Introduction to Human Nutrition

34 Introduction to Human Nutrition

factors, the central nervous system, and organ systems
(e.g., brain and liver), and environmental and exter-
nal factors.
Appetite is usually defi ned as a psychological desire
to eat and is related to the pleasant sensations that are
often associated with specifi c foods. Scientifi cally,
appetite is used as a general term of overall sensations
related to food intake.
Hunger is usually defi ned as the subjective feeling
that determines when food consumption is initiated
and can be described as a nagging, irritating feeling
that signifi es food deprivation to a degree that the
next eating episode should take place.
Satiety is considered as the state of inhibition over
eating that leads to the termination of a meal, and is
related to the time interval until the next eating episode.
Thus, hunger and satiety are more intrinsic instincts,
whereas appetite is often a learned response.
The internal factors that regulate the overall feeling
of hunger and satiety include the central nervous
system (primarily the hypothalamus and the vagus
nerve), the major digestive organs such as the stomach
and liver, and various hormones. In addition, envi-
ronmental factors (e.g., meal pattern and composi-
tion, food availability, smell and sight of foods,
climate), emotional factors (e.g., stress), and some
diseased states (e.g., anorexia, trauma, infection) may
infl uence the feelings of both hunger and appetite.
The factors that infl uence appetite include factors
external to the individual (e.g., climate, weather),
specifi c appetite cravings, specifi c learned dislikes or
avoidance (e.g., alcohol), intrinsic properties of food
(e.g., taste, palatability, texture), cultural practices or
preferences, specifi c effects of some drugs and dis-
eases, and metabolic factors such as hormones and
neurotransmitters. Some of these factors are described
in further detail below.
The classic way to describe the complex appetite-
regulating system is the satiety cascade put forth by
John Blundell. The satiety cascade describes four dis-
tinctly different but overlapping categories of mecha-
nisms involved in acute within-meal feeling of satiety
(referred to as satiation) and the inbetween-meal
satiety (Figure 3.1).

Factors infl uencing food intake

Digestive factors
Several factors in the digestive system exert a short-
term infl uence over food intake. The presence of food

and drink in the stomach and intestine and the resul-

tant pressure that they exert may regulate food intake.

This effect is known as gastrointestinal distension.

In addition, the stomach produces a hormone called

cholecystokinin (CCK) in response to food intake,

which may, in turn, regulate food intake. Furthermore,

when subjects have fat or carbohydrate infused

directly into the small intestine, they report feelings

of satiety. This suggests that factors in the intestine

regulate food intake. Indeed, receptors in the intes-

tine have been identifi ed that recognize the presence

of specifi c macronutrients; these receptors are linked

to the brain and therefore can communicate directly

with the central nervous system, resulting in regula-

tion of energy balance. In addition, other gastrointes-

tinal hormones, such as glucagon-like peptide-1 and

-2 (GLPs), CCK, and glucose-dependent insulinotro-

pic polypeptide (GIP) are likely to play a role in the

mediation of gut events and brain perception of

hunger and satiety.

Central nervous system factors

The main contributory factor regulating food intake

in the central nervous system is the hypothalamus.

The hypothalamus is linked to specifi c parts of the

Sensory Cognitive Postabsorptive

Postingestive

Food

Satiation Satiety

Early Late

Figure 3.1 The satiety cascade by John Blundell (1987). The four

categories of mechanisms are sensory, cognitive, postingestive, and

postabsorptive. The sensoric phase includes stimuli mediated via

sensory fi bers in the cranial nerves and relates to the palatability of

the ingested meal including smell, taste, temperature, and texture. The

cognitive phase of the satiety cascade represents learned, known, and

assumed properties of the ingested food. In the postingestive phase

the gastrointestinal tract is involved in a number of satiety signals both

via chemo- and mechanoreceptors and appetite-regulating peptides

from the gut and pancreas either entering circulation and acting

directly in the CNS or signaling via the vagus nerve. Important satiety

signals in the postabsorptive phase include circulating nutrients,

signals derived from differences in energy turnover, substrate oxida-

tion, and neurohormonal factors. Reprinted from Blundell et al.

(1987), copyright 1987 with permission of Elsevier.