588
Chapter 16
With the epidemic of obesity in the United States and
other countries, it is hoped that drugs can be developed
that inhibit the ghrelin-sensitive neurons in the hypothala-
mus, limiting hunger and, hopefully, allowing weight loss.
Interestingly, stomach bypass surgery for the morbidly obese
decreases ghrelin—this may be one mechanism that makes
this surgery successful in facilitating the initial weight loss and
maintaining it later.
Overweight and Obesity
The defi nition of
overweight
is functional, a state in which
an increased amount of fat in the body results in a signifi -
cant impairment of health from a variety of diseases, nota-
bly hypertension, atherosclerosis, heart disease, diabetes, and
sleep apnea.
Obesity
denotes a particularly large accumulation
of fat—that is, extreme overweight. The diffi culty has been
establishing just how much fat constitutes “overweight”—that
is, in determining at what point fat accumulation begins to
constitute a health risk. This is evaluated by epidemiological
studies that correlate disease rates with some measure of the
amount of fat in the body. Currently, the preferred simple
method for assessing the latter is not the body weight but the
body mass index (BMI),
which is calculated by dividing the
weight (in kilograms) by the square of the height (in meters).
For example, a 70-kg person with a height of 180 cm would
have a BMI of 21.6 (70/1.8
2
).
Current National Institutes of Health guidelines cat-
egorize BMIs of greater than 25 as overweight (i.e., as hav-
ing some increased health risk because of excess fat) and
those greater than 30 as obese, with a markedly increased
health risk. According to these criteria, more than half of U.S.
women and men age 20 and older are now considered to be
overweight and one-quarter or more to be clinically obese!
Even more troubling is that the incidence of childhood obe-
sity is increasing in the United States and other countries.
These guidelines, however, are controversial. First, the epide-
miological studies do not always agree as to where along the
continuum of BMIs between 25 and 30 health risks begin to
occur. Second, even granting increased risk above a BMI of
25, the studies do not always account for confounding factors
associated with being overweight or even obese, particularly a
sedentary lifestyle. Instead, the increased health risk may be at
least partly due to lack of physical activity, not body fat per se.
To add to the complexity, there is growing evidence that
not just total fat but where the fat is located has important
consequences. Specifi
cally, people with mostly abdominal fat
(“apples”) are at greater risk for developing serious conditions
such as diabetes and cardiovascular diseases than people whose
fat is mainly in the lower body (“pears”)—on the buttocks
and thighs. There is currently no agreement as to the explana-
tion of this phenomenon, but there are important differences
in the physiology of adipose tissue cells in these regions. For
Plasma
glucose
Plasma
insulin
Plasma
glucagon
Conditioned
responses
Body
temperature
or
or
Palatability
of food
Stress
Plasma
GI hormones
Plasma
leptin
Plasma
ghrelin
Activation of stretch receptors
and chemoreceptors in
stomach and duodenum
+
+
+
+
Brain
Hunger
Figure 16–15
Short-term inputs controlling appetite and, consequently, food intake. The minus signs denote hunger suppression, and the plus signs denote
hunger stimulation.
Figure 16–15
physiological
inquiry
As shown, stretch receptors in the gut after a meal can suppress hunger. Would drinking a large glass of water before a meal be an effective
means of dieting?
Answer can be found at end of chapter.
previous page 616 Vander's Human Physiology The Mechanisms of Body Function read online next page 618 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off