Eating

1. 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

2. 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.

4. Hypothalamus Food Intake Energy Expenditure Reproduction Leptin Energy Availability High Low Fat Cell

5. 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!

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

8. Anorexia nervosa • Anxiety and fear of obesity • Disturbed attitudes toward body weight & shape • Body weight < 85% • Hypothalamic amenorrhea .

9. 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

11. Stress-induced eating • Stress --> 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)

12. 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

13. Homeostatic Theory

15. 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

16. 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

17. 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!!

18. Sensory-specific satiety • You always have room for dessert, even if you are done with the main dish

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

20. 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.