M

metabolic equivalent (MET) A unit of measure
for the amount of oxygen the body uses during
physical activity. One MET is equivalent to the
oxygen an adult requires when sitting quietly for
one minute. A four-MET activity, such as brisk
walking or riding a bicycle on a level surface,
requires four times the amount of energy (oxygen
consumption) as the one-MET activity of sitting
quietly. An eight-MET activity, such as running or
riding a bicycle uphill, requires eight times as
much energy as sitting quietly. METs are the basis
for calculating the number of calories that particu-
lar activities burn. Activities that require three to
six METs are considered to be of moderate inten-
sity and burn 3.5 to 7 calories per minute. Activi-
ties that require greater than six METs are
considered to be of vigorous intensity and burn
more than 7 calories per minute.
Knowing an activity’s MET value helps an indi-
vidual calculate how long to participate in that
activity to meet a desired level of CALORIEcon-
sumption. For example, a person who wants to
burn 150 calories a day (the minimum recom-
mendation for adults) may choose to walk at three
and half miles per hour, a four-MET activity, for
40 minutes or run at five miles per hour, an eight-
MET activity, for 20 minutes, depending on the
desired level of intensity.
See also AEROBIC CAPACITY; AEROBIC EXERCISE; BODY
MASS INDEX(BMI); CONDITIONING; FITNESS LEVEL; PHYSI-
CAL EXERCISE RECOMMENDATIONS; TRAINING; WALKING
FOR FITNESS.

metabolism The processes through which cells
convert NUTRIENTSto energy. In the most basic
sense, metabolism is the point of transition from
energy intake (food consumption) to energy
expenditure (molecular conversion). Multiple

mechanisms within the body regulate the complex

chemical interactions that constitute metabolism,

with the HYPOTHALAMUSand the endocrine system

taking the lead roles. Hormones such as THYROXIN

(T 4 ), which the THYROID GLAND produces, and

INSULIN, which the ISLETS OFLANGERHANSin the PAN-

CREASproduce, determine the rate at which cells

convert GLUCOSE(one of two fuel sources for cells)

to energy. The hormones of the body’s stress

response—CORTISOL, EPINEPHRINE, and NOREPINEPH-

RINE—also can accelerate metabolism, usually as a

short burst; though in times of trauma the STRESS

RESPONSE HORMONAL CASCADEcan alter the body’s

metabolism on a long-term basis as a mechanism

to facilitate HEALING.

The common perception of metabolism is as a

rate that represents a balance between calories

consumed and calories expended. Metabolism

may also refer to the processes that occur during

digestion to convert foods and drugs into chemical

molecules the body can use. Metabolism has two

primary modes: anabolism and catabolism.

Anabolism is energy expended toward construc-

tion (building tissue) and catabolism is energy

expended toward destruction (breaking down tis-

sue). Energy needs increase when a person is

recovering from major surgery, injury, or illness,

as the processes of healing engage the body in

extra anabolic (constructive) effort.

Measuring Metabolic Rate

The fundamental measure of metabolism is the

basal metabolic rate (BMR), which identifies the

amount of energy, in terms of calories, that the

body requires over 24 hours to function at

absolute rest. Nutritionists and exercise physiolo-

gists generally use mathematical formulas to cal-

culate BMR because its actual measurement is

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