Eating
• Homeostatic theory
– Leptin / Anorexia
– Stress-induced eating
• Positive Incentive Theory
– Memory for the last meal
• Sensory-specific satiety Theory
• Obesity
• Low calorie diet & aging
• System variable: A variable controlled by a regulatory mechanism
(e.g., temperature, weight)
• Set point: The optimal value (e.g., 72 degrees)
• Detector: signals deviations from the set point.
• Correctional mechanism: changes the value (e.g., increases temp)
• Negative feedback: A process whereby the effect produced by an
action serves to diminish or terminate that action.
Hypothalamus
Food Intake
Energy Expenditure
Leptin
Reproduction
Energy
Availability
High
Low
Fat Cell
Leptin
– secreted by adipose tissue
– in direct proportion to amount of energy
stored in fat
– Receptors in hypothalamus
– for food intake and
– for sexual behavior,
– for control of reproductive hormones
– But obese humans have high levels of
circulating leptin!
Ob mouse: A strain of mouse whose obesity
and low metabolic rate is caused by a
mutation that prevents the production of leptin
• Anorexia nervosa
1. Anxiety and fear of obesity
2. Disturbed attitudes toward body weight &
shape
3. Body weight cortisol
– lipolysis & gluconeogenesis --> increase
glucose
– Glucocorticoid antagonists prevent obesity
– Exogenous cortisol --> increased food
intake in humans
– In rats, glucocorticoids increase preference
for carbohydrates (gimme a break, tic-tac
bar)
Stress-induced eating (cont’d)
• Women had to ‘give a talk’
(stressful situation)
– Salivary cortisol was measured
– Choice of snack was offered
– Those most stressed (higher levels of
cortisol) chose the sweet/high calorie
snack (granola bar) more often than the
control snack (rice cake)
– A non-stress day revealed no differences
in snack choice
Homeostatic Theory
Problems for Homeostatic theory
• Guinea pigs fed ad lib (food available throughout the
day) always have food in their stomachs.
• Meal size is unrelated to the length of food deprivation
• Glucose is reduced before a meal (supporting the
homeostatic theory) but it might be due to insulin release
that is related to intention to eat (diabetics?)
• Cue effects on eating / amnesic patients
Positive Incentive Theory
• we eat three meals a day out of habit (ie. Cued by
external stimuli such as the time of day, or the sight and
smell of food)
• classically conditioned eating in humans and animals
are consistent with this idea
Memory for when a last meal has been
eaten
• What starts and end a meal?
– physiological mechanisms (textbook)
– cognitive causes (this is your third plate!)
– Patients with severe long-term memory loss
due to hippocampus damage eat a second
dinner 20 mins after the first one!!
Sensory-specific satiety
• You always have room for dessert, even if you are done
with the main dish
Obesity
• Strong genetic influence. This explains individual
differences
• There are environmental factors (current epidemic)
• Pima Indians (in the West of USA and Mexico)
Obesity (cont’d)
• Easy access to french fries. Fat contains twice as many calories per gram than do proteins or carbohydrates.
Thus it is easy to see how humans (and other animals) would learn at an early age to prefer high-fat foods. Such
foods were not easy to find in the environment in which humans evolved. However, these foods are now easily
and cheaply available in industrialized countries
• Candy is not a fruit. In nature, the taste of sweet is often associated with a high concentration of quickly available
sugar and thus with readily available calories. In the environment in which humans evolved, one concentrated and
relatively quick source of sugar and therefore of calories was ripe fruit, which is characterized by a sweet taste....A
preference for sweet foods and drinks that would encourage consumption of ripe fruit was probably advantageous
to our early ancestors ...Thus it would have been adaptive for humans and other omnivores to have evolved with
an innate preference for the taste of sweet.
• Can you pass me the salt, please?･ ...salt is essential for the body to function properly. This is true for humans
as well as for other species. Although salt is necessary for the body to function properly, it is not easily available in
the wild. Prior to industrialization, humans sometimes had great difficulty obtaining enough salt. Many species
must constantly consume salt in order to have sufficient amounts. Therefore it would not be surprising that natural
selection would result in an innate preference for salt and that this preference would be present in most species.･
• Are you gonna eat that? Weight Gain and Loss: In an environment in which there is a limited or erratic food
supply, it would be adaptive for animals to take in as much food as they can, whenever it is available. Then, if
possible, these same animals should retain (as opposed to use) the calories thus consumed, as insurance against
future periods of food scarcity. From this perspective, and given that humans evolved in an environment in which
there was indeed limited and erratic access to food, it can be seen that a number of facts about human weight
regulation are all directed towards the maximization of stored energy.