Eating

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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

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• 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.

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Hypothalamus
Food Intake Energy Expenditure Reproduction
Leptin

• Energy Availability

High
Low
Fat Cell
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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!

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• Ob mouse A strain of mouse whose obesity and low
  metabolic rate is caused by a mutation that
  prevents the production of leptin

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• Anorexia nervosa
• Anxiety and fear of obesity
• Disturbed attitudes toward body weight shape
• Body weight lt 85
• Hypothalamic amenorrhea .

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• Anorexia nervosa
• Reduced body weight ?
• changes in levels of leptin ?
• Hypothalamic amenorrhea
• Replacement therapy with leptin restores the
  menstrual cycle in anorexic patients
• This is also true in other stress situations
  (malnourished nursing mothers, strenuous exercise
  in women athletes)
• Anorexia insulin levels change as in normal
  subjects after exposure to yummy food

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Stress-induced eating

• Stress --gt cortisol
• lipolysis gluconeogenesis --gt increase glucose
• Glucocorticoid antagonists prevent obesity
• Exogenous cortisol --gt increased food intake in
  humans
• In rats, glucocorticoids increase preference for
  carbohydrates (gimme a break, tic-tac bar)

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Stress-induced eating (contd)

• 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

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• Homeostatic Theory

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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

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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

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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!!

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Sensory-specific satiety

• You always have room for dessert, even if you are
  done with the main dish

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Obesity

• Strong genetic influence. This explains
  individual differences
• There are environmental factors (current
  epidemic)
• Pima Indians (in the West of USA and Mexico)

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Obesity (contd)

• 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.