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Summary 1.4 the human body problem 6; bon appetite!
- Instelling
- Erasmus Universiteit Rotterdam (EUR)
in this summary the human digestive system is explained.
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notes problem 6the incidence for obesity is bigger for men but the incidence of dieting is bigger with women.
Digestion and energy flow
the primary purpose
The primary purpose of eating is to supply the body with energy it needs to survive and function.
Digestion is the gastrointestinal process of breaking down food and absorbing its constituents into
the body. When food is digested energy is delivered in three forms: 1. Lipids (fats), 2. Amino acids
(breakdown products of proteins) and 3. Glucose (sugar). The energy is stored in three different
forms: 1. Fats, 2. Glycogen and 3. Fats. Most of the body’s energy is stored as fats and relatively little
as glycogen and proteins, this is because fats can hold the most energy with taking up the least
amount of space and thus making it very efficient for storing energy and glycogen attracts and holds
a lot of water which would mean that if your fat calories would all be stored as glycogen you would
likely weigh well above 275kg. The changes in the body weight are largely a consequence of changes
in the amount of body fat.
glycogen
glycogen is stored in the liver and readily available to be converted in glucose- the body’s main
directly utilizable source of energy-
Three phases of energy metabolism
The energy of metabolism are the chemical changes by which energy is made available for an
organism’s use you have three phases:
1. Cephalic phase
The preparatory phase which usually starts with the sight, smell or even just the thought of food and
it ends when the food is absorbed into the bloodstream.
2. Absorptive phase
The period during which the energy absorbed into the bloodstream from the meal meeting the
body’s immediate energy needs.
3. Fasting phase
The period during which all of the un-stored energy from the previous meal has been used and the
body is withdrawing energy from its reserves to meet its immediate energy requirements, it ends
with the beginning of the next cephalic phase to continue the cycle.
During periods of rapid weight gain, people often go directly from one absorptive phase into the next
cephalic phase without experiencing an intervening fasting phase. The flow of energy during these
phases are controlled by two pancreatic hormones: 1. Insulin and 2. Glucagon.
- during the cephalic and absorptive phases the pancreas releases a great deal of insulin into the
bloodstream and very little glucagon.
The insulin does three things:
1. It promotes the use of glucose as the primary source of energy by the body.
2. It promotes the conversion of blood-borne fuels to forms that can be stored: glucose to
glycogen and fat and amino acids to proteins.
3. It promotes storage of glycogen in the liver and muscles, fat in adipose tissue and proteins in
muscles.
, In short the function of insulin during the cephalic phase is to lower the levels of blood-borne
fuels, primarily glucose, in anticipation of the impending influx, and its function during the
absorptive phase is to minimize the increasing levels of blood-borne fuels by utilizing and storing
them.
Whereas the fasting phase is characterised by high blood levels of glucagon and low levels of
insulin, glucose has difficulty entering most body cells, thus glucose stops being the body’s
primary fuel. Hereby this saves the body’s glucose for the brain, because insulin is not required
for glucose to enter most brain cells. The low levels of insulin also promote the conversion of
glycogen and protein to glucose, this is called gluconeogenesis.
On the other hand the high levels of the fasting phase glucagon promote the release of free fatty
acids from adipose tissue and their use as the body’s primary fuel. The high glucagon levels also
stimulate the conversion of free fatty acids to ketones, which are used by muscles as a source of
energy during the fasting phase.
After a prolonged period without food the brain also starts to use ketones and thus further
conserving the body’s resources of glucose.
Theories of hunger and eating
Set-point assumption: most people attribute hunger to the presence of an energy deficit, and
they view eating as the means by which the energy resources of the body are returned to their
optimal level, which is their energy set-point.
After a meal your energy resources are thought to be near their set point and to decline after as
the body uses energy to fuel the physiological processes. When the level of the body’s energy
falls far enough below the set-point we become motivated by hunger to eat, we eat till the
energy set-point is reached and we no longer feel hungry. this works kind of like a thermostat
in the house. All set-point mechanisms are negative feedback systems!
Homeostasis: a constant internal environment e.g. your body temperature.
All set-point systems have three components:
1. A set-point mechanism: defines the set-point.
2. A detector mechanism: detects deviations from the set-point.
3. An effector mechanism: acts to eliminate the deviations.
Gluco-static set -point theory: we become hungry when our blood glucose levels drop below
their set-point and we become full filled when we reach the set-point again.
Lipo-static theory: every person has a set-point for body fat and deviations from this set-point
make us eat to reach the set point again.
support: body weight of adults stays constant.
The two theories were seen as complementary and not mutually exclusive (one or the other).
Criticism on the theories:
1. The theories are inconsistent with the basic eating-related evolutionary pressures. Our
ancestors didn’t have the consistency and predictability of a food supply. Thus it was important
for them to eat as much as they could when food was available otherwise they wouldn’t survive
because they didn’t know when food was available again.
2. Major predictions of the theories have not been confirmed. Early studies seemed to support
them by showing large reductions of body fat produced by starvation or large reductions in
glucose levels. The problem is that reductions of the magnitude needed to reliably induce eating
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