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3.9 - Hunger and the Chemical Senses.pdf

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McMaster University
Joe Kim

Arnav Agarwal 2011 Hunger and the Chemical Senses Module 1: Introduction to Hunger and Satiety Introduction - Seeking out food and drink is a fundamental goal-directed behavior o Moving body needs regular nourishment to function optimally o Result: much of the day is structured around times of feeding - Food and drink are more than nutrition o Have a social aspect, bringing family and friends together o Involves pleasure - How does hunger drive feeding behavior? How do satiety signals cue you to stop feeding? - Life is dominated by consumption of food and drink - Most of human evolutionary past: food sources scarce, behaviors motivated by constant need to obtain food and nutrients essential for survival - Present: Calories are available cheaply and easily; evolutionary-driven behaviors out of place - Feeding behaviors are partly motivated by hunger and satiety signals - Food doesn’t taste the same without sense of smell o Feeding guided largely by interaction of taste and smell senses Module 2: Hunger and Satiety Introduction to Hunger and Satiety - Morning breakfast preceded by longest period of fasting in daily cycle (overnight) Glucose and Glycogen Balance - When fasting, one of the main reasons for hunger is low blood glucose levels - Glucose: o Keeps body’s functions operating o Preferred source of energy for brain  Brain cannot use fat energy stores for fuel like other organs and tissues can  Regulating glucose levels is therefore top priority - High sensitivity to blood glucose levels -> directly relates to feelings of hunger - To keep brain constantly supplied with energy while performing other tasks, body stores glucose as glycogen which can be released between meals o Glycogen storage: mainly liver, also muscles  Liver: can be readily converted back into glucose when circulating blood glucose levels are low - Glucose-glycogen balance mediated a lot by the liver and insulin (pancreatic hormone) Arnav Agarwal 2011 Glucose and Glycogen Balance - Food intake -> increase in blood sugar -> pancreas secretes insulin -> uptake of glucose by cells promoted, excess glucose storage as glycogen stimulated - As time goes by: blood sugar levels begin decreasing -> reaches a point low enough -> liver breaks down stored glycogen into glucose -> glucose released into circulation - As glycogen reserves in liver get depleted, status signal is sent to brain - When glycogen levels are too low: feeling of hunger - Liver and pancreas therefore help buffer extreme blood glucose level swings - Glucose: for use now - Glycogen: for use later NPY - Neuropeptide Y is another hunger cue - High levels of NPY activity in the hypothalamus -> increased appetite and food-seeking behavior o Eg: heading to the kitchen - NPY affects feeding behavior similarly in fish, reptiles, birds and other non-human mammals - NPY is a potent appetite stimulant Arnav Agarwal 2011 Satiety and the Liver - Liver can send signals to the brain to trigger hunger, but can also trigger satiety this way o If glucose is injected into a vein connecting directly to a liver of eating animal, it will stop o If glucose is injected into any other vein, it will continue eating - Liver monitors glycogen stores and blood sugar levels o Low blood glucose and low glycogen levels -> signals of hunger o High glucose levels and high glycogen stores -> signals of satiety Arnav Agarwal 2011 CCK and Meal Duration - Small intestine produces CCK (Cholecystokinin) as food moves from stomach to gut - CCK: hormone responsible for feelings of satiety/fullness after a meal o Brain receptors detect CCK -> signal to stop eating - Individuals with CCK injected in feel satiated sooner - CCK administered to rats: shorter meal duration compared to controls o However, more total meals per day than the controls -> total daily food intake was actually the same  Therefore, CCK is short-term - CCK is a SHORT-TERM SATIETY SIGNAL and regulates SATIETY on a short-term basis - CCK regulates short-term feeding behaviors, not long-term energy consumption Concept Check: 1) Unlike other tissues in the body, the brain primary source of energy is: a. Glycogen b. Glucose i. Correct. Glucose is energy that fuels the brain. c. Carbohydrates d. Adipose tissue 2) Glucose is stored in the liver and muscles in the form of: a. Glycogen 3) The _______ secrtes insulin and the ___________ breaks down glycogen into glucose a. Adipose tissue; muscles b. Pancreas; liver Arnav Agarwal 2011 i. Correct. The liver breaks down glycogen and the pancreas produces insulin. c. Liver; pancreas d. Adipose tissue; NPY 4) High levels of NPY in the ________________ are associated with feelings of _____________ a. Thalamus; satiety b. Amygdala; satiety c. Hippocampus; hunger d. Hypothalamus; hunger 5) Which of the following best describes the results of the CCK study in mice? a. Giving CCK injections to mice increases their average meal duration b. Giving CCK injections to mice decreased their satiety and increased levels of NPY in the brain c. Giving CCK injections to mice decreases their average meal duration d. Giving CCK injections to mice decreases their average meal duration, but the rats increased their meal frequency i. Correct. This is why the treatment rats consumed the same amount of calories as control rats in the study mentioned in Module 2. Module 3: Long-Term Weight Regulation Long-term Energy Storage - Long-term energy storage takes place in the form of fat (ie: adipose tissue) - Both short- and long-term mechanisms interact to regulate overall energy balance and body weight Adipose Energy Stores - Why as fat and not glycogen, a quickly transferable source of energy? o Fat has twice the energy that carbohydrates (eg: glycogen) have  1 gram of fat = 9 units of kilocalories  1 gram of carbs = 4 units of kilocalories o Fat is found in virtually all body parts unlike glycogen  70 kg man -> 1200 kcal of glycogen (12-18 hours of activity) and 120,000 kcal of fat (couple of months of activity) stored  Fat clearly