17 Lecture (3/31/09): Motivation & Eating
Motivation
Some general considerations
Eating
Ultimate motives More immediate biological influence The role of learning The recent rise in obesity (Thursday)
Motivation
Motivation is defined in the dictionary (Oxford) as the stimulus or state or cause that drives action toward a goal. The factors that give purpose or direction to human or animal behavior. The reason for acting in a particular way. But virtually all psychological phenomena are motivated
Instincts Conditioned responses Reinforced behavior
Some limitations
Text (Gazzaniga & Heathertons) list of motivational topics---very long and diverse Class topics
Some general distinctions
Proximal and ultimate causes
Eating Sex
Thus, we can talk about motivation at different levels of generality Logical possibility that proximal and ultimate causes are at odds
Order of topics on eating
The ultimate goals of eating Proximal biological controls Role of learning Economic factors (price, availability) Recent national changes in body weight
Questions that we will address:
How do we know what to eat? Why do some people like aversive foods, such as chili peppers? Why is obesity on the rise?
Eating: Ultimate issues
Cells require energy and essential biochemicals for their structural properties: glucose, fats, and vitamins But cells cant move and food exists in many forms, not just glucose and fat. Thus, energy and vitamins have to be extracted from environment, processed, and delivered Solution: send out for supplies and metabolize them into correct form
The conventional approach to motivation: homeostasis
A classic example: body temperature and feeling cold and hot If cold, reflexive shivering and decreased blood flow to the skin, if hot start then reflexive sweating Discomfort reinforces actions that provide protection from the cold
The homeostatic view of eating
Low on Energy/Essential Amino Acids
Hunger
Eat
Stop Eating
Correct Deficits
iClicker
Whats wrong with the homeostatic account of eating?
A = obesity B = hunger strikes C = Nothing, it works D=A&B
National (US) Obesity Rates*
35%
% BMI > 30 ("obese")
BMI =
30% 25% 20% 15%
*CDC Data in Finkelstein & Zuckerman,
Wt (lbs) x 703/ins squared
10%
19 60 -6 2 19 76 -8 0 71 -7 4 88 -9 4 19 19
2008
Year
20 01 -2 0
04
Why has obesity increased recently in the US? All answers honored
A = Changes in television watching patterns B = Decrease in family meal traditions C = Changes in food availability D = all of the above E = B & C but not A
Why nature is likely to have gone beyond the homeostatic control of eating
Cells need energy continuously Complex creatures cant eat continuously Food is not always available We cant afford to let energy and vitamin stores get too low
The engineering solution
Solution: (1) Store energy and vitamins
If it takes energy to get energy then one must always have energy available, you can never let the tank go to zero
(2) Make food reinforcing so that it attracts behavior even when not essential
Reinforcing value should increase at first signs of biological need, or perhaps---to be safe---remain above zero at all times Thus one eats more than needed when food is available and stores the extra energy for when food is not available and/or its time to do something other than eat What feature(s) of food should be reinforcing?
Major components of the biological solution to the challenge of supplying cells with a continuous supply of energy
digestive tract metabolism of food storage of food as glycogen and fat
Digestive system and metabolism
Breaks down and delivers food Mouth: grind and dissolve (carbohydrates) Stomach: continues the dissolve process (proteins) Small intestine interfaces circulatory with system Circulatory system: capillaries, cells, receptors Insulin
Metabolized food takes 1 or 2 pathways
Utilized immediately by cells as fuel or building blocks Stored as glycogen in the liver or fat in the fat cells
Simplified picture of food metabolism: In addition digestion transforms proteins into amino acids that are used to build cells or if no immediate need then stored as fat.
tio n
Eat
D
ig es
Glucose & FAT Glycogen/ShortTrm Store/Liver Fat/Long-Trm Store
Fuel/Immediate Use
Glucose Fuel
Glucose Fuel
Fatty Acids Fuel
Fuel use is prioritized: brain is "first in line." If brain has enough fuel then muscles get fuel. If enough fuel for immediate needs, then glucose is stored in short-term storage (liver). If short-term storage is full then glucose is transformed into fat and stored in fat cells. Fat goes directly into fat cells. Number of fat cells remains more or less constant but can greatly increase in size. When you wake up, you typically start drawing on your long-term storage of fat cells.
Predictions:
Biological processes will be very good at initiating eating Biological processes will be lax when it comes to stopping eating Reward will play a large role in motivating eating and will be based in part on taste and other sensory properties If food becomes plentiful (and sweet & fatty & somewhat salty) then
self-control will be needed to curtail overeating Individual differences in fat storage and energy use will influence eating/body weight
Biological signals that initiate eating
Metabolic signals indicating glucose levels, fat levels and amount of food in digestive tract are sent to hypothalamus---hypothalamus then sends signals that are interpreted as hunger
Hypoglycemia (low blood glucose) powerful stimulus for eating Lipoprivation (depriving cells of lipids) stimulates eating Empty stomach releases hormones that promote eating (gherlin) Other factors: decrease in body temperature, slow down of metabolism, release of insulin
Homeostatic factors that slow eating, shortterm factors
Intestinal factors: hormones that are correlated with amount of nutrients in the small intestine
CCK depends largely on fat content of intestine
injections of CCK slow eating rats without CCK receptors become obese
PYY: levels are proportional to caloric content of food in intestine
Water does not stimulate PYY release Individual differences in PYY correlated with body weight (see next slide)
Homeostatic factors that slow eating, longterm factors
The body has a long-term system for detecting a correlate of body weight: leptin Fat cells secrete leptin (a hormone), more fat cells more leptin If leptin production is halted-obesity, diabetes, decrease in motor activity
Long-term satiation signals: The ob mouse and leptin
Genetic Leptin Free Mouse
Genetic Leptin Free Mouse with Leptin Injections
Leptin is secreted by triglyceride filled fat cells. Acts on hypothalamus Increases activity level and body temperature, decreases eating
Leptin in humans
To the disappointment of pharmaceutical industry, most people who are overweight do not have a leptin problem (that is, they have lots of leptin but are still overweight) There is a very small number of leptin insensitive people (and they are overweight). They are short on leptin receptors Leptin treatment only works in people who have become leptin resistant (age and genetics)
Role of innate taste sensitivities in eating
New born infants prefer sweet tastes, avoid bitter tastes All animals prefer somewhat salty foods to foods without salt All species eat more in the short-term and long-term when food has higher fat content, is sweeter, and is varied---see text To be continued.we will turn out attention to experiential factors in eating.