586
Chapter 16
This equation includes no term for loss of fuel from the
body via excretion of nutrients because normally only negli-
gible losses occur via the urine, feces, and sloughed hair and
skin. In certain diseases, however, the most important being
diabetes mellitus, urinary losses of organic molecules may be
quite large and would have to be included in the equation.
Rearranging the equation to focus on energy storage
gives:
Energy stored
=
Energy from food intake
(Internal heat produced
+
External work)
Thus, whenever energy intake differs from the sum of inter-
nal heat produced and external work, changes in energy stor-
age occur; that is, the total-body energy content increases or
decreases. Normally, energy storage is mainly in the form of
fat in adipose tissue.
It is worth emphasizing at this point that “body weight”
and “total-body energy content” are not synonymous. Body
weight is determined not only by the amount of fat, carbo-
hydrate, and protein in the body, but also by the amounts of
water, bone, and other minerals. For example, an individual
can lose body weight quickly as the result of sweating or an
unusual increase in urinary output, or can gain large amounts
of weight as a result of water retention, as occurs, for exam-
ple, during heart failure. Moreover, even focusing only on the
nutrients, a constant body weight does not mean that total-
body energy content is constant. The reason is that 1 g of fat
contains 9 kcal, whereas 1 g of either carbohydrate or protein
contains 4 kcal. Thus, for example, aging is usually associated
with a gain of fat and a loss of protein; the result is that even
though the person’s body weight may stay constant, the total-
body energy content has increased. Apart from these qualifi -
cations, however, in the remainder of this chapter changes in
body weight are equated with changes in total-body energy
content and, more specifi cally, changes in body fat stores.
Body weight in adults is usually regulated around a sta-
ble set point. Theoretically this regulation can be achieved by
refl exly adjusting caloric intake and/or energy expenditure in
response to changes in body weight. It had long been assumed
that regulation of caloric intake was the only important adjust-
ment, and the next section will describe this process. However,
it is now clear that energy expenditure can also be adjusted in
response to changes in body weight.
A typical demonstration of this process in human beings
is as follows: Total daily energy expenditure was measured in
nonobese subjects at their usual body weight and again after
they either lost 10 percent of their body weight by underfeed-
ing or gained 10 percent by overfeeding. At their new body
weight, the overfed subjects manifested a large (15 percent)
increase in both resting and nonresting energy expenditure,
and the underfed subjects showed a similar decrease. These
changes in energy expenditure were much greater than could
be accounted for simply by the altered metabolic mass of the
body or having to move a larger or smaller body.
The generalization that emerges from this and other
similar studies is that a dietary-induced change in total-body
energy stores triggers, in negative feedback fashion, an altera-
tion in energy expenditure that opposes the gain or loss of
energy stores. This phenomenon helps explain why some
dieters lose about 5 to 10 pounds of fat fairly easily and then
become stuck at a plateau. It also helps explain why some
very thin people have diffi
culty trying to gain much weight.
Another unsettled question is whether such “metabolic resis-
tance” to changes in body weight persists indefi nitely or is
only a transient response to rapid changes in body weight.
Control of Food Intake
The control of food intake can be analyzed in the same way
as any other biological control system. As the previous sec-
tion emphasized, the variable being maintained in this sys-
tem is total-body energy content or, more specifi cally, total
fat stores. An essential component of such a control system
is a hormone—
leptin
—synthesized by adipose tissue cells
themselves, and released from the cells in proportion to the
amount of fat in the adipose tissue. This hormone acts on
the hypothalamus to cause a reduction in food intake, in part
by inhibiting the release of
neuropeptide Y,
a hypothalamic
neurotransmitter that stimulates eating. Leptin also stimu-
lates the metabolic rate and, therefore, plays an important role
in the changes in energy expenditure that occur in response
to overfeeding or underfeeding, as described in the previous
section. Thus, as illustrated in
Figure 16–14
, leptin func-
tions in a negative feedback system to maintain a stable total-
body energy content by “telling” the brain how much fat is
being stored.
Leptin appears to exert many other effects on the hypo-
thalamus and anterior pituitary. For example, during long-
Table 16–7
Energy Expenditure During Different
Types of Activity for a 70-kg (154-lb)
Person
Form of Activity
Energy kcal/h
Lying still, awake
77
Sitting at rest
100
Typewriting rapidly
140
Dressing or undressing
150
Walking on level ground at 4.3 km/h
(2.6mi/h)
200
Bicycling on level ground at 9 km/h
(5.3 mi/h)
304
Walking on 3 percent grade at 4.3 km/h
(2.6 mi/h)
357
Sawing wood or shoveling snow
480
Jogging at 9 km/h (5.3 mi/h)
570
Rowing at 20 strokes/min
828
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