complex and requires an overnight stay in a spe-
cial lab to capture measurements at precisely the
point of minimal metabolic activity. A common
formula for estimating BMR is the Harris-Benedict
equation. There are separate equations for women
and for men, accommodating gender differences
in the ratio of LEAN MUSCLE MASSto body fat.
Easier to measure directly is the resting meta-
bolic rate (RMR), which provides similar informa-
tion about the body’s energy requirements at
minimal activity. Many exercise physiology clinics
can measure RMR. More sophisticated methods
are available that allow determination of precise
metabolic measures for elite athletes as well as for
people who have severe health conditions. Most
people expend 50 to 75 calories per hour at rest,
so a rough generalization of metabolic rate is
1,200 to 1,800 calories. The larger a person, the
higher his or her metabolic rate, whether body
size results from MUSCLEmass or fat accumulation.
However, increased muscle mass further raises the
metabolic rate because muscle cells use more
energy than fat cells in the normal course of their
functions. Men generally have higher metabolic
rates than women because their bodies have
larger muscles and greater lean muscle mass.
Metabolism and Weight ManagementFrom a practical perspective either RMR or BMR
presents the body’s energy needs in terms of calo-
ries, allowing an individual to estimate daily
energy use (CALORIEexpenditure) to tailor daily
calorie intake. Activity factors and injury factors
further determine the body’s overall energy
expenditure and intake needs. A person whose
lifestyle is sedentary, for example, uses less energy
and consequently requires less intake than a per-
son whose lifestyle includes daily physical exer-
cise. However, the metabolic rate decreases at
about 5 percent per decade between the ages of 25
and 75, largely because lean muscle mass
decrease; thus at age 75 a person requires about a
third fewer calories each day than at age 25. With-
out a comparable increase in exercise, the differ-
ence can amount to a weight gain of four to seven
pounds a year. Regular physical activity boosts the
metabolic rate by maintaining a higher percentage
of lean muscle mass.
Metabolic Response to Trauma
When the body experiences significant physical
trauma, such as due to BURNSor major injuries, its
natural stress response initiates metabolic changes
that allow the body to rapidly convert protein to
amino acids (and subsequently to glucose) for the
body to use as energy. Major surgery may also ini-
tiate this response. The purpose of the metabolic
response to trauma is to muster every available
resource for healing; the result is rapid destruction
(catabolism) of muscle tissue. During healing the
metabolic rate rises significantly, reflecting the
body’s efforts to repair and reconstruct damaged
tissue (anabolism). However, the rate of catabo-
lism may be up to 10 times that of anabolism,
establishing an imbalance that makes it difficult
for the body to replace its protein stores.
Intensified nutritional support in combination
with physical exercise (particularly RESISTANCE
EXERCISE) can expedite muscle tissue restoration
and help metabolism return back to normal. The
most effective nutritional support incorporates a
high-protein diet (up to two times the recom-
mended dietary allowance) and NUTRITIONAL SUP-
PLEMENTSto supply increased amounts of certain B
vitamins that are essential for cellular energy pro-
duction and the efficiency with which cells can
use glucose. Physical activity stimulates muscle
cells to improve the efficiency with which they
contract and relax, and encourages development
of new muscle tissue. Sometimes doctors may
prescribe hormones to further stimulate muscle
growth.Metabolic Disorders
Metabolic disorders are health conditions that
alter the function of the body’s metabolic, or
energy-producing, pathways. Among the most
common metabolic disorders are DIABETES, HYPER-
THYROIDISM, HYPOTHYROIDISM, and PHENYLKETONURIA
(PKU). Though doctors understand the mecha-
nisms of most metabolic disorders, the causes
remain largely unknown. Genetic factors play a
significant role and may be the sole cause of cer-
tain metabolic conditions such as glycogen-storage
disorders (which affect the body’s ability to metab-
olize carbohydrates) and lipid-storage disorders
(which affect the body’s ability to metabolize fats).226 Fitness: Exercise and Health