Fats

Question 1

What is the diffence in energy stored in fats versus proteins or carbohydrates?

Answer 1

Fats are the primary storage ofrm of energy in the human body - energy is more densely stored in fats than proteins or carbohydrates

Metabolic oxidation of fats = 37kJ/g

Carbohydrates = 17 kJ/g

Proteins = 17 kJ/g

BUT: the brain cannot use fats for energy; under conditions of starvation the brain can adjust to ketone bodies that are derived from fatty acids

Question 2

Discuss the absorption and transport process of dietary fats

Answer 2

1. Dietary bile emulsifies dietary fats in the small intestine
2. Intestinal lipases degrade TAGs into fatty acids and glycerols
3. Fatty acids and glycerols absorbed through into intestinal mucosa cells where they are reformed into TAGs
4. TAGs are incorporated into chylomicrons along with cholesterol and apolipoproteins (APOC II)
5. Chylomicrons enter the lymphatic system (via lacteals) and enter the blood circulation without hepatic processing
6. APOC II of the chylomicron activates lipoportein lipase in capillaries and converts TAGS into fatty acids + glycerol
7. Fatty acids (FFA) enter the cells
8. In muscle cells, FFA’s are used for energy
9. In adipocyte cells, FFA’s are stored for energy

Question 3

Discuss the stucture and function of an adipocyte cell

Answer 3

Adipocytes are one of the most innactive cells in the body.

They have no internal cell structure or cytoskeleton which allows it to expand up to 20x normal volume for the storage of esterfied fatty acids (fat).

Has a lipid droplet, cytosol, nucleus and mitochondrion

Produces leptin and adiponectin hormones that signal to the body how much fat is in the body

Question 4

How are fatty acids mobilised from adipocyte stores for use in respiring/active cells?

Answer 4

Adrenaline and glucagon are fat mobilising hormones that share cAMP as a second messenger that is responsible fo rthe activation of triacylglycerol lipase that breaks down TAGs in adipocytes

Glycerol and FFA are released into the blood stream

FFAs are carried by serum albumin because they are highly acidic and hydrophobic - are detergents.

Most tissues are capable of using fat as a fuel of energy production (except brain, RBCs and testis). Absorbed FFAs undergo beta-oxidation within mitochondria to form Acetyl-CoA for the Krebs Cycle.

Note: in rare cases the mobilisation of FFAs exceed the carrying capacity of albumin and this may contribute to heart attack following sustained physical stress.

Question 5

How are fatty acids transported into the mitochondria of fuel-requiring cells?

Answer 5

Acetyl CoA binds FFAs in the cytosol and presents FFAs to L-carnitine in the cytosol.

Carnitine carries the FFA accross the mitochondrial membranes into the mitochondrial matrix

Carnitine dependent transport is the rate limiting step in beta-oxidation.

Carnitine deficiency leads to muscle weakness

Question 6

How body cells use FFAs to produce energy?

How do cells form fatty acids to store as fat?

Answer 6

There is an equilibrium existing between making and using fat within the mitochondria of body cells: the equilibrium between Fatty Acids and Acetyl CoA.

Using Fat

• Beta-oxidation in mitochondria produces acetly CoA that enters the Kreb cycle
  
  • the most commonly stored FFA is palmitate -> it produces 106 ATP per molecule.

Making Fat

• Citrate within the mitochondria exit to the cytosol to generate acetyl CoA
• Acetyl CoA is polymerised into palmitate FFA
• Palmitate can then be transported to adipocytes for storage

Question 7

What are ketone bodies?

Answer 7

Ketone Bodies are made by liver mitochondria as fuel for other organs during starvation.

Ketone bodies are a mixture of acetoacetate, acetate and beta-hydroxybutyrate

The acetyl CoA is derived from beta-oxidation of fatty acids, but instead of entering the Krebs Cycle it forms ketone bodies.

Acetone can be smelt on the breath of starving and diabetic patients -> diabetes turns to starvation mechanism when glucose cannot be obtained by cells.

Ketone bodies are small and capable of diffusing accross the BBB to supply the brain in starvation

Question 8

How do ketone bodies lead to ketoacidosis?

Answer 8

Ketone bodies are strong organic acids that when in circulation can result in acidosis of the blood (<7.0)

This disrupts nerve function and can result in coma

Question 9

Describe the processes that occur in starvation leading to changes in fuel utilisation

Answer 9

In starvation, circulating insulin falls BUT glucagon rises

**Glucagon **is a fat mobilising hormone that results in the release of FFAs (and glycerol carbohydrate) from TAGs stored in adipocytes.

These FFA are used by most cells to produce acetyl-CoA for Kreb Cycle energy production…except for the brain, RBCs and testis.

Liver mitochondria produce ketone bodies from FFAs that are utilised by the brain, RBCs and testis as a secondary fuel source. Note that muscle cells can also use ketone bodies as fuel.

Question 10

Describe some of the beneficial effects of omega-3 fats as an anti-inflammatory and anti-thrombotic factor

Answer 10

In the inflammatory cascade arising from arachadonic acid, omega-3 fats:

• inhibit formation of _leukotrienes _
• inhibits formation of thromboxanes that mediate platelet agggregation and thrombosis
• promote formation of prostacyclins that prevent platelet thrombosis