228
U N I T 2
Integrative Body Functions
Regulatory Mechanisms
The control of food intake can be divided into short-
term regulation, which is concerned with the amount
of food that is consumed at a meal or snack, and inter-
mediate and long-term regulation, which is concerned
with the maintenance of constant stores of nutrients in
the tissues, preventing them from becoming too low or
too high.
1
Short-Term Regulation.
The short-term regulation of
food intake provides a person with the feeling of sati-
ety and turns off the desire for eating when adequate
food has been consumed. It requires rapid feedback
mechanisms that signal the adequacy of food intake
before digestion has taken place and nutrients have been
absorbed into the blood. These mechanisms include
stretch receptors that monitor filling of the gastrointes-
tinal tract, oral receptors that monitor food intake, and
gastrointestinal tract hormones that suppress or increase
food intake. Stretch receptors in the gastrointestinal
tract monitor gastrointestinal filling and send inhibitory
impulses by way of the vagus nerve to the feeding center
to suppress the desire for food. Oral receptors monitor
the amount of food that passes through the mouth and
sends impulses to the feeding center to suppress food
intake. This effect occurs despite the fact that the gastro-
intestinal tract has not become the least bit filled and is
thought to result from various oral factors such as tast-
ing, chewing, and swallowing that meter food intake.
1
However, the inhibition caused by this mechanism is
considerably less intense and of shorter duration than
that caused by gastrointestinal filling.
The gastrointestinal tract hormones that contribute
to the short-term regulation of food intake include cho-
lecytokinin (CCK), glucagon-like peptide 1 (GLP-1), and
ghrelin.
10,11
Cholecytokinin, which is released in response
to fat in the duodenum, and GLP-1, which is released
from the lower small bowel in response to nutrients, espe-
cially carbohydrates, have a strong suppressant effect on
the hypothalamic feeding center. Ghrelin is a hormone
released mainly from the stomach and to a lesser extent by
the intestine. Its levels peak just before eating and then fall
after a meal, suggesting that it may also stimulate appetite
and eating.
10
The presence of food in the stomach increases
the release of insulin and glucagon, both of which sup-
press the neurogenic feeding signals from the brain.
1
Intermediate and Long-Term Regulation.
The inter-
mediate and long-term regulation of food intake is
determined by the amount of nutrients that are in the
blood and in storage sites. It has long been known that a
decrease in blood glucose causes hunger. In contrast, an
increase in breakdown products of lipids such as ketoac-
ids produces a decrease in appetite. A ketogenic weight
loss diet (the Atkins diet) relies partly on the appetite
suppressant effects of ketones in the blood.
Recent evidence suggests that the hypothalamus also
senses the amount of energy through a hormone called
leptin
that is produced by fat cells.
1
The stimulation of
leptin receptors in the hypothalamus has been shown to
produce a decrease in appetite and food intake as well as
an increase in metabolic rate and energy consumption. It
also produces a decrease in insulin release from the beta
cells of the pancreas, which decreases energy storage in
fat cells.
Assessment of Energy Stores and
Nutritional Status
The nutritional status of an individual can be assessed
in a number of ways including a history of weight gain
or loss, dietary intake, gastrointestinal symptoms that
affect food intake, functional capacity, and physical
signs of fat loss and muscle wasting. This information
is usually combined with other objective measures of fat
stores and skeletal muscle mass.
Anthropometric Measurements
Anthropometric measurements provide a means for
assessing body composition, particularly fat stores and
skeletal muscle mass.
12
This is done by measuring height,
weight, body circumferences, and thickness of various
skinfold areas. These measurements commonly are used
to determine growth patterns in children and appropri-
ateness of current weight in adults.
Body weight is the most frequently used method of
assessing nutritional status; it should be used in combi-
nation with measurements of body height to establish
whether a person is underweight or overweight. For
weight measurement, subjects should ideally be in light
clothing and bare feet, fasting, and with an empty blad-
der. Repeat measurements are best made at the same time
of the day.
The body mass index (BMI) uses height and weight
to determine healthy weight (Table 10-2). It is calcu-
lated by dividing the weight in kilograms by the height
in meters squared (BMI = weight [kg]/height [m
2
]).
13
A
BMI less than 18.5 is classified as being underweight
and one between 25 and 29.9 is considered over-
weight.
13
A BMI at or greater than 30.0 is diagnosed as
obesity and is furthered classified into classes I (BMI 30.0
to 34.9), II (BMI 35.0 to 39.9), and III or extreme obesity
(BMI > 40). Body weight reflects both lean body mass
and adipose tissue and cannot be used as a method for
describing body composition or the percentage of fat tis-
sue present. Statistically, the best percentage of body fat
for men is between 12% and 20%, and for women, it
is between 20% and 30%.
14
During physical training,
body fat usually decreases, and lean body mass increases.
Among the methods used to estimate body fat are
body circumferences, skinfold thickness, bioelectrical
impedance, computed tomography (CT), and magnetic
resonance imaging (MRI).
12
The measurement of
body
circumferences
has received attention because excess
visceral (or intra-abdominal) fat is closely associated
with metabolic syndrome (i.e., a syndrome described
by a collection of cardiovascular risk factors).
15,16
Measurements of
skinfold thickness
can provide a rea-
sonable assessment of body fat, particularly if taken at
