Homeostasis & Hunger: Your Body’s Constant Drive to Stay in Balance.

Homeostasis & Hunger:

Your Body’s Constant Drive to Stay in

Balance

Homeostasis

• A tendency to maintain a balanced or constant internal state

• The regulation of any aspect of body chemistry, such as blood glucose, around a particular level

• Any change in levels, up or down, results in being motivated to bring the level back to normal.

Energy Balance•Positive energy balance occurs when caloric intake exceeds calories expended for energy. The excess glucose is converted to body fat.

•Negative energy balance occurs when caloric intake falls short of the calories expended for energy. Body fat stores shrink as the reserve energy in fat cells is used

Basal Metabolic Rate (BMR)

• The body’s resting rate of energy expenditure

• The rate at which the body uses energy for vital functions while at rest

• Factors that influence BMR– Age– Gender– Size– Genetics– Food intake

There is a steep decline in the rate at which your body uses energy for vital functions, such as heartbeat, breathing, and body heat.

Your BMR continues to decrease by about 2 to 3 percent during each decade of adulthood.

At all points in the lifespan, women’s metabolic rate is 3 to 5 percent lower than men’s

Body Mass Index:A measure for human body shape based on an individual’s weight and height.

(This equation doesn’t account for differences in muscle vs. fat.)

Set Point• A person’s average weight long term• (NOT necessarily a person’s ideal weight!)• When the body falls below this weight,

increased hunger and a lower basal metabolic rate (BMR) may act to restore the lost weight.

Physiology of Hunger

Hypothalamus – The Control Center

States of the Brain

• The hub of many central drive systems lies in the hypothalamus

Cerebral cortex

Portion of

limbic system

Hypothalamus

Pituitary

glandBrainstem

Hunger Drive

• Two areas of the hypothalamus, the lateral and ventromedial areas, play a central role in the hunger drive

Hypothalamus

Hypothalamus

Ventromedial Area

Hypothalamus

Hypothalamus

• Decreases Hunger• Lesions (tissue damage)

alter digestive and metabolic processes

• Food is converted into fat rather than energy molecules, causing animal to eat much more than normal and gain weight

• Lesions = Fat rats!!

Lateral Area

• Increases Hunger• Lesions reduce

hunger drive as well as general arousal

• Lesions = Skinny rats!

Hypothalamus

Hypothalamus

Hunger DriveA variety of stimuli act on the brain to increase or

decrease hunger:– satiety signals from the stomach (Cholecystokinin)– signals indicating the amount of food

molecules in the blood (insulin)– leptin, a hormone indicating the amount

of fat in the body– External/environmental factors

Physiological changes that predict eating (Short Term)

• About 30 minutes before you eat, you experience…• A slight increase in blood levels of insulin and • A slight decrease in blood levels of glucose. • Even if you do not eat, glucose will return to its

baseline level. • Prior to eating, body temperature increases and

metabolism decreases. • As the meal is consumed, this internal physiological

pattern reverses.

Glucose• Form of sugar which circulates through the body• Glucose is converted by the liver and stored as glycogen • One feels hunger when the levels become low.

Insulin• Hormone which allows the body to

use glucose for energy or fat production

• Secreted by the pancreas• Controls blood levels of glucose and

promotes the uptake of glucose by the muscles and other body tissues

• As insulin levels increase, glucose levels decrease.

Satiation signals: Sensing when to stop

• Satiation is the feeling of fullness and diminished desire to eat that accompanies eating a meal

Satiation signals include:1. taste buds that activate chemical signals in the stomach

and intestine to prepare for digesting food 2. stretch receptors in the stomach that communicate

sensory information to the brainstem 3. hormone cholecystokinin (CCK) that is secreted by the

intestines; it promotes satiation and reduces or stops eating

4. sensory-specific satiety - the reduced desire to continue consuming a particular food

The Appetite Hormones• Insulin: Hormone secreted by pancreas; controls blood

glucose. • Leptin: Protein secreted by “bloated” fat cells; when

abundant, sends a message to “stop eating” to brain that increases metabolism and decreases hunger.

• Orexin: Hunger - triggering hormone secreted by hypothalamus. As glucose levels drop, orexin levels increase and person feels hungry

• Ghrelin: Hormone secreted by empty stomach; sends "I'm hungry" signals to the brain.

• Corticotropin-Releasing Hormone (CRH): hormone in hypothamalus that sends "I'm not hungry" signals to the brain.

Psychological factors that trigger eating

• Through classical conditioning, the time of day at which you normally eat acts as a conditioned stimulus and elicits reflexive internal physiological changes (the CR), which increases your hunger

• Positive reinforcement plays a role in eating; voluntary eating behaviors are followed by the addition of a reinforcing stimulus—food

• Due to prior learning experiences, certain tastes, especially sweet, salty, and fatty tastes, hold greater positive incentive value

Environment and Hunger

Cultural Influences on Eating

• Cultural views on obesity can vary• Eating disorders are more prevalent in the

West than in East although the more developed a country becomes the more people are seen to develop them.

• Culture influences the foods we like and dislike.

The Obesity Epidemic

Factors Contributing to Being Overweight

• Highly palatable food—we eat because it tastes so good

• SuperSize It—food portions are larger than necessary for health

• Cafeteria Diet Effect—more food and more variety leads us to eat more

• Snacking—does not cause us to eat less at dinner• BMR—changes through the lifespan• Sedentary lifestyles

Factors in Obesity• Genetic susceptibility—some people are

more likely to be predisposed to obesity• Leptin resistance—condition where higher-

than-normal levels of leptin do not produce desired physiological response

• Weight cycling—repeated dieting, weight loss and weight gain tends to result in higher weight and reduced BMR.

We’re # 1!

Epidemic of Overweight and Obese Americans

• The problem of being overweight escalates during young and middle adulthood.

• Males outpace females in being overweight in each age group.

• However, although the data are not shown, more women than men are obese in each age group.

Eating Disorders

Eating Disorders• Anorexia nervosa —characterized by

excessive weight loss, irrational fear of gaining weight and distorted body image

• Bulimia nervosa —characterized by binges of extreme overeating followed by self-induced purging such as vomiting, laxatives

• Binge-eating —disorder characterized by recurring episodes of binge eating without purging.

Men’s & Women’s Body Images

According to surveys on body image, people in our society are much more dissatisfied with their bodies now than they were a generation ago. Women are still more dissatisfied than men, but today’s men are more dissatisfied with their bodies than the men of a generation past.

Anorexia NervosaKey Features

1. Refusal to maintain a normal body weight

2. Intensely afraid of being overweight.

3. Suffer from delusions of being overweight.

4. Denies there is a problem.• Usually in adolescent females• May put themselves on self-

starvation regimens• May become dangerously

underweight

Bulimia Nervosa• An eating disorder characterized by

episodes of overeating (usually high calorie foods)

• Overeating is followed by vomiting, using laxatives, fasting, or excessive exercise

• Usually stay within their normal weight.• Usually recognize they have an eating

disorder.

PicaStrange but True!

• Hippocrates the first to describe the disorder• People display a compulsive craving for inedible

substances such as clay, dirt, laundry starch, chalk, buttons, paper, dried paint, burnt matches, ashes, sand, oyster shells or broken crockery.

• Seen most often in pregnant women or nursing women but also with people with severe mental disorders.

• Could be a behavioral response to stress.