Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
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24
a comfortable 20–25°C. Te measurement obtained under these
circumstances, the
basal metabolic rate (BMR)
, reflects the en-
ergy the body needs to perform only its most essential activities,
such as breathing and maintaining resting levels of organ function.
Although named the
basal
metabolic rate, this measurement is
not
the lowest metabolic state of the body. Tat situation occurs dur-
ing sleep, when skeletal muscles are completely relaxed.
Te BMR, oFen referred to as the “energy cost of living,” is
reported in kilocalories per square meter of body surface per
hour (kcal/m
2
/h). A 70-kg adult has a BMR of approximately 66
kcal/h. You can approximate your BMR by multiplying weight
in kilograms (2.2 lb
5
1 kg) by 1 if you are male and by 0.9 if
you are female.
Several factors influence BMR, including body surface
area, age, gender, body temperature, stress, and the hormone
thyroxine.
Body Surface Area
BMR is related to overall weight and size,
but the critical factor is body surface area. As the ratio of body
surface area to body volume increases, heat loss to the environ-
ment increases and the metabolic rate must be higher to replace
the lost heat. ±or this reason, if two people weigh the same, the
taller or thinner person will have the higher BMR.
Age and Gender
In general, the younger the person, the higher
the BMR. Children and adolescents require large amounts of
energy for growth. In old age, BMR declines dramatically as
skeletal muscles begin to atrophy. (Tis helps explain why el-
derly people who fail to reduce their caloric intake gain weight.)
Gender also plays a role. Metabolic rate is disproportionately
higher in males than in females, because males typically have
more muscle, which is very active metabolically even during
rest. ±atty tissue, present in greater relative amounts in females,
is metabolically more sluggish than muscle.
Body Temperature
Body temperature and BMR tend to rise
and fall together. ±ever markedly increases metabolic rate above
basal levels.
Stress
Whether physical or emotional, stress increases BMR
by mobilizing the sympathetic nervous system. As norepineph-
rine and epinephrine (released by adrenal medullary cells)
flood into the blood, they raise BMR primarily by stimulating
fat catabolism.
Thyroxine
Te amount of
thyroxine
produced by the thyroid
gland is probably the most important hormonal factor in de-
termining BMR. ±or this reason, thyroxine has been dubbed
the “metabolic hormone.” Its direct effect on most body cells
(except brain cells) is to increase O
2
consumption and heat pro-
duction, presumably by accelerating the use of A²P to operate
the sodium-potassium pump. As A²P reserves decline, cellular
respiration accelerates. Tus, the more thyroxine produced, the
higher the BMR.
Homeostatic Imbalance 24.5
Hyperthyroidism
causes a host of problems resulting from the
excessive BMR it produces. Te body catabolizes stored fats and
tissue proteins and, despite increased hunger and food intake,
point. ±urthermore, individuals who are obese have higher-than-
normal leptin blood levels, but for some unknown reason, they are
resistant to its action. Now the consensus is that leptin’s main role
is to protect against weight loss in times of nutritional deprivation.
Although leptin has received the most attention as a long-
term appetite and metabolism regulator, there are several other
players. Insulin, like leptin, inhibits NPY release in non-insulin-
resistant individuals, but its effect is less potent.
Additional Regulatory Factors
Tere is no easy answer to explain how body weight is regulated,
but theories abound. Rising ambient temperature discourages
food seeking, whereas cold temperature activates the hunger
center. Depending on the individual, stress may increase or de-
crease food-seeking behavior, but chronic stress in combination
with a junk food (high-fat and -sugar) diet promotes sharply
increased release of NPY.
Psychological factors are thought to be important in people
who are obese, but even when psychological factors contribute
to obesity, individuals do not continue to gain weight endlessly.
Controls still operate but at a higher set-point weight. Certain ad-
enovirus infections, sleep deprivation, and even the composition
of gut bacteria are additional factors that may affect a person’s fat
mass and weight regulation, according to some clinical studies.
Whew! How does all this information fit together? So far we
only have bits and pieces of the story, but ±igure 24.23 shows the
best current model based on animal studies.
Check Your Understanding
28.
What three groups of stimuli influence short-term regulation
of feeding behavior?
29.
What is the most important long-term regulator of feeding
behavior and appetite?
For answers, see Appendix H.
Metabolic Rate and Heat Production
Define basal metabolic rate and total metabolic rate. Name
factors that influence each.
Te body’s rate of energy output, the
metabolic rate
, is the total
heat produced by all the chemical reactions and mechanical work
of the body. We can measure metabolic rate directly or indirectly.
In the
direct method
, a person enters a chamber called a
calorim-
eter
and water circulating around the chamber absorbs heat liber-
ated by the body. Te rise in water temperature is directly related
to the heat produced by the person’s body. Te
indirect method
uses a
respirometer
to measure oxygen consumption, which is
directly proportional to heat production. ±or each liter of oxygen
used, the body produces about 4.8 kcal of heat.
Basal Metabolic Rate (BMR)
Because many factors influence metabolic rate, it is usually mea-
sured under standardized conditions. Te person is in a postab-
sorptive state (has not eaten for at least 12 hours), is reclining, and
is mentally and physically relaxed. Te temperature of the room is
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