Lecture 6: Food Intake Flashcards
What are the 3 energy sources in the body?
- Triglycerides; fatty acids
- Carbohydrates; glucose
- Protein; amino acids
Describe carbohydrates: glucose as an energy source for the body:
- Glucose is the principal circulating sugar in the blood - a major energy source of the body
- It is the ONLY source of energy that the brain can use - ketones
- Glucose metabolism maintains blood glucose levels at 4-6 mM
- Other sugars: fructose & galactose
How is glucose stored in the body?
- Shortly after we have eaten glucose is readily available in the blood
- Excess glucose is stored as glycogen in the body - lots of glucose molecules joined together into a giant branching molecule
- When blood glucose levels drop glycogen is broken down to release glucose
Describe fats: free fatty acids as an energy source for the body:
- Fatty acids are a major energy source of the body
- Slower source of ATP production than glucose BUT each fatty acid molecule generates a lot more ATP
How are fatty acids stored in the body?
- Shortly after we have eaten fatty acids are readily available in the blood
- Excess fatty acids are stored as triglycerides, mostly in adipose tissue
- Stored triglycerides are broken down into 3 free fatty acids and 1 glycerol molecule
What is a triglyceride made up of?
3 fatty acid molecules and 1 glycerol molecule joined together
Describe protein: amino acids as an energy source for the body:
- Amino acids are the 3rd energy source of the body
BUT - In a normal well-fed state, amino acids are mostly used to make new body proteins, like muscle tissue - any excess will be made into fatty acids for storage in adipose tissue
Describe the energy stores of protein in the body:
- In the state of STARVATION, the body can break down body proteins into amino acids
- Amino acids can be used for gluconeogenesis
What occurs in the absorptive state?
Lasts for ~ 4 hours after a meal
* ANABOLIC: High insulin
* Amino acids -> Proteins
* α-glycerol phosphate + fatty acids -> Triglyceride
* Glucose -> CO2 + H2O + energy by most cells
* Glucose -> glycogen and fat in the liver
What occurs in the post-absorptive state?
4+ hours after a meal
* CATABOLIC: Low insulin
* Proteins -> amino acids
* Triglyceride -> glycerol + fatty acids
* glycogen -> glucose
* Fatty acids + ketones -> CO2 + H2O + energy by most cells - NOT the brain
* Pyruvate; lactate; glycerol; amino acids -> glucose in the liver
Describe the endocrine functions of the pancreas:
- Endocrine (2%): secretes hormones into the blood
- Endocrine portion consists of 1-3 million Islets of Langerhans (pancreatic islets)
- Within Islets:
- α cells secrete glucagon
- β cells secrete insulin
- 60–80% of cells within Islets are β cells
Describe the exocrine function of the pancreas:
- Exocrine (98%): secretes digestive enzymes & bicarbonate into the small intestine via a duct
How does insulin secretion control glucose levels in the body?
*↑ Plasma glucose
*↑ Insulin secretion by pancreatic islet β cells
* ↑ Plasma insulin
* ↑ Glucose uptake by adipocytes & muscle; ↑ Cessation of glucose output/net glucose uptake
* Restoration of plasma glucose to normal
- Insulin is the only regulator that decreases glucose levels
Describe the structure of insulin:
- Insulin is a peptide hormone so it can be made and stored in vesicles within β cells
- Secreted as a prohormone
- Cleaved to form the active hormone
- Composed of 2 short peptide chains which are joined together
- A peptide
- B peptide
Describe the structure and function of an insulin receptor:
- Homodimer (2 identical units):
-α-subunits: ligand binding
-β-subunits: signal transduction - tyrosine kinase domain & phosphorylation sites - Insulin receptor is a receptor tyrosine kinase
- Autophosphorylates its own tyrosine residues
- Phosphorylates ‘insulin receptor substrates’
How does insulin trigger the cellular uptake of glucose?
- Insertion of glucose transporters (GLUT4) via exocytosis into the cellular membrane
- Transporters are constantly recycled back into the endosome: endocytosis
- Then reformed into vesicles, and inserted into the membrane
- This cycle continues while insulin is present
Note: Cells in the brain have a different type of glucose transporter that is not insulin sensitive
What happens to the cellular uptake of glucose when insulin isn’t present?
- When blood glucose levels drop, insulin secretion stops, and insulin concentration drops
- Glucose transporters are no longer inserted into the membrane
- But they continue to be removed by endocytosis
- Glucose uptake stops
Describe the structure of glucagon:
Glucagon is a peptide hormone produced by A (α) cells in the Islets of Langerhans
What effect does glucagon have on blood glucose?
Glucagon acts to increase blood glucose levels to within the homeostatic range (4-6mM)
* If blood glucose levels drop too low glucagon acts on the liver to:
- Increase glycogenolysis (glycogen breakdown), releasing glucose
- Increase gluconeogenesis (synthesizing new glucose)
- Which will increase blood glucose concentration
What happens to glucagon and insulin levels in the absorptive state?
After a meal blood glucose is high
* Suppresses the release of glucagon
* Stimulates the release of insulin
What happens to glucagon and insulin levels in the post-absorptive state?
After the glucose from the meal has been absorbed, blood glucose drops
* Stimulates the release of glucagon
* Suppresses the release of insulin
What are satiety signals?
Anorexigenic signal: a signal that promotes satiety and the cessation of food intake
* These signals act in the brain to decrease our feelings of hunger
What are hunger signals?
Orexigenic signal: a signal that stimulates hunger and food intake
* These signals act in the brain to increase our feelings of hunger
Describe the structure and function of leptin:
- Leptin is a peptide hormone
- Secreted by fat (adipose) cells - amount of leptin secreted is proportional to amount of body fat
- Leptin secretion also increases after a meal
- Leptin changes the activity of neurons in hypothalamus resulting in reduced hunger/increased metabolic rate: Anorexigenic
- Obesity is associated with leptin resistance
What happens if there is a mutation in the ob gene?
- Leptin is produced by the ob gene
- Mutations in ob gene cause severe obesity in mice
- Mutations in the ob gene are not common in humans
- Polymorphisms in ob gene do not seem to be related to obesity in humans
Describe the control of leptin secretion:
Energy intake > Energy expenditure
↓
Adipose tissue: ↑ fat deposition & ↑ leptin secretion
↓
↑ Plasma leptin concentration
↓
Hypothalamus: Altered activity of integrating centres
↓
↓ Energy intake & ↑ metabolic rate (negative feedback to the start)
Describe the structure and function of grehlin:
- Peptide hormone
- Secreted by endocrine cells in the oxyntic glands of stomach
- Ghrelin changes the activity of neurons in hypothalamus resulting in increased hunger: Orexigenic
- Stimulates: Hunger; Food intake; Gastric emptying
- Suppresses fat utilization in adipose tissue
When do ghrelin levels rise/fall?
Rise prior to meals and decrease following meals
What happens if food intake > energy expenditure?
- Leptin is released which is anorexigenic (satiety) and acts on the hypothalamic integrator to decrease food intake and increase energy expenditure
- This leads to food intake = energy expenditure
What happens if food intake < energy expenditure?
- Ghrelin is released which is orexigenic (hunger) and acts on the hypothalamic integrator to increase food intake and decrease energy expenditure
- This leads to food intake = energy expenditure
