Regulation of Organic Metabolism and Energy Balance
585
edly increase hunger and food intake. The greater intake often
remains inadequate to meet metabolic needs. The resulting net
catabolism of protein and fat stores leads to loss of body weight.
Of importance is the fact that the more metabolically active a
particular cell is, the greater its requirements for vitamin cofac-
tors. Therefore, even with increased dietary intake, the onset of
hyperthyroidism may result in symptoms of vitamin defi ciency.
Also, the greater heat production activates heat-dissipating
mechanisms, such as skin vasodilation and sweating, and the
person feels intolerant to warm environments. In contrast,
the hypothyroid person may experience cold intolerance.
The calorigenic effect of TH is only one of a wide variety
of effects these hormones exert. The major functions of the
thyroid hormones have all been described in this chapter and
in Chapter 11 and are listed for reference in
Table 16–6
.
As described in Chapter 11, secretion of the thyroid
hormones is stimulated by the anterior pituitary hormone
thyroid-stimulating hormone (TSH), itself stimulated by the
hypophysiotropic hormone, thyrotropin-releasing hormone
(TRH). The thyroid hormones, in turn, exert a negative feed-
back effect on the hypothalamo-pituitary system. What is
unusual about this hormonal system is that there is no known
stimulus that activates the negative feedback elimination of
the stimulus (as, for example, the way changes in plasma glu-
cose infl uence insulin secretion). It is thought that the thyroid
hormones set a background for the various parameters, such
as BMR, that they infl uence. Starvation, however, is associ-
ated with decreased TH production, which tends to slow the
consumption rate of endogenous fuel stores.
Epinephrine
Epinephrine is another hormone that exerts a calorigenic effect.
This effect may be related to its stimulation of glycogen and tri-
glyceride catabolism, as ATP hydrolysis and energy liberation
occur during both the breakdown and subsequent resynthesis
of these molecules. Thus, when epinephrine secretion by the
adrenal medulla is stimulated, the metabolic rate rises. This
accounts for part of the greater heat production associated
with emotional stress.
Food-Induced Thermogenesis
The ingestion of food rapidly increases the metabolic rate by
10 to 20 percent for a few hours after eating. This effect is
known as
food-induced thermogenesis.
Ingested protein
produces the greatest effect, while carbohydrate and fat pro-
duce less. Most of the increased heat production is caused by
the processing of the absorbed nutrients by the liver, not by
the energy expended by the gastrointestinal tract in digestion
and absorption. Because of the contribution of food-induced
thermogenesis, BMR tests must be performed in the postab-
sorptive state.
Food-induced thermogenesis is the rapid increase in
energy expenditure in response to ingestion of a meal. As we
will see,
prolonged
alterations in food intake (either increased
or decreased total calories) also have signifi cant effects on
metabolic rate.
Muscle Activity
The factor that can most increase metabolic rate is altered
skeletal muscle activity. Even minimal increases in muscle
contraction signifi cantly increase metabolic rate, and strenu-
ous exercise may raise energy expenditure more than 15-fold
(
Table 16–7
). Thus, depending on the degree of physical
activity, total energy expenditure may vary for a healthy young
adult from a value of approximately 1500 kcal/24 h to more
than 7000 kcal/24 h (for a lumberjack). Changes in muscle
activity also account in part for the changes in metabolic
rate that occur during sleep (decreased muscle contraction)
and during exposure to a low environmental temperature
(increased muscle contraction due to shivering).
Regulation of Total-Body
Energy Stores
Under normal conditions for body weight to remain stable,
the total energy expenditure (metabolic rate) of the body must
equal the total energy intake. We have already identifi ed the
ultimate forms of energy expenditure: internal heat produc-
tion, external work, and net molecular synthesis (energy stor-
age). The source of input is the energy contained in ingested
food. Therefore:
Energy from food intake
=
Internal heat produced
+
External work
+
Energy stored
Table 16–6
Major Functions of the Thyroid
Hormones (TH)
1.
Required for normal maturation of the nervous system in
the fetus and infant
Defi
ciency:
Mental retardation (cretinism)
2.
Required for normal bodily growth because they facilitate
the secretion of and response to growth hormone
Defi
ciency:
Defi cient growth in children
3.
Required for normal alertness and refl exes at all ages
Defi
ciency:
Mentally and physically slow and lethargic;
delayed refl exes
Excess:
Restless, irritable, anxious, wakeful; hyper-refl exic
4.
Major determinant of the rate at which the body produces
heat during the basal metabolic state
Defi
ciency:
Low BMR, cold intolerance; decreased food
appetite
Excess:
High BMR, heat intolerance; increased food
appetite, increased catabolism of nutrients
5.
Facilitates the activity of the sympathetic nervous system
by stimulating the synthesis of one class of receptors (beta
receptors) for epinephrine and norepinephrine
Excess:
Symptoms similar to those observed with activation
of the sympathetic nervous system (e.g., increased heart rate)
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