708
UNIT FIVE
a holdover from thousands of years ago, when the ability to
store energy in the form of fat was a survival advantage when
food supplies were scarce or erratic. Today in many African
nations, natural famines combined with political unrest cause
mass starvation. Starvation is considered later in the chapter.
It is difF cult to determine a desirable body weight. In the
past, weight standards were based on average weights and
heights in a certain population, and the degrees of underweight
and overweight were expressed as percentage deviations from
these averages. These standards refl ected the gradual gain in
weight as people age. Then medical researchers recognized
that such an increase in weight after the age of twenty-F ve
to thirty years is not necessary and may not be healthy. This
lead to standards of
desirable weights.
Today a measurement
termed
body mass index
(BMI) is used to assess weight con-
sidering height, and has become the basis of classifying a per-
son as underweight, normal weight, overweight, or obese.
Overweight
is deF ned as exceeding desirable weight by
10% to 20%, or a BMI between 25 and 30. A person more
than 20% above the desired weight or with a BMI over 30, is
obese,
although
obesity
(o-be
¯s
ı˘-te) is more correctly deF
ned
as excess adipose tissue. Therefore, overweight and obesity
are not the same. ±or example, as
f
gure 18.9
shows, an ath-
lete or a person whose work requires heavy muscular activ-
ity may be overweight, but not obese. Clinical Application
18.1 discusses obesity.
When a person needs to gain weight, diet can be
altered to include more calories and to emphasize par-
ticular macronutrients. ±or example, a person recovering
Energy Balance
A state of
energy balance
exists when caloric intake in the
form of foods equals caloric output from the basal metabolic
rate and muscular activities. Under these conditions, body
weight remains constant, except perhaps for slight variations
due to changes in water content.
If, however, caloric intake exceeds output, a
positive
energy balance
occurs, and tissues store excess nutrients.
This increases body weight because 3,500 excess calories is
stored as a pound of fat. Conversely, if caloric output exceeds
input, the energy balance is negative, and stored materials
are mobilized from the tissues for oxidation, causing weight
loss. All diet plans, no matter which foods they stress, boil
down to this fact: to maintain weight, calories in must equal
calories out.
PRACTICE
23
What is basal metabolic rate?
24
What factors inF
uence the BMR?
25
What is energy balance?
Desirable Weight
The most obvious and common nutritional disorders refl ect
calorie imbalances, which may result from societal and geo-
graphic factors. Obesity is prevalent in nations where food is
plentiful and diverse. The tendency to become obese may be
FIGURE 18.9
±Weight.±(
a
) An obese person is overweight and has excess
adipose tissue. (
b
) An athlete may be overweight due to muscle overgrowth but is
not considered obese. Many athletes have very low percentages of body fat.
(a)
(b)
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