Physiology of obesity and apetite Flashcards
What is obesity?
- It is an excessive and abnormal increase in body fat that can be stored in the adipocytes, subcutaneous tissue, and intraperitoneal cavity
- Obesity is linked with an increased number and size of the adipocytes
1 GRAM OF FAT HAS HOW MANY CALORIES APPROXIMATELY?
9.3 CALORIES
What is meant by starvation?
The depletion of food STORES in the body
What is meant by hunger?
- Craving/Needing food (the need to eat)
- Physiologically it is associated with the release of ghrelin, rhythmical contractions of the stomach
What are the physiological factors that are associated with hunger?
1) Ghrelin
2) Rhythmical contractions of the stomach
What is meant by appetite?
- Desire for food
- It is associated with sensory experiences like sight, smell, and emotional factors
What are the sensory experiences associated with appetite?
1) Sight
2) Smell
3) Emotional factors
What is meant by satiety?
The satisfied feeling of being full after eating
What is the neural control of food intake?
1) Feeding center
- FYI: Lesions of the Dorsomedial nucleus (DMN) usually depresses the eating behaviors
2) Satiety center
- FYI: Lesions of the paraventricular nuclei often cause excessive eating
What are the nuclei of the feeding center?
The lateral nuclei of the hypothalamus (LNH)
What is the function of the lateral nuclei of the hypothalamus?
1) Activation: Increases food intake (hyperphagia)
2) Destruction: Decreases food intake and lack of desire for food
What are the satiety center nuclei?
Ventromedial nuclei of the hypothalamus
What is the function of the ventromedial nuclei of the hypothalamus?
1) Activation: Complete satiety (aphagia)
2) Destruction: Continuation of eating and obesity
What are the neural signals that control food intake?
- The hypothalamus receives several types of signals:
1) Neural from GIT (provides info about stomach filling)
2) Chemical from nutrients
3) GIT hormones
4) Adipose tissue hormone
5) Cerebral cortex (sight, smell, and taste)
What is the main integrative centre in the hypothalamus for the hormones released by the GIT and adipose tissue?
Arcuate nucleus
What do the pro-opiomelanocortin neurons release?
1) a-melanocytic-stimulating hormone (a-MSH)
2) Cocaine/amphetamine-related transcript (CART)
What will happen if we activate the Pro-opimelanocortin neurons?
- Decrease the food intake and increase energy expenditure
What are the main types of neurons found in the arcuate nucleus?
1) Pro-opimelanocortin (POMC) neurons (releases a-MSH and cocaine/amphetamine-related transcript “CART”)
- POMC/CART
2) Neurons release orexigenic substances (NPY “Neuropeptide-Y”, and agouti-related protein “AGRP”)
- AGRP/NPY
What is the function of the a-melanocyte stimulating hormone?
- It acts on the melanocortin receptors (MCR) found in the PVN (MCR3 & 4) which will decrease the food intake and increase the expenditure of energy
- The PVN has a lot of melanocortin receptors of which MCR3 & 4 are v.importnat
What will happen if there is a mutation in the POMC and MCR4 receptors?
Obesity in children
What would happen if the MCR receptors were activated excessively?
Anorexia
What will happen if we activate the NPY-AGRP?
1) Increase the food intake
2) Decreases the energy expenditure
What hormones regulate the POMC/CART, AGRP/NPY neurons of the arcuate nucleus?
1) Ghrelin
2) Insulin
3) Leptin
4) CCK
What is the mechanism of action of the agouti-related protein?
It is an antagonist of the MCR3 & 4, increasing the feeding by inhibiting the effects of the a-MSH to stimulate the MCR
What will happen if the agouti-related protein is mutated?
There will be an excessive formation of AGRP increasing the food intake and thus obesity
What is the neurohormonal control of food intake?
Hypothalamic feeding and satiety center have receptors for neurotransmitters and hormone (anorexigenic and orexigenic substances) which influences feeding behaviours
What is a anorexigenic substancec?
They are substances that inhibit the feeding
What is an orexigenic substance?
They are substances that stimulate feeding
What are the anorexigenic substances?
- Neurotransmitter + Hormones:
1) a-MSH
2) Leptin
3) Serotonin
4) Norepinephrine
5) Corticotrophin-releasing hormone
6) Insulin
7) CCK
8) Glucagon-like peptide
9) CART (Cocaine/amphetamine-related transcript)
10) PPY
- All of these decrease the sensation of feeding
What are the different orexigenic substances?
- Neurotransmitter + Hormones:
1) AGRP
2) Melanin-concentrating hormones
3) Orexins A and B
4) Endorphins
5) Galanin
6) Amino acids (glutamate/GABA)
7) Cortisol
8) Ghrelin
9) Endocannabinoidsll of them increase the sensation of feeding
What is the function of ghrelin?
