Human Nutrition

REASONS FOR EATING ❚
Metabolism
The blood levels of metabolites are regulated
by the liver and peripheral tissues, which remove
them from the circulation and may also have an
effect on feeding behaviour.
It is becoming clear that metabolism of the
energy-providing nutrients is regulated with different
levels of precision. Alcohol, as a potential
toxin, must be oxidized completely and removed
as quickly as possible. Its metabolic regulation is
perfect and all alcohol is completely metabolized.
The capacity of the body to store carbohydrate
and protein is limited, and blood levels
of glucose and amino acids are carefully controlled.
It is now believed that under normal circumstances
very little carbohydrate is actually
converted to fat. The conversion of carbohydrate
to fat is very inefficient, with approximately 25
per cent of the potential energy wasted as heat.
Metabolism of both glucose and amino acids is
thought to ‘autoregulate’ to match the intake
level.
Fat metabolism, however, exhibits no such
‘autoregulation’, probably because of the large
capacity for fat storage in the body. Therefore, fat
metabolism does not correlate well with fat
intake and there is no evidence that fat oxidation
adjusts when intake increases. Consequently, one
can conclude that fat intake plays a smaller role
in the control of food intake than do either
carbohydrates or proteins. This may provide an
explanation for the ‘fattening’ effects of high-fat
diets, which can be consumed without any consequent
change to fat oxidation. Also evidence
from feeding studies of diets where fat content
has been covertly increased shows that a change
in fat levels has no effect on satiety and subsequent
food intakes. This means that such diets
are easy to consume leading to ‘passive overconsumption’,
and may be a significant contributory
factor in obesity.
Integration by the brain
Early research identified hunger and satiety
centres in the hypothalamus; these could be
artificially stimulated or destroyed, resulting in
starvation or overeating. The function of these
centres was thought to be the maintenance of
adequate levels of energy-providing nutrients in
the blood. The dietary macronutrients, carbohydrates,
fats and proteins, were the primary
candidates for these regulatory factors. It is now
clear that this is an oversimplified picture. The
brain receives information from receptors and
metabolites about the whole feeding process,
from the initial thought about food to the final
metabolism of its breakdown products. Changes
in plasma concentrations of nutrients resulting
from metabolism in the liver and peripheral tissues
are also monitored. A number of these have
been mentioned in the earlier sections. In particular,
leptin levels are believed to be important,
through their action on neuropeptide Y, causing
inhibition of food intake. Low levels of leptin are
thought to cause a number of adaptive changes
to minimize weight loss, including increased food
intake. In addition, levels of serotonin, which
promotes feelings of satiety, are influenced by
the amounts of tryptophan crossing the blood–
brain barrier. By these means, food intake and
metabolic processes can be regulated to match the
body’s needs. This complex integration occurs
in the brain, probably by appropriate changes in
the levels of neurotransmitters.
In addition, it has been proposed that unconscious
reflex pathways are established, particularly
during childhood, whereby the body
‘learns’ the metabolic consequences of particular
eating patterns and responds accordingly. If such
reflexes are not established, perhaps due to erratic
eating behaviour in childhood, then control
mechanisms remain less efficient. Much remains
to be learned about the complex control of eating.
Habit as an influence on eating behaviour
In parts of the world where food is readily available,
intake could occur at any time of day or
night. Most people do not eat continuously, but
usually at fairly clearly defined ‘mealtimes’,
with ‘snack times’ interspersed between them.
This behaviour has become a habit, and many
Westerners believe that they should have three
meals a day together with two or three snacks,
with the main meal either in the middle of the
day or in the evening (see Figure 2.2). In other
societies, especially among the poor, fewer
meals are eaten, maybe only one or at most two
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