more useful - Fat is not just passive energy storehouse; it is an active component of regulatory physiology and was recently classified as an endocrine organ too Leptin - Adipose tissue secretes leptin (hormone) - Leptin involved in long-term energy balance; correlated with fat mass Arnav Agarwal 2011 - When leptin levels rise -> act on hypothalamus receptors -> appetite reduction -> food consumption reduction - Leptin production is controlled by the OB gene - In genetically-altered knockout mice lacking an OB gene: o Leptin production stops -> appetite regulation not present -> fat storage increases -> mouse becomes obese o Regular leptin injections -> condition reversed -> eating behavior and weight return to normal OB Gene Revisited - Studies suggest obesity in humans might be linked to defective OB genes or receptors o Not supported by clinical findings: very few obese people have known leptin signaling system defects - Humans and other animals are capable of becoming leptin-resistant; leptin’s ability to inhibit appetite beyond a point is reduced o Giving additional leptin to animal: no reliable link to normal weight loss levels - Original adaptive purpose of leptin must have been to indicate low energy stores, not to reduce food intake o Most of evolutionary history: calories were limited; rarely would one consume too many at a time o Low leptin levels -> increase foraging effort, conserve energy o Individuals would rarely have had high leptin levels or suffered from excess adipose tissue’s effects NPY - NPY activity in the hypothalamus stimulates appetite - High levels of leptin inhibit NPY - NPY activity in the hypothalamus = “on” switch; leptin inhibits NPY actions (“off” switch) - NPY mediated increase in appetite is prevented by leptin -> decreased appetite and energy consumption o Result: regulated optimal weight Arnav Agarwal 2011 Maladaptive Feeding and Neuropeptide Y - Rats: NPYergic neurons can specifically affect reward-driven feeding for high-calorie foods like sucrose - Experiments: NPY injected into rats’ brains who were satiated by previous food consumption o Increase in intake of sucrose observed o Begin to work harder for a cue associated with sucrose o Increased consumption of saccharin (similar taste to sucrose, no calories) o Preferentially choose a diet of carbs over proteins, or fat - Conclusions: o NPY action promotes unconditional and conditional behaviors specifically leading to increased carb consumption - Rats’ pre-existing preferences play role in NPY-induced increase in carb preference: rats showing higher baseline preference for carbs showed greatest preference for carbs following injection Endogenous Opioids (involved in hypothesis for overeating) - Naturally-occurring chemical substances that have morphine-like analgesic actions in the body Arnav Agarwal 2011 - Contributes to palatability and reward-driven feeding - Blocking the receptors with naloxone (drug) reduces intake of rewarding foods (eg: saccharin, sucrose, saline) - Knock-in mice which have been genetically-modified to lack the opioid receptor show lower preference for saccharin than control mice -> in line with expectations - Overeating in some people might be due to maladaptive opioid-mediated reward-driven feeding Conclusion - Energy balance and body weight regulation by hormones and mechanisms is very asymmetric o Body defends itself against weight loss more than it does against weight gain  Evolutionary perspective: calories and nutrition were less certain o Can have a maladaptive expression in a modern, fast-food nation  Calories are cheap and physical exertion is minimal Summary Concept Check Arnav Agarwal 2011 1. Gram for gram, fat is a superior form of energy storage than glycogen because: a. Glycogen (a carbohydrate) has 9 kcal/g whereas fat has only 4 kcal/g b. Fat has 19 kcal/g whereas glycogen (a carbohydrate) has only 14 kcal/g c. Glycogen (a carbohydrate) has a 19 kcal/g whereas fat has only 14 kcal/g d. Fat has 9 kcal/g whereas glycogen (a carbohydrate) has only 4 kcal/g 2. The hormone leptin is secreted from: a. The liver b. The small intestine c. The pancreas d. Adipose tissue i. Correct. Fat tissue actually produces hormones! 3. OB-gene knockout mice become obese, but giving them injections of leptin causes them to: a. Eat less and consequently return to a normal weight b. Become more active and consequently return to a normal weight c. Continue increasing in weight. Exogenous leptin does not result in weight loss. d. Return to a normal weight. 4. Which of the following best describes the evolutionary significance of leptin? a. High levels of leptin strongly reduce food-seeking and eating behaviors because excess fat is strongly correlated with cardiovascular disease b. The primary adaptive function of leptin is to serve as a signal to reduce food intake when energy stores are replenished c. Low levels of leptin serve as signals to increase foraging effort and/or decrease activity levels in order to conserve energy i. Correct. As a signaling molecule, it is believed that leptin involved to indicate low energy stores (adipose), which were much more common than having excess fat d. Low levels of leptin serve as signals of excessive energy stores and serve as signals to the brain to reduce food intake and increase energy expenditure to maintain homeostasis 5. Blocking opioid receptors in the brain with naloxone, would result in: a. Increased intake of rewarding food like sucrose b. Increased consumption of carbohydrates and proteins c. Reduced consumption of all foods d. Reduced intake of rewarding foods like sucrose Module 4: Taste Preferences and Food Selection Significance of Tastes - Throughout evolution, bitter and sour foods are associated with flavors not enjoyed -> indicative of toxins or noxious foods - Special acquired tastes for bitter and sour must be learned through experience Arnav Agarwal 2011 - Foods that are sweet, salty or savory are associated with craved flavors -> indicative of safe, nutritious, rich in energy -> eg: fruits and proteins - Individuals who could distinguish between bi
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