Chapter 4 | 19518.0 MJ/kg for carbohydrates, 22.2 MJ/kg for proteins, and 39.8 MJ/kg for
fats. These food groups are not entirely metabolized in the human body,
however. The fraction of metabolizable energy contents are 95.5 percent for
carbohydrates, 77.5 percent for proteins, and 97.7 percent for fats. That is,
the fats we eat are almost entirely metabolized in the body, but close to one
quarter of the protein we eat is discarded from the body unburned. This cor-
responds to 4.1 Calories/g for proteins and carbohydrates and 9.3 Calories/g
for fats (Fig. 4–41) commonly seen in nutrition books and on food labels.
The energy contents of the foods we normally eat are much lower than the
values above because of the large water content (water adds bulk to the food
but it cannot be metabolized or burned, and thus it has no energy value).
Most vegetables, fruits, and meats, for example, are mostly water. The aver-
age metabolizable energy contents of the three basic food groups are 4.2
MJ/kg for carbohydrates, 8.4 MJ/kg for proteins, and 33.1 MJ/kg for fats.
Note that 1 kg of natural fat contains almost 8 times the metabolizable
energy of 1 kg of natural carbohydrates. Thus, a person who fills his stomach
with fatty foods is consuming much more energy than a person who fills his
stomach with carbohydrates such as bread or rice.
The metabolizable energy content of foods is usually expressed by nutri-
tionists in terms of the capitalized Calories. One Calorie is equivalent to one
kilocalorie(1000 calories), which is equivalent to 4.1868 kJ. That is,
The calorie notation often causes confusion since it is not always followed in
the tables or articles on nutrition. When the topic is food or fitness, a calorie
normally means a kilocalorie whether it is capitalized or not.
The daily calorie needsof people vary greatly with age, gender, the state
of health, the activity level, the body weight, and the composition of the
body as well as other factors. A small person needs fewer calories than a
larger person of the same sex and age. An average man needs about 2400 to
2700 Calories a day. The daily need of an average woman varies from 1800
to 2200 Calories. The daily calorie needs are about 1600 for sedentary
women and some older adults; 2000 for sedentary men and most older
adults; 2200 for most children, teenage girls, and active women; 2800 for
teenage boys, active men, and some very active women; and above 3000 for
very active men. The average value of calorie intake is usually taken to be
2000 Calories per day. The daily calorie needs of a person can be determined
by multiplying the body weight in pounds (which is 2.205 times the
body weight in kg) by 11 for a sedentary person, 13 for a moderately active
person, 15 for a moderate exerciser or physical laborer, and 18 for an
extremely active exerciser or physical laborer. The extra calories a body
consumes are usually stored as fat, which serves as the spare energy of the
body for use when the energy intake of the body is less than the needed
amount.
Like other natural fat, 1 kg of human body fat contains about 33.1 MJ of
metabolizable energy. Therefore, a starving person (zero energy intake) who
uses up 2200 Calories (9211 kJ) a day can meet his daily energy intake
requirements by burning only 9211/33,100 0.28 kg of body fat. So it is no
surprise that people are known to survive over 100 days without eating.
(They still need to drink water, however, to replenish the water lost through
the lungs and the skin to avoid the dehydration that may occur in just a few
1 Cal 1 Calorie 2 1000 calories1 kcal 1 kilocalorie 2 4.1868 kJFIGURE 4 –41
Evaluating the calorie content of one
serving of chocolate chip cookies
(values are for Chips Ahoy cookies
made by Nabisco).
© Vol. 30/PhotoDisc