.2 Energy intake
Sources of dietary energy
As mentioned above, the sources of energy in the food
we eat include the major macronutrients: protein, car-
bohydrate, and fat, as well as alcohol. Carbohydrate
and protein provide 16.8 kJ of energy for each gram;
alcohol provides 29.4 kJ/g, whereas fat is the most
energy dense, providing 37.8 kJ/g. Note that 4.2 kJ is
defi ned as the amount of heat that is required to raise
the temperature of 1 liter of water by 1°C. The energy
content of food can be measured by bomb calori-
metry, which involves combusting a known weight of
food inside a sealed chamber and measuring the
amount of heat that is released during this process.
Thus, 1 g of pure fat would release 37.8 kJ during its
complete combustion, whereas 1 g of pure carbohy-
drate would release 16.8 kJ. Thus, if the gram quanti-
ties of any type of food are known, the energy content
can easily be calculated. For example, if a protein-rich
nutrition snack contains 21 g of carbohydrate, 6 g of
fat, and 14 g of protein, then the total energy content
is (21 × 16.8) + (6 × 37.8) + (14 × 16.8) = 814.8 kJ. The
macronutrient composition of food is typically
assessed in the percentage contribution of each mac-
ronutrient to the total number of calories. If a food has
a carbohydrate content of 21 g, which is 352.8 kJ, and
the total energy content is 820 kJ the proportion of
energy derived from carbohydrate is 43%; the fat
content is 6 g, or 226.8 kJ, equivalent to 28% of the
energy; and the protein contributes 14 g, 235.2 kJ and
29% of the energy.Regulation of food intake
Appetite, hunger, and satiety
The quality and quantity of food that is consumed are
closely regulated by the body. Food intake is regulated
by a number of factors involving complex inter-
actions among various hormones, neuroendocrine