Energy Metabolism 45muscle. Glycogen provides a very small and short-
term energy store, which can easily be depleted after
an overnight fast or after a bout of exercise. Most
carbohydrate that is consumed is immediately used
for energy. Contrary to popular belief, humans cannot
convert excess carbohydrate intake to fat. Instead,
when excess carbohydrates are consumed, the body
adapts by preferentially increasing its use of carbohy-
drate as a fuel, thus, in effect, burning off any
excessive carbohydrate consumption. Large excesses
of carbohydrate may induce de novo lipogenesis,
but normally this process is quantitatively minor.
However, no such adaptive mechanism for fat exists.
In other words, if excess fat is consumed, there is no
mechanism by which the body can increase its use of
fat as a fuel. Instead, when excess fat calories are con-
sumed, the only option is to accumulate the excess fat
as an energy store in the body. This process occurs at
a very low metabolic cost and is therefore an extremely
effi cient process. To store excess carbohydrate as
glycogen is much more metabolically expensive and
therefore a less effi cient option. There is another
important reason why the body would prefer to store
fat rather than glycogen. Glycogen can only be stored
in a hydrated form that requires 3 g of water for each
gram of glycogen, whereas fat does not require any
such process. In other words, for each gram of glyco-
gen that is stored, the body has to store an additional
3 g of water. Thus, for each 4 g of storage tissue, the
body stores only 16.8 kJ, equivalent to just 4.2 kJ/g,
compared with the benefi t of fat which can be stored
as 37.8 kJ/g.
Thus, a typical adult with 15 kg of fat carries
567.0 MJ of stored energy. If the adult did not eat and
was inactive, he or she might require 8.4 MJ/day for
survival, and the energy stores would be suffi cient for
almost 70 days. This length is about the limit of human
survival without food. Given that glycogen stores
require 4 g to store 4.2 kJ (3 g of water plus 1 g of gly-
cogen = 16.8 kJ), we can calculate that to carry this
much energy in the form of glycogen requires 135 kg
of weight. It is no wonder therefore that the body’s
metabolism favors fat as the preferred energy store.
Defi nition of obesity
Obesity has traditionally been defi ned as an excess
accumulation of body energy, in the form of fat or
adipose tissue. Thus, obesity is a disease of positive
energy balance, which arises as a result of dysregula-
tion in the energy balance system – a failure of the
regulatory systems to make appropriate adjustments
between intake and expenditure. It is now becoming
clear that the increased health risks of obesity may be
conferred by the distribution of body fat. In addition,
the infl uence of altered body fat and/or body fat dis-
tribution on health risk may vary across individuals.
Thus, obesity is best defi ned by indices of body fat
accumulation, body fat pattern, and alterations in
health risk profi le.
The body mass index (BMI) is now the most
accepted and most widely used crude index of obesity.
This index classifi es weight relative to height squared.
The BMI is therefore calculated as weight in kilo-
grams divided by height squared in meters, and
expressed in the units of kg/m^2. Obesity in adults is
defi ned as a BMI above 30.0 kg/m^2 , while the normal
range for BMI in adults is 18.5–24.9 kg/m^2. A BMI in
the range of 25–30 kg/m^2 is considered overweight. In
children, it is more diffi cult to classify obesity by BMI
because height varies with age during growth; thus,
age-adjusted BMI percentiles must be used.
One of the major disadvantages of using the BMI
to classify obesity is that this index does not distin-
guish between excess muscle weight and excess fat
weight. Thus, although BMI is strongly related to
body fatness, at any given BMI in a population, there
may be large differences in the range of body fatness.
A classic example of misclassifi cation that may arise
from the use of the BMI is a heavy football player or
body-builder with a large muscle mass who may have
a BMI above 30 kg/m^2 but is not obese; rather, this
man has a high body weight for his height resulting
from increased FFM.
Since the health risks of obesity are related to body
fat distribution, and in particular to excess abdominal
fat, other anthropometric indices of body shape are
useful in the defi nition of obesity. Traditionally, the
waist-to-hip ratio has been used as a marker of upper
versus lower body-fat distribution. More recent
studies suggest that waist circumference alone pro-
vides the best index of central body-fat pattern and
increased risk of obesity-related conditions. The rec-
ommended location for the measurement of waist
circumference is at the midpoint between the lowest
point of the rib cage and the iliac crest. The risk of
obesity-related diseases is increased above a waist cir-
cumference of 94 cm in men and above 80 cm in
women.