It is an appetite-stimulating hormone
What are the appetite-inhibiting hormones?
1) CKK
2) PYY
3) GLP-1
What is the function of leptin?
It inhibits energy intake
Which hormone is responsible for stimulating appetite?
Ghrelin
What is the function of CKK, PYY, & GLP-1?
They are appetite-inhibiting hormones
What is an example of a hormone that inhibits the intake of energy?
Leptin
What is the role of adipocytes (adipose tissue)?
Before we viewed them as an inert organ for energy stores and now we view them as an endocrine gland that communicates with the hypothalamus
How does leptin regulate food intake, what happens if the lepton receptors are triggered?
1) Leptin helps the hypothalamus to sense energy storage through the actions of leptin
- Stimulation of the leptin receptors decreases the fat storage by:
1) Decreased production of NPY & AGRP
2) Activation of the POMC neurons which releases a-MSH and activate the melanocortin receptors
3) Increased production of corticotropin-releasing hormone (decreasing the intake of food)
4) Increases the sympathetic activity (via the neurons projecting from the hypothalamus to the vasomotor centers) increasing the basal metabolic rate and energy expenditure
5) Decreases the secretion of insulin and thus energy storage
What will happen if there are mutations that inhibit the production of leptin or expresses defective leptin?
Morbid obesity
What is meant by leptin resistance?
- Obesity seems due to it
- It is when the leptin receptors or the post-receptor signaling pathways resist the getting activated by leptin
What are the different regulatory systems of food intake?
1) Long-term regulation
2) Short-term regulation
What is the long-term regulation of food intake?
- They are nutritional feedback mechanisms that maintain a constant store of nutrients in the tissue
There are many theories explaining it like the glucostatic theory (as blood glucose decreases hunger increases), lipostatic theory, and the aminostatic theory which all state that the desire for feeding increases as blood levels of glucose, amino acids, and lipids decrease
What is the short-term regulation of food intake?
- Appropriate food intake in every meal session
- Steady pace to allow for appropriate digestion and absorption
- Initiates satiety after adequate food intake in every meal
What is the relationship between food intake and blood glucose?
An increase in the levels of blood glucose will increase the firing rate of the glucoreceptor neurons in the VMN and PVN nuclei (satiety center) and decrease the firing rate of the glucosensitive neurons in the hunger center of the lateral hypothalamus (feeding center)
What is the thermal regulation of food intake?
- It describes the interaction between the temperature regulatory system (hypothalamic thermostat) and the food intake regulatory system (lateral and ventromedial nuclei of the hypothalamus), it states that:
- An increase in temperature will decrease the food intake, while a low temperature will enhance food intake by increasing the metabolic rate, increasing the fat for insulation
What turns of the desire to eat?
- Short-term regulation
1) GIT filling
- Negative feedback pathways which are mediated by the activation of the stretch receptors in the stomach which activates the vagal afferent and inhibits the food intake by suppressing the feeding center
2) Hormones
- Hormonal involvement in short-term regulation is inhibitory in nature, suppressing the food intake and enhancing satiety
- CCK, Peptide YY, Glucagon-like peptide, Insulin (which is released by the ingestion of food and suppresses further feeding
- Leptin which is produced by fat cells and inhibits food intake
What are the cells that release ghrelin?
Oxyntic cells of the stomach and to a much lesser extent the intestine
- It is an orexigenic hormone *stimulates food intake)
Describe the levels of ghrelin during various physiological state
- Appetite-stimulating hormone
1) Rises during fast
2) Peaks before eating
3) Decreases after meals
What secretes the CCK?
Enteroendocrine cells in the duodenum
What are the functions of the CCK?
- It influences several GI functions:
1) Inhibits gastric emptying
2) Regulates the motility of the gut
3) Decreases the gastric acid secretion
- It communicates with the brain via the vagus nerve contributing to the satiation and meal cessation
What secretes Polypeptide YY?
The entire GIT especially the ileum and the colon
- It is released postprandial in proportion to the calorie ingested, there is a high level of PYY in response to the increased number of calories and high-fat meal (peak level of PYY is observed 1-2 hours after meal)
- PYY can inhibit food intake for 12 hours
What are the hormones that inhibit the appetite center in the brain and where are the released from?
1) Leptin (adipocytes)
2) Insulin/Amylin (B-cells of the pancreas)
3) PYY, Oxyntomodulin, GLP-1 (from the L-cells of the L.I)
4) GIP, CCK (From the - and K-cells of the S.I
What are the hormones that stimulate the appetite center in the brain and where is it released from?
Ghrelin (G-cells of the stomach)
What is the function of incretins?
Released in response to ingesting glucose, they stimulate the secretion of insulin and inhibits the secretion of glucagon from the alpha-cells of the pancreas
