Hunger and Thirst

Israel Kamakawiwo’ole

Obese, 757 lbs

Died of heart failure at 38

Homeostatic systems use our behavior to keep things _________

Balanced

Negative feedback systems

Main homeostatic mechanisms
Desired set point deviations triggers compensatory actions

Types of thirst

Hypovolemic:

• Stimulated by low extracellular/intravascular volume

Osmotic thirst:

• Stimulated by high extracellular solute concetration

Hypovolemic thirst

Baroreceptors in blood vessels and heart detect initial drop → brain activates thirst and salt craving → arteries constrict to raise BP

Hypovolemia → vasopressin release

• Constricts blood vessels and reduces blood flow to the bladder

Vasopressin deficiency

Kidneys send more urine to the bladder → chronic thirst

Angiotensin Cascade

Kidneys release renin wen blood volume decreases → formation of angiotensin II

• Which constrict blood vessels

• Circumventricular organs trigger drinking

• Vasopressin is released

• Aldosterone is released

Angiotensiogen → Angiotensin I → Angiotensin II → angiotensin III

Brain control of drinking

Circulating angiotensin II acts in the subfornical organ to signal other brain sites to initiate drinking.

Osmotic thirst

Osmosensory neurons in hypothalamus (OVLT) respond to rise in blood osmotic pressure (salt)

• Respond to increased osmotic pressure by causing pituitary to release antidiuretic hormone.

Cell membranes shrink → opening mechanical-gated Na+ channels

Dehydration and rehydration summary

Why don’t diets work?

Energy expenditure adjusts in response of nutrition
BMR falls at the start of a diet to prevent losing weight

Obesity and reduced metabolism

BMR:

• Energy required to fuel brain/body and maintain temperature

• 75% EE in average sedentary student

Heredity accounts for 40% of BMR

Activity can increase BMR

Energy storage/Utilization

Glucose: the principal fuel for energy.

Glycogen: glucose stored for short term in the liver

Glycogenesis: Converting glucose to glycogen using pancreas hormone insulin

Lipids: Long term storage, fat tissue

Metabolic rate variations

Up until age 1 calorie burning is at its peak

From age 1 to about age 20, metabolism gradually slows

From age 20 to 60, it holds steady

After age 60, it declines about 1 percent a year

Brain ________ insuline and glucose levels with other signals to decide when to start/stop eating.

integrates

Leptin

Fat cells produce and secrete them into the bloodstream.
Defects in leptin production/sensitivity give a false low report of body fat.
Obese people are leptin-resistant
Overnutrition inflames the hypothalamus → obesity, diabetes, and heart disease.

Ghrelin

Appetite stimulant
Released by stomach/gut endocrine cells
Rises during fasting

Drops after eating

Prader-Willi syndrome

Causes a sense of hunger.

Genetic disease

Ghrelin levels are elevated

Hypothalamus as the hunger control center

Lateral:

• Lesions lead to the refusal to eat, destroy hunger-causing LHA

• Resume and stabilize their weight at new, lower level

Ventromedial:

• Lesions lead to obesity (eating too much), destroy hunger ending PVN

• Increased weight stabilizes

• New weight maintained even after food challenges

Neurons groups in hypothalamus

NPY/AgRP → neuropeptide Y and agouti-related peptide

• Stimulate appetite and lower metabolism

• Ghrelin stimulates them.

POMC/CART → pro-opiomelanocortin & cocaine and amphetamine-related transccript

• Inhibite appetite

• Raise metabolism

• Stimulates by leptin

Brain damage from overeating

Hypothalamic inflammation → inhibits neurogenesis, resets set point.

Hypothalamic scarring, microglial activation, fewer POMC neurons

Can recover if overeating stops

• Newborn hypothalamic cells can become POMC neurons.Anore

Anorexia Nervosa

Restricting or binge eating/purging

• Refuse to maintain body weight

• Fear of gaining

• Body image disturbance

• Highest mortality rate

Bulimia

• Recurrent binge eating

• Recurrent inappropriate compensatory behavior

• 2x/wk for 3 months

Women with anorexia/bulimia

40% childhood anxiety disorders

90% depression

246 women with eating disorder

30% attempted suicide

5% died

Teen girls with anorexia

Larger insula → disgust

larger orbitofrontal cortex → you shouldn’t do that

Obesity treatment

Eat less

• deficit of 200 calories

Exercise

• Strenuous aerobic activity for over 200 minutes per week

• Long time

• with calorie restriction

• Walking doesn’t count

Glucagon-like peptide agonists

• Mounjaro

• Ozempic