Ganong's Review of Medical Physiology, 23rd Edition

462 SECTION V Gastrointestinal Physiology

The metabolic rate determined at rest in a room at a com-
fortable temperature in the thermoneutral zone 12 to 14 h
after the last meal is called the basal metabolic rate (BMR).
This value falls about 10% during sleep and up to 40% during
prolonged starvation. The rate during normal daytime activi-
ties is, of course, higher than the BMR because of muscular
activity and food intake. The maximum metabolic rate
reached during exercise is often said to be 10 times the BMR,
but trained athletes can increase their metabolic rate as much
as 20-fold.

The BMR of a man of average size is about 2000 kcal/d.

Large animals have higher absolute BMRs, but the ratio of

BMR to body weight in small animals is much greater. One

variable that correlates well with the metabolic rate in differ-

ent species is the body surface area. This would be expected,

since heat exchange occurs at the body surface. The actual

relation to body weight (W) would be

BMR = 3.52W0.67

However, repeated measurements by numerous investiga-

tors have come up with a higher exponent, averaging 0.75:

BMR = 3.52W0.75

Thus, the slope of the line relating metabolic rate to body

weight is steeper than it would be if the relation were due

solely to body area (Figure 27–9). The cause of the greater

slope has been much debated but remains unsettled.

For clinical use, the BMR is usually expressed as a percent-

age increase or decrease above or below a set of generally used

standard normal values. Thus, a value of +65 means that the

individual’s BMR is 65% above the standard for that age and

sex.

The decrease in metabolic rate is part of the explanation of

why, when an individual is trying to lose weight, weight loss is

initially rapid and then slows down.

ENERGY BALANCE

The first law of thermodynamics, the principle that states that

energy is neither created nor destroyed when it is converted

from one form to another, applies to living organisms as well

as inanimate systems. One may therefore speak of an energy

FIGURE 27–8 Diagram of a modified Benedict apparatus, a recording spirometer used for measuring human O 2 consumption, and
the record obtained with it. The slope of the line AB is proportionate to the O 2 consumption. V: one-way check valve.

A

B

Pulleys

Oxygenbell

Water seal

Inhalation tube

Exhalation tube

Mouthpiece

Pulley

Rotating

drum

Volume

Time

v

v

Breathing

chamber

CO 2

absorber

TABLE 27–2 Factors affecting the metabolic rate.

Muscular exertion during or just before measurement

Recent ingestion of food

High or low environmental temperature

Height, weight, and surface area

Sex

Age

Growth

Reproduction

Lactation

Emotional state

Body temperature

Circulating levels of thyroid hormones

Circulating epinephrine and norepinephrine levels