Summary Block 4 The Human Body Problem 6

Problem 6 – Pinel + Kalat
 The primary purpose of hunger is to increase the probability of eating, and the
primary purpose of eating is to supply the body with the molecular building blocks
and energy it needs to survive and function.
 Energy is delivered to the body in 3 forms: lipids (fats), amino acid (proteins), and
glucose (carbs, starch, sugar)
 The body uses energy continuously but its consumption is intermittent so the body
must store energy in 3 forms: fats, proteins and glycogen
 The changes in the body weights of humans are largely dependent on the amount of
stored body fat
 2 reasons why fat rather than glycogen is the body’s preferred primary energy
storage: 1. A gram of fat can store almost twice as much energy as a gram of
glycogen 2. Glycogen, unlike fat, attracts and hold substantial quantities of water

,  Energy metabolism: the chemical change by which the energy is made available for
an organism’s use, 3 phases:
o 1) cephalic phase: the preparatory phase, begins with the sight, smell or
thought of food and ends when the food starts to be absorbed into the
bloodstream
o 2) absorptive phase: the energy is absorbed into the bloodstream
o 3) fasting phase: all the unstored energy from the previous meal has been
used and the body is withdrawing energy from its reserves to meet its
immediate energy requirements, this ends with a new cephalic phase

 The flow of energy during these phases is controlled by 2 pancreatic hormones.
 During cephalic and absorptive phases: the pancreas releases high levels of insulin
and very little glucagon into the bloodstream.
 Insulin:
o 1) promotes the use of glucose as the primary source of energy
o 2) promotes the conversion of bloodborne fuels to forms that can be stored
(glucose  glycogen, fat & a.a.  protein)
o 3) promotes the storage of fat in adipose tissue (85%), proteins in muscle
(14.5%) and glycogen in and liver and muscle (0.5%)
 During the fasting phase: there are low levels of insulin and high levels of glucagon.
When the insulin levels are low, glucose has difficulty entering most body cells and
this stops it from being the body’s primary fuel. This saves the body’s glucose for
brain because insulin is not required to enter brain cells. The low levels of insulin also
promote the conversion of glycogen and protein to glucose (gluconeogenesis).
 The high levels of glucagon promote the release of free fatty acids from adipose
tissue and their use as the body’s primary fuel. Glucagon also stimulates the
conversion of free fatty acids to ketones (source of energy for muscles)

 Set-point assumption: hunger is the presence of an energy deficit and by eating,
energy sources of the body are returned to their optimal level – to the energy set
point. (thermostat regulated heating)
 3 components of all set point systems: 1) set point mechanism: defines the set point
2) detector mechanism: detects deviations from the set point 3) effector
mechanism: acts to eliminate deviations
 All set point systems also have negative feedback systems: feedback from changes in
one direction elicit compensatory effects in the opposite direction. This is in the form
of homeostasis in mammals – a stable internal environment
 Other set point theories:
o Glucostatic theory: Eating is regulated to maintain blood glucose set point.
We become hungry when our blood glucose levels drop below the set point
and we become satiated when eating returns our blood glucose levels to the
normal set point, account for meal initiation and termination
o Lipostatic theory: every person has a set point for body fat and deviations
from this set point produce the level of eating, account for long-term
regulation
 Set point theories are inconsistent and not supported anymore.