becomes less active, the total decrease is almost 5%
per decade.- Body configuration of adults—Tall, thin people
usually have higher metabolic rates than do short,
stocky people of the same weight. This is so because
the tall, thin person has a larger surface area (pro-
portional to weight) through which heat is con-
tinuously lost. The metabolic rate, therefore, is
slightly higher to compensate for the greater heat
loss. The variance of surface-to-weight ratios for
different body configurations is illustrated in
Fig. 17–5.- Sex hormones—Testosterone increases metabolic
activity to a greater degree than does estrogen, giv-
ing men a slightly higher metabolic rate than
women. Also, men tend to have more muscle, an
active tissue, whereas women tend to have more
fat, a relatively inactive tissue. - Sympathetic stimulation—In stress situations, the
metabolism of many body cells is increased. Also
contributing to this are the hormones epinephrine
and norepinephrine. As a result, metabolic rate
increases. - Decreased food intake—If the intake of food
decreases for a prolonged period of time, metabolic
rate also begins to decrease. It is as if the body’s
metabolism is “slowing down” to conserve what-
ever energy sources may still be available. (See also
Box 17–6: Leptin and Body-Mass Index.) - Climate—People who live in cold climates may
have metabolic rates 10% to 20% higher than peo-
ple who live in tropical regions. This is believed to
be due to the variations in the secretion of thyrox-
ine, the hormone most responsible for regulation
of metabolic rate. In a cold climate, the necessity
for greater heat production brings about an
412
Figure 17–5. Surface-to-weight ratios. Imagine that
the three shapes are people who all weigh the same
amount. The “tall, thin person” on the right has about
50% more surface area than does the “short, stocky per-
son” on the left. The more surface area (where heat is
lost), the higher the metabolic rate.
QUESTION:Which of these ratios best represents an
infant? (Rather than weight, think of inside-outside
proportion.)
Box 17–6 LEPTIN AND BODY-MASS INDEXdirectly decreases fat storage in cells, and improves
the efficiency of the pancreatic cells that produce
insulin. What was first believed to be a simple chem-
ical signal has proved to be much more complex.
A good measure of leanness or fatness is the
body-mass index.
To calculate: Multiple weight in pounds by 703.
Divide by height in inches.
Divide again by height in inches = body-mass
index
Example: A person five foot six weighing 130
pounds.
130 x 703 = 91,390
91,39066 = 1385
1385 66 = 20.98
The optimal body-mass index is considered to be- Any index over 25 is considered overweight.
The 1994 discovery of the hormone leptin was
reported to the general public in 1995, along with
speculation that leptin could become an anti-obe-
sity medication, which it has not. Leptin is a protein
produced by fat cells, and signals the hypothalamus
to release a chemical that acts as an appetite sup-
pressant. It seems to inform the brain of how much
stored fat the body has, and is therefore involved in
the regulation of body weight (along with many
other chemicals, some still unknown).
Another likely role for leptin is as a contributor to
the onset of puberty, especially in females. Girls
who are very thin, with little body fat, tend to have
a later first menstrual period than girls with average
body fat, and a certain level of body fat is necessary
for continued ovulation. Leptin may be the chemi-
cal mediator of this information.
The most recent research indicates that leptin