Human Physiology, 14th edition (2016)

674 Chapter 19

In summary, leptin and insulin provide long-term regula-

tion of hunger and body weight, whereas several hormones

from the gastrointestinal tract provide sensory signals that reg-

ulate hunger on a shorter term, meal-related basis ( fig. 19.4 ).

Caloric Expenditures

The caloric energy expenditure of the body has three compo-

nents ( fig. 19.5 ):

1. Basal metabolic rate (BMR) is the energy expenditure

of a relaxed, resting person who is at a neutral ambient

temperature (about 28 8 C) and who has not eaten in 8 to

12 hours. This constitutes the majority (about 60%) of the

total calorie expenditure in an average adult.

2. Adaptive thermogenesis is the heat energy expended in

response to (a) changes in ambient temperature and (b) the

digestion and absorption of food. This constitutes about

10% of the total calorie expenditure, although this contri-

bution can change in response to cold and diet.

response to the satiety hormones (CK, GLP-1, and PPY),
which causes the person to eat smaller meals. Conversely, if
a person is deficient in leptin, or the leptin fails to work ade-
quately for some reason (as appears to be the case for most
obese people), the hunger-promoting effects of ghrelin on the
orexigenic neurons is enhanced, leading to greater food intake.
Insulin, secreted by the b -cells of the pancreatic islets, has
long been suspected of being a satiety factor. The beta cells
secrete more insulin in obese than in lean people, because more
insulin is required to maintain homeostasis of blood glucose
as the stored fat increases. Because insulin levels, like leptin
secretion, increase with increasing adiposity, insulin could act
like leptin to suppress appetite. Like leptin, insulin can cross
the blood-brain barrier in proportion to its concentration gradi-
ent and enter the hypothalamus. Also like leptin, insulin sup-
presses the release of neuropeptide Y to reduce appetite (see
fig.  19.3 ). Although leptin and insulin together work in the
negative feedback control of body fat, evidence suggests that
leptin exerts a more significant effect.

Figure 19.4 Hormonal

signals that regulate

feeding and energy

expenditures. Ghrelin

(green) is the only hormone

that stimulates hunger; the

other hormones (red) suppress

appetite. The central nervous

system (CNS) integrates this

sensory information with other

information (smell, taste, and

psychological factors) to help

regulate hunger and satiety,

energy expenditures, as well as

growth and reproduction.

CNS

Adipose tissue

Leptin

Insulin

Pancreas

Long-term

sensory

signals

PYY

CCK

Others

Ghrelin

Stomach and

small intestine

Meal-related

sensory

signals

Effects

Hunger and satiety

Energy expenditure

Growth and reproduction

Figure 19.5 Homeostasis of body weight depends on caloric balance. Energy is taken into the body by eating, and
energy is expended in basal metabolism (the basal metabolic rate, or BMR), exercise, and adaptive thermogenesis. The latter includes
calories expended by the diet (the digestion and absorption of food) and metabolic heat production induced by cold.

Feeding Basal metabolism

Physical activity

Adaptive thermogenesis

Diet-induced

Cold-induced

Energy intake Energy expenditure